Introduction

ICCROMUNESCO

WHC

INTROD

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CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

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This publication has received financial support from the World Heritage Fund

ISBN 92-9077-157-7

copy ICCROM 1999International Centre for the Study of the Preservationand Restoration of Cultural PropertyVia di San Michele 13 00153 Rome Italy

Printed in Italy by ATEL SpA ~ RomaGraphic Design wwwocomit ~ RomaEditing Christopher McDowall and Cynthia RockwellCover Design Andrea Urland

Photographs ICCROM Archive ~ Cover E Borrelli MT JaquintaW Schmid A Urland

INTROICCROM PREFACE

In recent decades the conservation of architec-tural heritage has increasingly drawn on newknowledge arising from the development of spe-cialized scientific disciplinesCondition assessment diagnosis conservation andrestoration treatment as well as long-term moni-toring of performance often call for specific inves-tigation techniques and interdisciplinary studywith specialized conservation laboratoriesOver the years conservation scientists workingwith cultural heritage have developed analyticalmethods procedures and testing techniques forthe study of materials their characteristics caus-es of deterioration alteration and decay process-es Such data are of essential support to planningany conservation or restoration activity as well aspreventive measuresInformed use of the various available testing tech-niques and measuring procedures combined withthe findings of other surveys and past experienceallow for establishing a more complete basis forcorrect diagnosis sound judgement and decision-making in the choice of the most appropriateconservation and restoration treatments withinthe general strategy for the historic buildingConservation professionals today are fully awareof the fundamental role materials science andlaboratory analysis play in the conservationprocessThus a certain knowledge of the basic and mostfrequently used laboratory tests and a capacity tointerpret their results should be an integral part ofthe general preparation of any professionalworking in the field of conservationWith the Laboratory Handbook ICCROM hopesto fulfil the long-felt need to provide a simple andpractical guide where basic concepts and practi-cal applications are integrated and explained

MARC LAENEN

DIRECTOR-GENERAL

INTROINTRODUCTION

INTRODUCTION

ConceptThe ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field ofconservation and restoration of architectural heritage

Target groupIt has been prepared principally for architects and engineers but may also be relevant for conservator-restorers archaeologists and others

Aimsbull To offer simplified and selected material structured to the needs of the target group an overview

of the problem area combined with laboratory practicals and case studiesbull To describe some of the most widely used practices and illustrate the various approaches to the

analysis of materials and their deterioration processesbull To facilitate interdisciplinary teamwork among scientists and other professionals involved in the

conservation process

ContextThe handbooks have evolved from lecture and laboratory handouts developed and constantlyupdated for ICCROMrsquos international or regional training programmes principally the following mid-career professional courses- Conservation of Architectural Heritage and Historic Structures- Technology of Stone Conservation- Conservation of Mural Paintings and Related Architectural Surfaces- Conservation of Architectural Surfaces as well as in a series of collaborative laboratory activities and consultancy and research projects

BackgroundThe Laboratory Handbook builds on valuable past experience Certain ICCROM publications alongsimilar lines namely ldquoPorous Building Materials Materials Science for Architectural Conservationrdquo byGiorgio Torraca (1982) and ldquoA Laboratory Manual for Architectural Conservatorsrdquo by Jeanne MarieTeutonico (1988) have been among the main referencesThe long-term experience accumulated from other institutions that of renowned experts and con-servationrestoration practice in general have naturally also been a point of reference as has theprocess of designing laboratory session modules for ICCROM courses in recent years the latter hasbeen enriched by feedback both from participants and from contributing leading specialists

StructureThe concept behind the Laboratory Handbook is modularThus it has been conceived as a set ofindependent volumes each of which will address a particular subject areaThe volumes relate par-ticularly to topics covered in the International Refresher Course on Conservation of ArchitecturalHeritage and Historic Structures (ARC) which are as followsbull DECAY MECHANISMS DIAGNOSIS guiding principles of materials science external climatic risk factors

atmospheric pollution humidity bio-deterioration investigation techniques surveying

[ 2 ]

[ 3 ]

bull CONSERVATION AND RESTORATION TREATMENTS OF BUILDING MATERIALS AND STRUCTURES

THEIR TESTING MONITORING AND EVALUATION timber earthen architectural heritage brick stonemortars metals modern materials surface finishesAs an issue of broader interest practical information and guidelines for setting up architecturalconservation laboratories are also scheduled for publication

In general each volume includesbull introductory informationbull explanations of scientific terms usedbull examples of common problemsbull types of analysis (basic principles)bull practicals (laboratory tests)bull applications (case study)bull bibliography

Principles are explained and some longstanding methodologies described Up-to-date informationon techniques instruments and widely used reference standards has been includedFurther information on practical techniques not strictly related to laboratory work is also providedin some casesThe practicals - involving basic tests and simple analyses - are conceived as part of ICCROMrsquos labo-ratory sessionsIt is understood that architects will not generally be performing laboratory analyses on their ownHowever knowledge of the types of analysis available for obtaining specific data their cost reliabilityand limitations is essential for todayrsquos conservation architectsThey should also be aware of samplingrequirements and techniques able to understand and interpret results and effectively communicatethem to other colleagues in interdisciplinary teamsThe individual volumes are being prepared by various authors depending on the subject and theirspecializationThe Scientific Committee is composed of specialists in the relevant fields who also havestrong ties with ICCROM through direct involvement in the training courses

Special featuresProgress in conservation science and technology means that currently available information must beregularly evaluated and updatedThis is why the Handbook has been structured as a series of volumesThis will allowbull the authors to periodically update specific volumes to reflect changing methodology and tech-

nology in an easy time-saving and economical way thanks to the digital reprinting processbull the users to work selectively with the volume relating to the particular problem they are facingbull new volumes to be gradually added to the set in line with developing needs until all the relevant

subjects have been covered by the series

We are aware that the information provided is not all-embracing but selective It is hoped that feed-back from users will assist us in improving and continually adapting the project to changing needs

The project team - Ernesto Borrelli and Andrea Urland - welcomes any constructive criticism com-ments and suggestions that might help us achieve this goal

ANDREA URLAND

ARC PROJECT MANAGER

INTRO

INTRO

INTRO

[ 4 ]

SCIENTIFIC COMMITTEE

ERNESTO BORRELLI (ex officio)ICCROM Laboratory Coordinator

GIACOMO CHIARI

Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic SciencesUniversity of Turin (Dipartimento di Scienze Mineralogiche e Petrologiche Universitagrave degli Studi diTorino)

MARISA LAURENZI TABASSO

Assistant to the Director-General ICCROM (Former Head of the Science and TechnologyProgramme) Chemist PhD

JEANNE MARIE TEUTONICO

Senior Architectural Conservator English HeritageGIORGIO TORRACA

Chemist University of Rome Faculty of Engineering (former ICCROM Deputy Director)ANDREA URLAND (ex officio)

ICCROM Architectural Conservation Project Manager Architect PhD

ACKNOWLEDGEMENTS

This publication was made possible thanks to the financial contribution of the UNESCO WorldHeritage Centre

The authors would especially like to express their gratitude to the members of the ScientificCommittee who kindly agreed to give this publication their support sharing their expertise byreviewing the draft texts and providing valuable comments and suggestionsThe authors further wish to recognize the scientific collaboration of Beatrice Muscatello the preciouscontribution of the advisors and the text editing by Christopher McDowall and Cynthia RockwellThey are equally grateful to the ICCROM staff for their whole-hearted collaboration and to thoseat the UNESCO World Heritage Centre for their supportAnd finally we wish to thank all the others who have in some way contributed to the preparationand completion of this publication

1998 - 99 VOLUMES

1 Introduction2 Porosity3 Salts4 Binders5 Colour specification and measurement

Dr HaroldPlenderleith (1898-1997)

INTROICCROMrsquos LABORATORYA BRIEF HISTORY

The new ICCROM Laboratory inaugu-rated in 1997 is dedicated to Dr HaroldPlenderleith the Organizationrsquos foundingdirectorIn 1966 during his tenure a first basiclaboratory was set up for didactic pur-poses at the original headquarters in ViaCavourIn the 1980s after ICCROM had movedto Via di San Michele a new and better-equipped laboratory was installed underthe coordination of Dr Giorgio Torracathen ICCROMrsquos Deputy Director Manyactivities including research werelaunched in the framework of the newlycreated ldquoTraining and Research UnitsrdquoLater a smaller specialized laboratorywas also developed by Jeanne MarieTeutonico for the architectural conser-vation coursesICCROMrsquos current Laboratory (about200 m2) is organized as a series of spe-cialized areas grouping together all theprevious activitiesThe works were sub-sidized by a major financial contributionfrom the Italian GovernmentThe improved level of equipment andfurnishing offers opportunities toexpand and further raise the standard ofwork

INTROICCROM

The International Centre for theStudy of the Preservation andRestoration of CulturalProperty (ICCROM) is a leadingvoice in the conservation of cul-tural heritage around the worldFounded by UNESCO in 1956and based in Rome ICCROM isthe only intergovernmentalorganization concerned withconserving all types of movableor immovable heritage

ICCROM takes action today forheritage tomorrow Its broad-ranging activities span five conti-nents and cover a wide range ofcultural heritage from cavepainting to sculpture ear thenarchitecture plastered faccediladeslibraries archives and entire his-toric centres

ICCROM tackles the challengesfacing cultural heritage ndashneglect pollution catastrophetheft or decay ndash and strengthensthe conditions for its effectiveconservation It provides aninternational platform fordebate while also tailoring activ-ities to regional needs

It achieves this by collecting anddisseminating information coor-dinating research offering con-sultancy and cooperation in fieldconservation and restorationproviding professional trainingand promoting awareness of thesocial value of cultural heritagepreservation amongst all sectorsof society

Above all ICCROM integratesthe conservation of our culturalheritage into global approachesto sustainable human develop-ment and community service

International Centre for the study

of the preservation and restoration

of cultural property

Via di San Michele 13I-00153 Rome RM Italy

e-mail iccromiccromorgwwwiccromorg

ISBN

92-9077-157-7

Porosity

Ernesto Borre l l i

ICCROMUNESCO

WHC

PORO

SITY

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

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Scientific CommitteeGiacomo Chiari (Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic Sciences University of Turin)Marisa Laurenzi Tabasso (Assistant to the Director-General ICCROM) Jeanne Marie Teutonico (Senior Architectural ConservatorEnglish Heritage) Giorgio Torraca (Chemist University of Rome Faculty of Engineering)Ex officioErnesto Borrelli (ICCROM Laboratory Coordinator)Andrea Urland (ICCROM Architectural Conservation Project Manager)

ICCROM Project Team Andrea Urland Ernesto BorrelliCollaboration Beatrice MuscatelloAdvice Rocco Mazzeo Paolo Saturno Photographs E Borrelli A Ortolan

Cover M Alonso Campoy E Borrelli MT Jaquinta R Lujan A Ortolan

ISBN 92-9077-157-7

copy ICCROM 1999International Centre for the Study of the Preservationand Restoration of Cultural PropertyVia di San Michele 13 00153 Rome Italy

Printed in Italy by ATEL SpA ~ RomaGraphic Design wwwocomit ~ RomaEditing Christopher McDowallCover Design Andrea Urland

This publication has received financial support from the World Heritage Fund

The ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field of conserva-tion of architectural heritage and historic structures It has been prepared mainly for architects and engineersbut may also be relevant for conservator-restorers or archaeologistsIt aims to

- offer an overview of each problem area combined with laboratory practicals and casestudies

- describe some of the most widely used practices and illustrate the various approaches tothe analysis of materials and their deterioration

- facilitate interdisciplinary teamwork among scientists and other professionals involved inthe conservation process

The Handbook has evolved from lecture and laboratory handouts that have been developed for the ICCROMtraining programmes It has been devised within the framework of the current courses principally theInternational Refresher Course on Conservation of Architectural Heritage and Historic Structures (ARC)

The general layout of each volume is as follows introductory information explanations of scientific termi-nology the most common problems met types of analysis laboratory tests case studies and bibliography

The concept behind the Handbook is modular and it has been purposely structured as a series ofindependent volumes to allow

- authors to periodically update the texts- users to work selectively with the volume relating to the particular problem they are facing- new volumes to be gradually added in line with developing needs

1998 - 99 volumes(1) Introduction (2) Porosity (3) Salts (4) Binders (5) Colour specification and measurement

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

ARCLaboratory Handbook

Porosity

Ernesto Borrel l i

R o m e 1 9 9 9

1 INTRODUCTION 3

2 CLASSIFICATION OF PORES 3

Typology 3

Geometry 3

Size 4

3 BASE TERMS 5

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS 6

In liquid form 6

In vapour form 6

5 DETERIORATION 6

6 METHODS OF MEASUREMENT 7

Direct methods 7

Petrography microscope analysis 7

Scanning electron microscopyanalysis (SEM) 7

Indirect methods 8

Mercury porosimetry 8

Nitrogen adsorption 8

Simple methods 9

Water absorption by total immersion 9

Water desorption 9

Water absorption by capillarity 9

Water vapour permeability 9

7 PRACTICALS 10

1 Measuring apparent volume and open pore volume 10

2 Measuring water absorption by total immersion 12

3 Measuring water desorption 15

8 CASE STUDY 17

9 BIBLIOGRAPHY 20

Footnotes11 Bibliographic references

CONTENTS

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

This publication has received financial support from the World Heritage Fund

ISBN 92-9077-157-7

copy ICCROM 1999International Centre for the Study of the Preservationand Restoration of Cultural PropertyVia di San Michele 13 00153 Rome Italy

Printed in Italy by ATEL SpA ~ RomaGraphic Design wwwocomit ~ RomaEditing Christopher McDowall and Cynthia RockwellCover Design Andrea Urland

Photographs ICCROM Archive ~ Cover E Borrelli MT JaquintaW Schmid A Urland

INTROICCROM PREFACE

In recent decades the conservation of architec-tural heritage has increasingly drawn on newknowledge arising from the development of spe-cialized scientific disciplinesCondition assessment diagnosis conservation andrestoration treatment as well as long-term moni-toring of performance often call for specific inves-tigation techniques and interdisciplinary studywith specialized conservation laboratoriesOver the years conservation scientists workingwith cultural heritage have developed analyticalmethods procedures and testing techniques forthe study of materials their characteristics caus-es of deterioration alteration and decay process-es Such data are of essential support to planningany conservation or restoration activity as well aspreventive measuresInformed use of the various available testing tech-niques and measuring procedures combined withthe findings of other surveys and past experienceallow for establishing a more complete basis forcorrect diagnosis sound judgement and decision-making in the choice of the most appropriateconservation and restoration treatments withinthe general strategy for the historic buildingConservation professionals today are fully awareof the fundamental role materials science andlaboratory analysis play in the conservationprocessThus a certain knowledge of the basic and mostfrequently used laboratory tests and a capacity tointerpret their results should be an integral part ofthe general preparation of any professionalworking in the field of conservationWith the Laboratory Handbook ICCROM hopesto fulfil the long-felt need to provide a simple andpractical guide where basic concepts and practi-cal applications are integrated and explained

MARC LAENEN

DIRECTOR-GENERAL

INTROINTRODUCTION

INTRODUCTION

ConceptThe ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field ofconservation and restoration of architectural heritage

Target groupIt has been prepared principally for architects and engineers but may also be relevant for conservator-restorers archaeologists and others

Aimsbull To offer simplified and selected material structured to the needs of the target group an overview

of the problem area combined with laboratory practicals and case studiesbull To describe some of the most widely used practices and illustrate the various approaches to the

analysis of materials and their deterioration processesbull To facilitate interdisciplinary teamwork among scientists and other professionals involved in the

conservation process

ContextThe handbooks have evolved from lecture and laboratory handouts developed and constantlyupdated for ICCROMrsquos international or regional training programmes principally the following mid-career professional courses- Conservation of Architectural Heritage and Historic Structures- Technology of Stone Conservation- Conservation of Mural Paintings and Related Architectural Surfaces- Conservation of Architectural Surfaces as well as in a series of collaborative laboratory activities and consultancy and research projects

BackgroundThe Laboratory Handbook builds on valuable past experience Certain ICCROM publications alongsimilar lines namely ldquoPorous Building Materials Materials Science for Architectural Conservationrdquo byGiorgio Torraca (1982) and ldquoA Laboratory Manual for Architectural Conservatorsrdquo by Jeanne MarieTeutonico (1988) have been among the main referencesThe long-term experience accumulated from other institutions that of renowned experts and con-servationrestoration practice in general have naturally also been a point of reference as has theprocess of designing laboratory session modules for ICCROM courses in recent years the latter hasbeen enriched by feedback both from participants and from contributing leading specialists

StructureThe concept behind the Laboratory Handbook is modularThus it has been conceived as a set ofindependent volumes each of which will address a particular subject areaThe volumes relate par-ticularly to topics covered in the International Refresher Course on Conservation of ArchitecturalHeritage and Historic Structures (ARC) which are as followsbull DECAY MECHANISMS DIAGNOSIS guiding principles of materials science external climatic risk factors

atmospheric pollution humidity bio-deterioration investigation techniques surveying

[ 2 ]

[ 3 ]

bull CONSERVATION AND RESTORATION TREATMENTS OF BUILDING MATERIALS AND STRUCTURES

THEIR TESTING MONITORING AND EVALUATION timber earthen architectural heritage brick stonemortars metals modern materials surface finishesAs an issue of broader interest practical information and guidelines for setting up architecturalconservation laboratories are also scheduled for publication

In general each volume includesbull introductory informationbull explanations of scientific terms usedbull examples of common problemsbull types of analysis (basic principles)bull practicals (laboratory tests)bull applications (case study)bull bibliography

Principles are explained and some longstanding methodologies described Up-to-date informationon techniques instruments and widely used reference standards has been includedFurther information on practical techniques not strictly related to laboratory work is also providedin some casesThe practicals - involving basic tests and simple analyses - are conceived as part of ICCROMrsquos labo-ratory sessionsIt is understood that architects will not generally be performing laboratory analyses on their ownHowever knowledge of the types of analysis available for obtaining specific data their cost reliabilityand limitations is essential for todayrsquos conservation architectsThey should also be aware of samplingrequirements and techniques able to understand and interpret results and effectively communicatethem to other colleagues in interdisciplinary teamsThe individual volumes are being prepared by various authors depending on the subject and theirspecializationThe Scientific Committee is composed of specialists in the relevant fields who also havestrong ties with ICCROM through direct involvement in the training courses

Special featuresProgress in conservation science and technology means that currently available information must beregularly evaluated and updatedThis is why the Handbook has been structured as a series of volumesThis will allowbull the authors to periodically update specific volumes to reflect changing methodology and tech-

nology in an easy time-saving and economical way thanks to the digital reprinting processbull the users to work selectively with the volume relating to the particular problem they are facingbull new volumes to be gradually added to the set in line with developing needs until all the relevant

subjects have been covered by the series

We are aware that the information provided is not all-embracing but selective It is hoped that feed-back from users will assist us in improving and continually adapting the project to changing needs

The project team - Ernesto Borrelli and Andrea Urland - welcomes any constructive criticism com-ments and suggestions that might help us achieve this goal

ANDREA URLAND

ARC PROJECT MANAGER

INTRO

INTRO

INTRO

[ 4 ]

SCIENTIFIC COMMITTEE

ERNESTO BORRELLI (ex officio)ICCROM Laboratory Coordinator

GIACOMO CHIARI

Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic SciencesUniversity of Turin (Dipartimento di Scienze Mineralogiche e Petrologiche Universitagrave degli Studi diTorino)

MARISA LAURENZI TABASSO

Assistant to the Director-General ICCROM (Former Head of the Science and TechnologyProgramme) Chemist PhD

JEANNE MARIE TEUTONICO

Senior Architectural Conservator English HeritageGIORGIO TORRACA

Chemist University of Rome Faculty of Engineering (former ICCROM Deputy Director)ANDREA URLAND (ex officio)

ICCROM Architectural Conservation Project Manager Architect PhD

ACKNOWLEDGEMENTS

This publication was made possible thanks to the financial contribution of the UNESCO WorldHeritage Centre

The authors would especially like to express their gratitude to the members of the ScientificCommittee who kindly agreed to give this publication their support sharing their expertise byreviewing the draft texts and providing valuable comments and suggestionsThe authors further wish to recognize the scientific collaboration of Beatrice Muscatello the preciouscontribution of the advisors and the text editing by Christopher McDowall and Cynthia RockwellThey are equally grateful to the ICCROM staff for their whole-hearted collaboration and to thoseat the UNESCO World Heritage Centre for their supportAnd finally we wish to thank all the others who have in some way contributed to the preparationand completion of this publication

1998 - 99 VOLUMES

1 Introduction2 Porosity3 Salts4 Binders5 Colour specification and measurement

Dr HaroldPlenderleith (1898-1997)

INTROICCROMrsquos LABORATORYA BRIEF HISTORY

The new ICCROM Laboratory inaugu-rated in 1997 is dedicated to Dr HaroldPlenderleith the Organizationrsquos foundingdirectorIn 1966 during his tenure a first basiclaboratory was set up for didactic pur-poses at the original headquarters in ViaCavourIn the 1980s after ICCROM had movedto Via di San Michele a new and better-equipped laboratory was installed underthe coordination of Dr Giorgio Torracathen ICCROMrsquos Deputy Director Manyactivities including research werelaunched in the framework of the newlycreated ldquoTraining and Research UnitsrdquoLater a smaller specialized laboratorywas also developed by Jeanne MarieTeutonico for the architectural conser-vation coursesICCROMrsquos current Laboratory (about200 m2) is organized as a series of spe-cialized areas grouping together all theprevious activitiesThe works were sub-sidized by a major financial contributionfrom the Italian GovernmentThe improved level of equipment andfurnishing offers opportunities toexpand and further raise the standard ofwork

INTROICCROM

The International Centre for theStudy of the Preservation andRestoration of CulturalProperty (ICCROM) is a leadingvoice in the conservation of cul-tural heritage around the worldFounded by UNESCO in 1956and based in Rome ICCROM isthe only intergovernmentalorganization concerned withconserving all types of movableor immovable heritage

ICCROM takes action today forheritage tomorrow Its broad-ranging activities span five conti-nents and cover a wide range ofcultural heritage from cavepainting to sculpture ear thenarchitecture plastered faccediladeslibraries archives and entire his-toric centres

ICCROM tackles the challengesfacing cultural heritage ndashneglect pollution catastrophetheft or decay ndash and strengthensthe conditions for its effectiveconservation It provides aninternational platform fordebate while also tailoring activ-ities to regional needs

It achieves this by collecting anddisseminating information coor-dinating research offering con-sultancy and cooperation in fieldconservation and restorationproviding professional trainingand promoting awareness of thesocial value of cultural heritagepreservation amongst all sectorsof society

Above all ICCROM integratesthe conservation of our culturalheritage into global approachesto sustainable human develop-ment and community service

International Centre for the study

of the preservation and restoration

of cultural property

Via di San Michele 13I-00153 Rome RM Italy

e-mail iccromiccromorgwwwiccromorg

ISBN

92-9077-157-7

Porosity

Ernesto Borre l l i

ICCROMUNESCO

WHC

PORO

SITY

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

LA

BO

RAT

OR

Y H

AN

DB

OO

K

2VOLUME

99

AR

C

Scientific CommitteeGiacomo Chiari (Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic Sciences University of Turin)Marisa Laurenzi Tabasso (Assistant to the Director-General ICCROM) Jeanne Marie Teutonico (Senior Architectural ConservatorEnglish Heritage) Giorgio Torraca (Chemist University of Rome Faculty of Engineering)Ex officioErnesto Borrelli (ICCROM Laboratory Coordinator)Andrea Urland (ICCROM Architectural Conservation Project Manager)

ICCROM Project Team Andrea Urland Ernesto BorrelliCollaboration Beatrice MuscatelloAdvice Rocco Mazzeo Paolo Saturno Photographs E Borrelli A Ortolan

Cover M Alonso Campoy E Borrelli MT Jaquinta R Lujan A Ortolan

ISBN 92-9077-157-7

copy ICCROM 1999International Centre for the Study of the Preservationand Restoration of Cultural PropertyVia di San Michele 13 00153 Rome Italy

Printed in Italy by ATEL SpA ~ RomaGraphic Design wwwocomit ~ RomaEditing Christopher McDowallCover Design Andrea Urland

This publication has received financial support from the World Heritage Fund

The ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field of conserva-tion of architectural heritage and historic structures It has been prepared mainly for architects and engineersbut may also be relevant for conservator-restorers or archaeologistsIt aims to

- offer an overview of each problem area combined with laboratory practicals and casestudies

- describe some of the most widely used practices and illustrate the various approaches tothe analysis of materials and their deterioration

- facilitate interdisciplinary teamwork among scientists and other professionals involved inthe conservation process

The Handbook has evolved from lecture and laboratory handouts that have been developed for the ICCROMtraining programmes It has been devised within the framework of the current courses principally theInternational Refresher Course on Conservation of Architectural Heritage and Historic Structures (ARC)

The general layout of each volume is as follows introductory information explanations of scientific termi-nology the most common problems met types of analysis laboratory tests case studies and bibliography

The concept behind the Handbook is modular and it has been purposely structured as a series ofindependent volumes to allow

- authors to periodically update the texts- users to work selectively with the volume relating to the particular problem they are facing- new volumes to be gradually added in line with developing needs

1998 - 99 volumes(1) Introduction (2) Porosity (3) Salts (4) Binders (5) Colour specification and measurement

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

ARCLaboratory Handbook

Porosity

Ernesto Borrel l i

R o m e 1 9 9 9

1 INTRODUCTION 3

2 CLASSIFICATION OF PORES 3

Typology 3

Geometry 3

Size 4

3 BASE TERMS 5

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS 6

In liquid form 6

In vapour form 6

5 DETERIORATION 6

6 METHODS OF MEASUREMENT 7

Direct methods 7

Petrography microscope analysis 7

Scanning electron microscopyanalysis (SEM) 7

Indirect methods 8

Mercury porosimetry 8

Nitrogen adsorption 8

Simple methods 9

Water absorption by total immersion 9

Water desorption 9

Water absorption by capillarity 9

Water vapour permeability 9

7 PRACTICALS 10

1 Measuring apparent volume and open pore volume 10

2 Measuring water absorption by total immersion 12

3 Measuring water desorption 15

8 CASE STUDY 17

9 BIBLIOGRAPHY 20

Footnotes11 Bibliographic references

CONTENTS

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

INTROICCROM PREFACE

In recent decades the conservation of architec-tural heritage has increasingly drawn on newknowledge arising from the development of spe-cialized scientific disciplinesCondition assessment diagnosis conservation andrestoration treatment as well as long-term moni-toring of performance often call for specific inves-tigation techniques and interdisciplinary studywith specialized conservation laboratoriesOver the years conservation scientists workingwith cultural heritage have developed analyticalmethods procedures and testing techniques forthe study of materials their characteristics caus-es of deterioration alteration and decay process-es Such data are of essential support to planningany conservation or restoration activity as well aspreventive measuresInformed use of the various available testing tech-niques and measuring procedures combined withthe findings of other surveys and past experienceallow for establishing a more complete basis forcorrect diagnosis sound judgement and decision-making in the choice of the most appropriateconservation and restoration treatments withinthe general strategy for the historic buildingConservation professionals today are fully awareof the fundamental role materials science andlaboratory analysis play in the conservationprocessThus a certain knowledge of the basic and mostfrequently used laboratory tests and a capacity tointerpret their results should be an integral part ofthe general preparation of any professionalworking in the field of conservationWith the Laboratory Handbook ICCROM hopesto fulfil the long-felt need to provide a simple andpractical guide where basic concepts and practi-cal applications are integrated and explained

MARC LAENEN

DIRECTOR-GENERAL

INTROINTRODUCTION

INTRODUCTION

ConceptThe ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field ofconservation and restoration of architectural heritage

Target groupIt has been prepared principally for architects and engineers but may also be relevant for conservator-restorers archaeologists and others

Aimsbull To offer simplified and selected material structured to the needs of the target group an overview

of the problem area combined with laboratory practicals and case studiesbull To describe some of the most widely used practices and illustrate the various approaches to the

analysis of materials and their deterioration processesbull To facilitate interdisciplinary teamwork among scientists and other professionals involved in the

conservation process

ContextThe handbooks have evolved from lecture and laboratory handouts developed and constantlyupdated for ICCROMrsquos international or regional training programmes principally the following mid-career professional courses- Conservation of Architectural Heritage and Historic Structures- Technology of Stone Conservation- Conservation of Mural Paintings and Related Architectural Surfaces- Conservation of Architectural Surfaces as well as in a series of collaborative laboratory activities and consultancy and research projects

BackgroundThe Laboratory Handbook builds on valuable past experience Certain ICCROM publications alongsimilar lines namely ldquoPorous Building Materials Materials Science for Architectural Conservationrdquo byGiorgio Torraca (1982) and ldquoA Laboratory Manual for Architectural Conservatorsrdquo by Jeanne MarieTeutonico (1988) have been among the main referencesThe long-term experience accumulated from other institutions that of renowned experts and con-servationrestoration practice in general have naturally also been a point of reference as has theprocess of designing laboratory session modules for ICCROM courses in recent years the latter hasbeen enriched by feedback both from participants and from contributing leading specialists

StructureThe concept behind the Laboratory Handbook is modularThus it has been conceived as a set ofindependent volumes each of which will address a particular subject areaThe volumes relate par-ticularly to topics covered in the International Refresher Course on Conservation of ArchitecturalHeritage and Historic Structures (ARC) which are as followsbull DECAY MECHANISMS DIAGNOSIS guiding principles of materials science external climatic risk factors

atmospheric pollution humidity bio-deterioration investigation techniques surveying

[ 2 ]

[ 3 ]

bull CONSERVATION AND RESTORATION TREATMENTS OF BUILDING MATERIALS AND STRUCTURES

THEIR TESTING MONITORING AND EVALUATION timber earthen architectural heritage brick stonemortars metals modern materials surface finishesAs an issue of broader interest practical information and guidelines for setting up architecturalconservation laboratories are also scheduled for publication

In general each volume includesbull introductory informationbull explanations of scientific terms usedbull examples of common problemsbull types of analysis (basic principles)bull practicals (laboratory tests)bull applications (case study)bull bibliography

Principles are explained and some longstanding methodologies described Up-to-date informationon techniques instruments and widely used reference standards has been includedFurther information on practical techniques not strictly related to laboratory work is also providedin some casesThe practicals - involving basic tests and simple analyses - are conceived as part of ICCROMrsquos labo-ratory sessionsIt is understood that architects will not generally be performing laboratory analyses on their ownHowever knowledge of the types of analysis available for obtaining specific data their cost reliabilityand limitations is essential for todayrsquos conservation architectsThey should also be aware of samplingrequirements and techniques able to understand and interpret results and effectively communicatethem to other colleagues in interdisciplinary teamsThe individual volumes are being prepared by various authors depending on the subject and theirspecializationThe Scientific Committee is composed of specialists in the relevant fields who also havestrong ties with ICCROM through direct involvement in the training courses

Special featuresProgress in conservation science and technology means that currently available information must beregularly evaluated and updatedThis is why the Handbook has been structured as a series of volumesThis will allowbull the authors to periodically update specific volumes to reflect changing methodology and tech-

nology in an easy time-saving and economical way thanks to the digital reprinting processbull the users to work selectively with the volume relating to the particular problem they are facingbull new volumes to be gradually added to the set in line with developing needs until all the relevant

subjects have been covered by the series

We are aware that the information provided is not all-embracing but selective It is hoped that feed-back from users will assist us in improving and continually adapting the project to changing needs

The project team - Ernesto Borrelli and Andrea Urland - welcomes any constructive criticism com-ments and suggestions that might help us achieve this goal

ANDREA URLAND

ARC PROJECT MANAGER

INTRO

INTRO

INTRO

[ 4 ]

SCIENTIFIC COMMITTEE

ERNESTO BORRELLI (ex officio)ICCROM Laboratory Coordinator

GIACOMO CHIARI

Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic SciencesUniversity of Turin (Dipartimento di Scienze Mineralogiche e Petrologiche Universitagrave degli Studi diTorino)

MARISA LAURENZI TABASSO

Assistant to the Director-General ICCROM (Former Head of the Science and TechnologyProgramme) Chemist PhD

JEANNE MARIE TEUTONICO

Senior Architectural Conservator English HeritageGIORGIO TORRACA

Chemist University of Rome Faculty of Engineering (former ICCROM Deputy Director)ANDREA URLAND (ex officio)

ICCROM Architectural Conservation Project Manager Architect PhD

ACKNOWLEDGEMENTS

This publication was made possible thanks to the financial contribution of the UNESCO WorldHeritage Centre

The authors would especially like to express their gratitude to the members of the ScientificCommittee who kindly agreed to give this publication their support sharing their expertise byreviewing the draft texts and providing valuable comments and suggestionsThe authors further wish to recognize the scientific collaboration of Beatrice Muscatello the preciouscontribution of the advisors and the text editing by Christopher McDowall and Cynthia RockwellThey are equally grateful to the ICCROM staff for their whole-hearted collaboration and to thoseat the UNESCO World Heritage Centre for their supportAnd finally we wish to thank all the others who have in some way contributed to the preparationand completion of this publication

1998 - 99 VOLUMES

1 Introduction2 Porosity3 Salts4 Binders5 Colour specification and measurement

Dr HaroldPlenderleith (1898-1997)

INTROICCROMrsquos LABORATORYA BRIEF HISTORY

The new ICCROM Laboratory inaugu-rated in 1997 is dedicated to Dr HaroldPlenderleith the Organizationrsquos foundingdirectorIn 1966 during his tenure a first basiclaboratory was set up for didactic pur-poses at the original headquarters in ViaCavourIn the 1980s after ICCROM had movedto Via di San Michele a new and better-equipped laboratory was installed underthe coordination of Dr Giorgio Torracathen ICCROMrsquos Deputy Director Manyactivities including research werelaunched in the framework of the newlycreated ldquoTraining and Research UnitsrdquoLater a smaller specialized laboratorywas also developed by Jeanne MarieTeutonico for the architectural conser-vation coursesICCROMrsquos current Laboratory (about200 m2) is organized as a series of spe-cialized areas grouping together all theprevious activitiesThe works were sub-sidized by a major financial contributionfrom the Italian GovernmentThe improved level of equipment andfurnishing offers opportunities toexpand and further raise the standard ofwork

INTROICCROM

The International Centre for theStudy of the Preservation andRestoration of CulturalProperty (ICCROM) is a leadingvoice in the conservation of cul-tural heritage around the worldFounded by UNESCO in 1956and based in Rome ICCROM isthe only intergovernmentalorganization concerned withconserving all types of movableor immovable heritage

ICCROM takes action today forheritage tomorrow Its broad-ranging activities span five conti-nents and cover a wide range ofcultural heritage from cavepainting to sculpture ear thenarchitecture plastered faccediladeslibraries archives and entire his-toric centres

ICCROM tackles the challengesfacing cultural heritage ndashneglect pollution catastrophetheft or decay ndash and strengthensthe conditions for its effectiveconservation It provides aninternational platform fordebate while also tailoring activ-ities to regional needs

It achieves this by collecting anddisseminating information coor-dinating research offering con-sultancy and cooperation in fieldconservation and restorationproviding professional trainingand promoting awareness of thesocial value of cultural heritagepreservation amongst all sectorsof society

Above all ICCROM integratesthe conservation of our culturalheritage into global approachesto sustainable human develop-ment and community service

International Centre for the study

of the preservation and restoration

of cultural property

Via di San Michele 13I-00153 Rome RM Italy

e-mail iccromiccromorgwwwiccromorg

ISBN

92-9077-157-7

Porosity

Ernesto Borre l l i

ICCROMUNESCO

WHC

PORO

SITY

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

LA

BO

RAT

OR

Y H

AN

DB

OO

K

2VOLUME

99

AR

C

Scientific CommitteeGiacomo Chiari (Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic Sciences University of Turin)Marisa Laurenzi Tabasso (Assistant to the Director-General ICCROM) Jeanne Marie Teutonico (Senior Architectural ConservatorEnglish Heritage) Giorgio Torraca (Chemist University of Rome Faculty of Engineering)Ex officioErnesto Borrelli (ICCROM Laboratory Coordinator)Andrea Urland (ICCROM Architectural Conservation Project Manager)

ICCROM Project Team Andrea Urland Ernesto BorrelliCollaboration Beatrice MuscatelloAdvice Rocco Mazzeo Paolo Saturno Photographs E Borrelli A Ortolan

Cover M Alonso Campoy E Borrelli MT Jaquinta R Lujan A Ortolan

ISBN 92-9077-157-7

copy ICCROM 1999International Centre for the Study of the Preservationand Restoration of Cultural PropertyVia di San Michele 13 00153 Rome Italy

Printed in Italy by ATEL SpA ~ RomaGraphic Design wwwocomit ~ RomaEditing Christopher McDowallCover Design Andrea Urland

This publication has received financial support from the World Heritage Fund

The ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field of conserva-tion of architectural heritage and historic structures It has been prepared mainly for architects and engineersbut may also be relevant for conservator-restorers or archaeologistsIt aims to

- offer an overview of each problem area combined with laboratory practicals and casestudies

- describe some of the most widely used practices and illustrate the various approaches tothe analysis of materials and their deterioration

- facilitate interdisciplinary teamwork among scientists and other professionals involved inthe conservation process

The Handbook has evolved from lecture and laboratory handouts that have been developed for the ICCROMtraining programmes It has been devised within the framework of the current courses principally theInternational Refresher Course on Conservation of Architectural Heritage and Historic Structures (ARC)

The general layout of each volume is as follows introductory information explanations of scientific termi-nology the most common problems met types of analysis laboratory tests case studies and bibliography

The concept behind the Handbook is modular and it has been purposely structured as a series ofindependent volumes to allow

- authors to periodically update the texts- users to work selectively with the volume relating to the particular problem they are facing- new volumes to be gradually added in line with developing needs

1998 - 99 volumes(1) Introduction (2) Porosity (3) Salts (4) Binders (5) Colour specification and measurement

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

ARCLaboratory Handbook

Porosity

Ernesto Borrel l i

R o m e 1 9 9 9

1 INTRODUCTION 3

2 CLASSIFICATION OF PORES 3

Typology 3

Geometry 3

Size 4

3 BASE TERMS 5

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS 6

In liquid form 6

In vapour form 6

5 DETERIORATION 6

6 METHODS OF MEASUREMENT 7

Direct methods 7

Petrography microscope analysis 7

Scanning electron microscopyanalysis (SEM) 7

Indirect methods 8

Mercury porosimetry 8

Nitrogen adsorption 8

Simple methods 9

Water absorption by total immersion 9

Water desorption 9

Water absorption by capillarity 9

Water vapour permeability 9

7 PRACTICALS 10

1 Measuring apparent volume and open pore volume 10

2 Measuring water absorption by total immersion 12

3 Measuring water desorption 15

8 CASE STUDY 17

9 BIBLIOGRAPHY 20

Footnotes11 Bibliographic references

CONTENTS

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

INTROINTRODUCTION

INTRODUCTION

ConceptThe ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field ofconservation and restoration of architectural heritage

Target groupIt has been prepared principally for architects and engineers but may also be relevant for conservator-restorers archaeologists and others

Aimsbull To offer simplified and selected material structured to the needs of the target group an overview

of the problem area combined with laboratory practicals and case studiesbull To describe some of the most widely used practices and illustrate the various approaches to the

analysis of materials and their deterioration processesbull To facilitate interdisciplinary teamwork among scientists and other professionals involved in the

conservation process

ContextThe handbooks have evolved from lecture and laboratory handouts developed and constantlyupdated for ICCROMrsquos international or regional training programmes principally the following mid-career professional courses- Conservation of Architectural Heritage and Historic Structures- Technology of Stone Conservation- Conservation of Mural Paintings and Related Architectural Surfaces- Conservation of Architectural Surfaces as well as in a series of collaborative laboratory activities and consultancy and research projects

BackgroundThe Laboratory Handbook builds on valuable past experience Certain ICCROM publications alongsimilar lines namely ldquoPorous Building Materials Materials Science for Architectural Conservationrdquo byGiorgio Torraca (1982) and ldquoA Laboratory Manual for Architectural Conservatorsrdquo by Jeanne MarieTeutonico (1988) have been among the main referencesThe long-term experience accumulated from other institutions that of renowned experts and con-servationrestoration practice in general have naturally also been a point of reference as has theprocess of designing laboratory session modules for ICCROM courses in recent years the latter hasbeen enriched by feedback both from participants and from contributing leading specialists

StructureThe concept behind the Laboratory Handbook is modularThus it has been conceived as a set ofindependent volumes each of which will address a particular subject areaThe volumes relate par-ticularly to topics covered in the International Refresher Course on Conservation of ArchitecturalHeritage and Historic Structures (ARC) which are as followsbull DECAY MECHANISMS DIAGNOSIS guiding principles of materials science external climatic risk factors

atmospheric pollution humidity bio-deterioration investigation techniques surveying

[ 2 ]

[ 3 ]

bull CONSERVATION AND RESTORATION TREATMENTS OF BUILDING MATERIALS AND STRUCTURES

THEIR TESTING MONITORING AND EVALUATION timber earthen architectural heritage brick stonemortars metals modern materials surface finishesAs an issue of broader interest practical information and guidelines for setting up architecturalconservation laboratories are also scheduled for publication

In general each volume includesbull introductory informationbull explanations of scientific terms usedbull examples of common problemsbull types of analysis (basic principles)bull practicals (laboratory tests)bull applications (case study)bull bibliography

Principles are explained and some longstanding methodologies described Up-to-date informationon techniques instruments and widely used reference standards has been includedFurther information on practical techniques not strictly related to laboratory work is also providedin some casesThe practicals - involving basic tests and simple analyses - are conceived as part of ICCROMrsquos labo-ratory sessionsIt is understood that architects will not generally be performing laboratory analyses on their ownHowever knowledge of the types of analysis available for obtaining specific data their cost reliabilityand limitations is essential for todayrsquos conservation architectsThey should also be aware of samplingrequirements and techniques able to understand and interpret results and effectively communicatethem to other colleagues in interdisciplinary teamsThe individual volumes are being prepared by various authors depending on the subject and theirspecializationThe Scientific Committee is composed of specialists in the relevant fields who also havestrong ties with ICCROM through direct involvement in the training courses

Special featuresProgress in conservation science and technology means that currently available information must beregularly evaluated and updatedThis is why the Handbook has been structured as a series of volumesThis will allowbull the authors to periodically update specific volumes to reflect changing methodology and tech-

nology in an easy time-saving and economical way thanks to the digital reprinting processbull the users to work selectively with the volume relating to the particular problem they are facingbull new volumes to be gradually added to the set in line with developing needs until all the relevant

subjects have been covered by the series

We are aware that the information provided is not all-embracing but selective It is hoped that feed-back from users will assist us in improving and continually adapting the project to changing needs

The project team - Ernesto Borrelli and Andrea Urland - welcomes any constructive criticism com-ments and suggestions that might help us achieve this goal

ANDREA URLAND

ARC PROJECT MANAGER

INTRO

INTRO

INTRO

[ 4 ]

SCIENTIFIC COMMITTEE

ERNESTO BORRELLI (ex officio)ICCROM Laboratory Coordinator

GIACOMO CHIARI

Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic SciencesUniversity of Turin (Dipartimento di Scienze Mineralogiche e Petrologiche Universitagrave degli Studi diTorino)

MARISA LAURENZI TABASSO

Assistant to the Director-General ICCROM (Former Head of the Science and TechnologyProgramme) Chemist PhD

JEANNE MARIE TEUTONICO

Senior Architectural Conservator English HeritageGIORGIO TORRACA

Chemist University of Rome Faculty of Engineering (former ICCROM Deputy Director)ANDREA URLAND (ex officio)

ICCROM Architectural Conservation Project Manager Architect PhD

ACKNOWLEDGEMENTS

This publication was made possible thanks to the financial contribution of the UNESCO WorldHeritage Centre

The authors would especially like to express their gratitude to the members of the ScientificCommittee who kindly agreed to give this publication their support sharing their expertise byreviewing the draft texts and providing valuable comments and suggestionsThe authors further wish to recognize the scientific collaboration of Beatrice Muscatello the preciouscontribution of the advisors and the text editing by Christopher McDowall and Cynthia RockwellThey are equally grateful to the ICCROM staff for their whole-hearted collaboration and to thoseat the UNESCO World Heritage Centre for their supportAnd finally we wish to thank all the others who have in some way contributed to the preparationand completion of this publication

1998 - 99 VOLUMES

1 Introduction2 Porosity3 Salts4 Binders5 Colour specification and measurement

Dr HaroldPlenderleith (1898-1997)

INTROICCROMrsquos LABORATORYA BRIEF HISTORY

The new ICCROM Laboratory inaugu-rated in 1997 is dedicated to Dr HaroldPlenderleith the Organizationrsquos foundingdirectorIn 1966 during his tenure a first basiclaboratory was set up for didactic pur-poses at the original headquarters in ViaCavourIn the 1980s after ICCROM had movedto Via di San Michele a new and better-equipped laboratory was installed underthe coordination of Dr Giorgio Torracathen ICCROMrsquos Deputy Director Manyactivities including research werelaunched in the framework of the newlycreated ldquoTraining and Research UnitsrdquoLater a smaller specialized laboratorywas also developed by Jeanne MarieTeutonico for the architectural conser-vation coursesICCROMrsquos current Laboratory (about200 m2) is organized as a series of spe-cialized areas grouping together all theprevious activitiesThe works were sub-sidized by a major financial contributionfrom the Italian GovernmentThe improved level of equipment andfurnishing offers opportunities toexpand and further raise the standard ofwork

INTROICCROM

The International Centre for theStudy of the Preservation andRestoration of CulturalProperty (ICCROM) is a leadingvoice in the conservation of cul-tural heritage around the worldFounded by UNESCO in 1956and based in Rome ICCROM isthe only intergovernmentalorganization concerned withconserving all types of movableor immovable heritage

ICCROM takes action today forheritage tomorrow Its broad-ranging activities span five conti-nents and cover a wide range ofcultural heritage from cavepainting to sculpture ear thenarchitecture plastered faccediladeslibraries archives and entire his-toric centres

ICCROM tackles the challengesfacing cultural heritage ndashneglect pollution catastrophetheft or decay ndash and strengthensthe conditions for its effectiveconservation It provides aninternational platform fordebate while also tailoring activ-ities to regional needs

It achieves this by collecting anddisseminating information coor-dinating research offering con-sultancy and cooperation in fieldconservation and restorationproviding professional trainingand promoting awareness of thesocial value of cultural heritagepreservation amongst all sectorsof society

Above all ICCROM integratesthe conservation of our culturalheritage into global approachesto sustainable human develop-ment and community service

International Centre for the study

of the preservation and restoration

of cultural property

Via di San Michele 13I-00153 Rome RM Italy

e-mail iccromiccromorgwwwiccromorg

ISBN

92-9077-157-7

Porosity

Ernesto Borre l l i

ICCROMUNESCO

WHC

PORO

SITY

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

LA

BO

RAT

OR

Y H

AN

DB

OO

K

2VOLUME

99

AR

C

Scientific CommitteeGiacomo Chiari (Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic Sciences University of Turin)Marisa Laurenzi Tabasso (Assistant to the Director-General ICCROM) Jeanne Marie Teutonico (Senior Architectural ConservatorEnglish Heritage) Giorgio Torraca (Chemist University of Rome Faculty of Engineering)Ex officioErnesto Borrelli (ICCROM Laboratory Coordinator)Andrea Urland (ICCROM Architectural Conservation Project Manager)

ICCROM Project Team Andrea Urland Ernesto BorrelliCollaboration Beatrice MuscatelloAdvice Rocco Mazzeo Paolo Saturno Photographs E Borrelli A Ortolan

Cover M Alonso Campoy E Borrelli MT Jaquinta R Lujan A Ortolan

ISBN 92-9077-157-7

copy ICCROM 1999International Centre for the Study of the Preservationand Restoration of Cultural PropertyVia di San Michele 13 00153 Rome Italy

Printed in Italy by ATEL SpA ~ RomaGraphic Design wwwocomit ~ RomaEditing Christopher McDowallCover Design Andrea Urland

This publication has received financial support from the World Heritage Fund

The ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field of conserva-tion of architectural heritage and historic structures It has been prepared mainly for architects and engineersbut may also be relevant for conservator-restorers or archaeologistsIt aims to

- offer an overview of each problem area combined with laboratory practicals and casestudies

- describe some of the most widely used practices and illustrate the various approaches tothe analysis of materials and their deterioration

- facilitate interdisciplinary teamwork among scientists and other professionals involved inthe conservation process

The Handbook has evolved from lecture and laboratory handouts that have been developed for the ICCROMtraining programmes It has been devised within the framework of the current courses principally theInternational Refresher Course on Conservation of Architectural Heritage and Historic Structures (ARC)

The general layout of each volume is as follows introductory information explanations of scientific termi-nology the most common problems met types of analysis laboratory tests case studies and bibliography

The concept behind the Handbook is modular and it has been purposely structured as a series ofindependent volumes to allow

- authors to periodically update the texts- users to work selectively with the volume relating to the particular problem they are facing- new volumes to be gradually added in line with developing needs

1998 - 99 volumes(1) Introduction (2) Porosity (3) Salts (4) Binders (5) Colour specification and measurement

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

ARCLaboratory Handbook

Porosity

Ernesto Borrel l i

R o m e 1 9 9 9

1 INTRODUCTION 3

2 CLASSIFICATION OF PORES 3

Typology 3

Geometry 3

Size 4

3 BASE TERMS 5

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS 6

In liquid form 6

In vapour form 6

5 DETERIORATION 6

6 METHODS OF MEASUREMENT 7

Direct methods 7

Petrography microscope analysis 7

Scanning electron microscopyanalysis (SEM) 7

Indirect methods 8

Mercury porosimetry 8

Nitrogen adsorption 8

Simple methods 9

Water absorption by total immersion 9

Water desorption 9

Water absorption by capillarity 9

Water vapour permeability 9

7 PRACTICALS 10

1 Measuring apparent volume and open pore volume 10

2 Measuring water absorption by total immersion 12

3 Measuring water desorption 15

8 CASE STUDY 17

9 BIBLIOGRAPHY 20

Footnotes11 Bibliographic references

CONTENTS

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

[ 3 ]

bull CONSERVATION AND RESTORATION TREATMENTS OF BUILDING MATERIALS AND STRUCTURES

THEIR TESTING MONITORING AND EVALUATION timber earthen architectural heritage brick stonemortars metals modern materials surface finishesAs an issue of broader interest practical information and guidelines for setting up architecturalconservation laboratories are also scheduled for publication

In general each volume includesbull introductory informationbull explanations of scientific terms usedbull examples of common problemsbull types of analysis (basic principles)bull practicals (laboratory tests)bull applications (case study)bull bibliography

Principles are explained and some longstanding methodologies described Up-to-date informationon techniques instruments and widely used reference standards has been includedFurther information on practical techniques not strictly related to laboratory work is also providedin some casesThe practicals - involving basic tests and simple analyses - are conceived as part of ICCROMrsquos labo-ratory sessionsIt is understood that architects will not generally be performing laboratory analyses on their ownHowever knowledge of the types of analysis available for obtaining specific data their cost reliabilityand limitations is essential for todayrsquos conservation architectsThey should also be aware of samplingrequirements and techniques able to understand and interpret results and effectively communicatethem to other colleagues in interdisciplinary teamsThe individual volumes are being prepared by various authors depending on the subject and theirspecializationThe Scientific Committee is composed of specialists in the relevant fields who also havestrong ties with ICCROM through direct involvement in the training courses

Special featuresProgress in conservation science and technology means that currently available information must beregularly evaluated and updatedThis is why the Handbook has been structured as a series of volumesThis will allowbull the authors to periodically update specific volumes to reflect changing methodology and tech-

nology in an easy time-saving and economical way thanks to the digital reprinting processbull the users to work selectively with the volume relating to the particular problem they are facingbull new volumes to be gradually added to the set in line with developing needs until all the relevant

subjects have been covered by the series

We are aware that the information provided is not all-embracing but selective It is hoped that feed-back from users will assist us in improving and continually adapting the project to changing needs

The project team - Ernesto Borrelli and Andrea Urland - welcomes any constructive criticism com-ments and suggestions that might help us achieve this goal

ANDREA URLAND

ARC PROJECT MANAGER

INTRO

INTRO

INTRO

[ 4 ]

SCIENTIFIC COMMITTEE

ERNESTO BORRELLI (ex officio)ICCROM Laboratory Coordinator

GIACOMO CHIARI

Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic SciencesUniversity of Turin (Dipartimento di Scienze Mineralogiche e Petrologiche Universitagrave degli Studi diTorino)

MARISA LAURENZI TABASSO

Assistant to the Director-General ICCROM (Former Head of the Science and TechnologyProgramme) Chemist PhD

JEANNE MARIE TEUTONICO

Senior Architectural Conservator English HeritageGIORGIO TORRACA

Chemist University of Rome Faculty of Engineering (former ICCROM Deputy Director)ANDREA URLAND (ex officio)

ICCROM Architectural Conservation Project Manager Architect PhD

ACKNOWLEDGEMENTS

This publication was made possible thanks to the financial contribution of the UNESCO WorldHeritage Centre

The authors would especially like to express their gratitude to the members of the ScientificCommittee who kindly agreed to give this publication their support sharing their expertise byreviewing the draft texts and providing valuable comments and suggestionsThe authors further wish to recognize the scientific collaboration of Beatrice Muscatello the preciouscontribution of the advisors and the text editing by Christopher McDowall and Cynthia RockwellThey are equally grateful to the ICCROM staff for their whole-hearted collaboration and to thoseat the UNESCO World Heritage Centre for their supportAnd finally we wish to thank all the others who have in some way contributed to the preparationand completion of this publication

1998 - 99 VOLUMES

1 Introduction2 Porosity3 Salts4 Binders5 Colour specification and measurement

Dr HaroldPlenderleith (1898-1997)

INTROICCROMrsquos LABORATORYA BRIEF HISTORY

The new ICCROM Laboratory inaugu-rated in 1997 is dedicated to Dr HaroldPlenderleith the Organizationrsquos foundingdirectorIn 1966 during his tenure a first basiclaboratory was set up for didactic pur-poses at the original headquarters in ViaCavourIn the 1980s after ICCROM had movedto Via di San Michele a new and better-equipped laboratory was installed underthe coordination of Dr Giorgio Torracathen ICCROMrsquos Deputy Director Manyactivities including research werelaunched in the framework of the newlycreated ldquoTraining and Research UnitsrdquoLater a smaller specialized laboratorywas also developed by Jeanne MarieTeutonico for the architectural conser-vation coursesICCROMrsquos current Laboratory (about200 m2) is organized as a series of spe-cialized areas grouping together all theprevious activitiesThe works were sub-sidized by a major financial contributionfrom the Italian GovernmentThe improved level of equipment andfurnishing offers opportunities toexpand and further raise the standard ofwork

INTROICCROM

The International Centre for theStudy of the Preservation andRestoration of CulturalProperty (ICCROM) is a leadingvoice in the conservation of cul-tural heritage around the worldFounded by UNESCO in 1956and based in Rome ICCROM isthe only intergovernmentalorganization concerned withconserving all types of movableor immovable heritage

ICCROM takes action today forheritage tomorrow Its broad-ranging activities span five conti-nents and cover a wide range ofcultural heritage from cavepainting to sculpture ear thenarchitecture plastered faccediladeslibraries archives and entire his-toric centres

ICCROM tackles the challengesfacing cultural heritage ndashneglect pollution catastrophetheft or decay ndash and strengthensthe conditions for its effectiveconservation It provides aninternational platform fordebate while also tailoring activ-ities to regional needs

It achieves this by collecting anddisseminating information coor-dinating research offering con-sultancy and cooperation in fieldconservation and restorationproviding professional trainingand promoting awareness of thesocial value of cultural heritagepreservation amongst all sectorsof society

Above all ICCROM integratesthe conservation of our culturalheritage into global approachesto sustainable human develop-ment and community service

International Centre for the study

of the preservation and restoration

of cultural property

Via di San Michele 13I-00153 Rome RM Italy

e-mail iccromiccromorgwwwiccromorg

ISBN

92-9077-157-7

Porosity

Ernesto Borre l l i

ICCROMUNESCO

WHC

PORO

SITY

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

LA

BO

RAT

OR

Y H

AN

DB

OO

K

2VOLUME

99

AR

C

Scientific CommitteeGiacomo Chiari (Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic Sciences University of Turin)Marisa Laurenzi Tabasso (Assistant to the Director-General ICCROM) Jeanne Marie Teutonico (Senior Architectural ConservatorEnglish Heritage) Giorgio Torraca (Chemist University of Rome Faculty of Engineering)Ex officioErnesto Borrelli (ICCROM Laboratory Coordinator)Andrea Urland (ICCROM Architectural Conservation Project Manager)

ICCROM Project Team Andrea Urland Ernesto BorrelliCollaboration Beatrice MuscatelloAdvice Rocco Mazzeo Paolo Saturno Photographs E Borrelli A Ortolan

Cover M Alonso Campoy E Borrelli MT Jaquinta R Lujan A Ortolan

ISBN 92-9077-157-7

copy ICCROM 1999International Centre for the Study of the Preservationand Restoration of Cultural PropertyVia di San Michele 13 00153 Rome Italy

Printed in Italy by ATEL SpA ~ RomaGraphic Design wwwocomit ~ RomaEditing Christopher McDowallCover Design Andrea Urland

This publication has received financial support from the World Heritage Fund

The ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field of conserva-tion of architectural heritage and historic structures It has been prepared mainly for architects and engineersbut may also be relevant for conservator-restorers or archaeologistsIt aims to

- offer an overview of each problem area combined with laboratory practicals and casestudies

- describe some of the most widely used practices and illustrate the various approaches tothe analysis of materials and their deterioration

- facilitate interdisciplinary teamwork among scientists and other professionals involved inthe conservation process

The Handbook has evolved from lecture and laboratory handouts that have been developed for the ICCROMtraining programmes It has been devised within the framework of the current courses principally theInternational Refresher Course on Conservation of Architectural Heritage and Historic Structures (ARC)

The general layout of each volume is as follows introductory information explanations of scientific termi-nology the most common problems met types of analysis laboratory tests case studies and bibliography

The concept behind the Handbook is modular and it has been purposely structured as a series ofindependent volumes to allow

- authors to periodically update the texts- users to work selectively with the volume relating to the particular problem they are facing- new volumes to be gradually added in line with developing needs

1998 - 99 volumes(1) Introduction (2) Porosity (3) Salts (4) Binders (5) Colour specification and measurement

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

ARCLaboratory Handbook

Porosity

Ernesto Borrel l i

R o m e 1 9 9 9

1 INTRODUCTION 3

2 CLASSIFICATION OF PORES 3

Typology 3

Geometry 3

Size 4

3 BASE TERMS 5

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS 6

In liquid form 6

In vapour form 6

5 DETERIORATION 6

6 METHODS OF MEASUREMENT 7

Direct methods 7

Petrography microscope analysis 7

Scanning electron microscopyanalysis (SEM) 7

Indirect methods 8

Mercury porosimetry 8

Nitrogen adsorption 8

Simple methods 9

Water absorption by total immersion 9

Water desorption 9

Water absorption by capillarity 9

Water vapour permeability 9

7 PRACTICALS 10

1 Measuring apparent volume and open pore volume 10

2 Measuring water absorption by total immersion 12

3 Measuring water desorption 15

8 CASE STUDY 17

9 BIBLIOGRAPHY 20

Footnotes11 Bibliographic references

CONTENTS

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

INTRO

INTRO

INTRO

[ 4 ]

SCIENTIFIC COMMITTEE

ERNESTO BORRELLI (ex officio)ICCROM Laboratory Coordinator

GIACOMO CHIARI

Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic SciencesUniversity of Turin (Dipartimento di Scienze Mineralogiche e Petrologiche Universitagrave degli Studi diTorino)

MARISA LAURENZI TABASSO

Assistant to the Director-General ICCROM (Former Head of the Science and TechnologyProgramme) Chemist PhD

JEANNE MARIE TEUTONICO

Senior Architectural Conservator English HeritageGIORGIO TORRACA

Chemist University of Rome Faculty of Engineering (former ICCROM Deputy Director)ANDREA URLAND (ex officio)

ICCROM Architectural Conservation Project Manager Architect PhD

ACKNOWLEDGEMENTS

This publication was made possible thanks to the financial contribution of the UNESCO WorldHeritage Centre

The authors would especially like to express their gratitude to the members of the ScientificCommittee who kindly agreed to give this publication their support sharing their expertise byreviewing the draft texts and providing valuable comments and suggestionsThe authors further wish to recognize the scientific collaboration of Beatrice Muscatello the preciouscontribution of the advisors and the text editing by Christopher McDowall and Cynthia RockwellThey are equally grateful to the ICCROM staff for their whole-hearted collaboration and to thoseat the UNESCO World Heritage Centre for their supportAnd finally we wish to thank all the others who have in some way contributed to the preparationand completion of this publication

1998 - 99 VOLUMES

1 Introduction2 Porosity3 Salts4 Binders5 Colour specification and measurement

Dr HaroldPlenderleith (1898-1997)

INTROICCROMrsquos LABORATORYA BRIEF HISTORY

The new ICCROM Laboratory inaugu-rated in 1997 is dedicated to Dr HaroldPlenderleith the Organizationrsquos foundingdirectorIn 1966 during his tenure a first basiclaboratory was set up for didactic pur-poses at the original headquarters in ViaCavourIn the 1980s after ICCROM had movedto Via di San Michele a new and better-equipped laboratory was installed underthe coordination of Dr Giorgio Torracathen ICCROMrsquos Deputy Director Manyactivities including research werelaunched in the framework of the newlycreated ldquoTraining and Research UnitsrdquoLater a smaller specialized laboratorywas also developed by Jeanne MarieTeutonico for the architectural conser-vation coursesICCROMrsquos current Laboratory (about200 m2) is organized as a series of spe-cialized areas grouping together all theprevious activitiesThe works were sub-sidized by a major financial contributionfrom the Italian GovernmentThe improved level of equipment andfurnishing offers opportunities toexpand and further raise the standard ofwork

INTROICCROM

The International Centre for theStudy of the Preservation andRestoration of CulturalProperty (ICCROM) is a leadingvoice in the conservation of cul-tural heritage around the worldFounded by UNESCO in 1956and based in Rome ICCROM isthe only intergovernmentalorganization concerned withconserving all types of movableor immovable heritage

ICCROM takes action today forheritage tomorrow Its broad-ranging activities span five conti-nents and cover a wide range ofcultural heritage from cavepainting to sculpture ear thenarchitecture plastered faccediladeslibraries archives and entire his-toric centres

ICCROM tackles the challengesfacing cultural heritage ndashneglect pollution catastrophetheft or decay ndash and strengthensthe conditions for its effectiveconservation It provides aninternational platform fordebate while also tailoring activ-ities to regional needs

It achieves this by collecting anddisseminating information coor-dinating research offering con-sultancy and cooperation in fieldconservation and restorationproviding professional trainingand promoting awareness of thesocial value of cultural heritagepreservation amongst all sectorsof society

Above all ICCROM integratesthe conservation of our culturalheritage into global approachesto sustainable human develop-ment and community service

International Centre for the study

of the preservation and restoration

of cultural property

Via di San Michele 13I-00153 Rome RM Italy

e-mail iccromiccromorgwwwiccromorg

ISBN

92-9077-157-7

Porosity

Ernesto Borre l l i

ICCROMUNESCO

WHC

PORO

SITY

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

LA

BO

RAT

OR

Y H

AN

DB

OO

K

2VOLUME

99

AR

C

Scientific CommitteeGiacomo Chiari (Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic Sciences University of Turin)Marisa Laurenzi Tabasso (Assistant to the Director-General ICCROM) Jeanne Marie Teutonico (Senior Architectural ConservatorEnglish Heritage) Giorgio Torraca (Chemist University of Rome Faculty of Engineering)Ex officioErnesto Borrelli (ICCROM Laboratory Coordinator)Andrea Urland (ICCROM Architectural Conservation Project Manager)

ICCROM Project Team Andrea Urland Ernesto BorrelliCollaboration Beatrice MuscatelloAdvice Rocco Mazzeo Paolo Saturno Photographs E Borrelli A Ortolan

Cover M Alonso Campoy E Borrelli MT Jaquinta R Lujan A Ortolan

ISBN 92-9077-157-7

copy ICCROM 1999International Centre for the Study of the Preservationand Restoration of Cultural PropertyVia di San Michele 13 00153 Rome Italy

Printed in Italy by ATEL SpA ~ RomaGraphic Design wwwocomit ~ RomaEditing Christopher McDowallCover Design Andrea Urland

This publication has received financial support from the World Heritage Fund

The ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field of conserva-tion of architectural heritage and historic structures It has been prepared mainly for architects and engineersbut may also be relevant for conservator-restorers or archaeologistsIt aims to

- offer an overview of each problem area combined with laboratory practicals and casestudies

- describe some of the most widely used practices and illustrate the various approaches tothe analysis of materials and their deterioration

- facilitate interdisciplinary teamwork among scientists and other professionals involved inthe conservation process

The Handbook has evolved from lecture and laboratory handouts that have been developed for the ICCROMtraining programmes It has been devised within the framework of the current courses principally theInternational Refresher Course on Conservation of Architectural Heritage and Historic Structures (ARC)

The general layout of each volume is as follows introductory information explanations of scientific termi-nology the most common problems met types of analysis laboratory tests case studies and bibliography

The concept behind the Handbook is modular and it has been purposely structured as a series ofindependent volumes to allow

- authors to periodically update the texts- users to work selectively with the volume relating to the particular problem they are facing- new volumes to be gradually added in line with developing needs

1998 - 99 volumes(1) Introduction (2) Porosity (3) Salts (4) Binders (5) Colour specification and measurement

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

ARCLaboratory Handbook

Porosity

Ernesto Borrel l i

R o m e 1 9 9 9

1 INTRODUCTION 3

2 CLASSIFICATION OF PORES 3

Typology 3

Geometry 3

Size 4

3 BASE TERMS 5

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS 6

In liquid form 6

In vapour form 6

5 DETERIORATION 6

6 METHODS OF MEASUREMENT 7

Direct methods 7

Petrography microscope analysis 7

Scanning electron microscopyanalysis (SEM) 7

Indirect methods 8

Mercury porosimetry 8

Nitrogen adsorption 8

Simple methods 9

Water absorption by total immersion 9

Water desorption 9

Water absorption by capillarity 9

Water vapour permeability 9

7 PRACTICALS 10

1 Measuring apparent volume and open pore volume 10

2 Measuring water absorption by total immersion 12

3 Measuring water desorption 15

8 CASE STUDY 17

9 BIBLIOGRAPHY 20

Footnotes11 Bibliographic references

CONTENTS

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

Dr HaroldPlenderleith (1898-1997)

INTROICCROMrsquos LABORATORYA BRIEF HISTORY

The new ICCROM Laboratory inaugu-rated in 1997 is dedicated to Dr HaroldPlenderleith the Organizationrsquos foundingdirectorIn 1966 during his tenure a first basiclaboratory was set up for didactic pur-poses at the original headquarters in ViaCavourIn the 1980s after ICCROM had movedto Via di San Michele a new and better-equipped laboratory was installed underthe coordination of Dr Giorgio Torracathen ICCROMrsquos Deputy Director Manyactivities including research werelaunched in the framework of the newlycreated ldquoTraining and Research UnitsrdquoLater a smaller specialized laboratorywas also developed by Jeanne MarieTeutonico for the architectural conser-vation coursesICCROMrsquos current Laboratory (about200 m2) is organized as a series of spe-cialized areas grouping together all theprevious activitiesThe works were sub-sidized by a major financial contributionfrom the Italian GovernmentThe improved level of equipment andfurnishing offers opportunities toexpand and further raise the standard ofwork

INTROICCROM

The International Centre for theStudy of the Preservation andRestoration of CulturalProperty (ICCROM) is a leadingvoice in the conservation of cul-tural heritage around the worldFounded by UNESCO in 1956and based in Rome ICCROM isthe only intergovernmentalorganization concerned withconserving all types of movableor immovable heritage

ICCROM takes action today forheritage tomorrow Its broad-ranging activities span five conti-nents and cover a wide range ofcultural heritage from cavepainting to sculpture ear thenarchitecture plastered faccediladeslibraries archives and entire his-toric centres

ICCROM tackles the challengesfacing cultural heritage ndashneglect pollution catastrophetheft or decay ndash and strengthensthe conditions for its effectiveconservation It provides aninternational platform fordebate while also tailoring activ-ities to regional needs

It achieves this by collecting anddisseminating information coor-dinating research offering con-sultancy and cooperation in fieldconservation and restorationproviding professional trainingand promoting awareness of thesocial value of cultural heritagepreservation amongst all sectorsof society

Above all ICCROM integratesthe conservation of our culturalheritage into global approachesto sustainable human develop-ment and community service

International Centre for the study

of the preservation and restoration

of cultural property

Via di San Michele 13I-00153 Rome RM Italy

e-mail iccromiccromorgwwwiccromorg

ISBN

92-9077-157-7

Porosity

Ernesto Borre l l i

ICCROMUNESCO

WHC

PORO

SITY

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

LA

BO

RAT

OR

Y H

AN

DB

OO

K

2VOLUME

99

AR

C

Scientific CommitteeGiacomo Chiari (Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic Sciences University of Turin)Marisa Laurenzi Tabasso (Assistant to the Director-General ICCROM) Jeanne Marie Teutonico (Senior Architectural ConservatorEnglish Heritage) Giorgio Torraca (Chemist University of Rome Faculty of Engineering)Ex officioErnesto Borrelli (ICCROM Laboratory Coordinator)Andrea Urland (ICCROM Architectural Conservation Project Manager)

ICCROM Project Team Andrea Urland Ernesto BorrelliCollaboration Beatrice MuscatelloAdvice Rocco Mazzeo Paolo Saturno Photographs E Borrelli A Ortolan

Cover M Alonso Campoy E Borrelli MT Jaquinta R Lujan A Ortolan

ISBN 92-9077-157-7

copy ICCROM 1999International Centre for the Study of the Preservationand Restoration of Cultural PropertyVia di San Michele 13 00153 Rome Italy

Printed in Italy by ATEL SpA ~ RomaGraphic Design wwwocomit ~ RomaEditing Christopher McDowallCover Design Andrea Urland

This publication has received financial support from the World Heritage Fund

The ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field of conserva-tion of architectural heritage and historic structures It has been prepared mainly for architects and engineersbut may also be relevant for conservator-restorers or archaeologistsIt aims to

- offer an overview of each problem area combined with laboratory practicals and casestudies

- describe some of the most widely used practices and illustrate the various approaches tothe analysis of materials and their deterioration

- facilitate interdisciplinary teamwork among scientists and other professionals involved inthe conservation process

The Handbook has evolved from lecture and laboratory handouts that have been developed for the ICCROMtraining programmes It has been devised within the framework of the current courses principally theInternational Refresher Course on Conservation of Architectural Heritage and Historic Structures (ARC)

The general layout of each volume is as follows introductory information explanations of scientific termi-nology the most common problems met types of analysis laboratory tests case studies and bibliography

The concept behind the Handbook is modular and it has been purposely structured as a series ofindependent volumes to allow

- authors to periodically update the texts- users to work selectively with the volume relating to the particular problem they are facing- new volumes to be gradually added in line with developing needs

1998 - 99 volumes(1) Introduction (2) Porosity (3) Salts (4) Binders (5) Colour specification and measurement

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

ARCLaboratory Handbook

Porosity

Ernesto Borrel l i

R o m e 1 9 9 9

1 INTRODUCTION 3

2 CLASSIFICATION OF PORES 3

Typology 3

Geometry 3

Size 4

3 BASE TERMS 5

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS 6

In liquid form 6

In vapour form 6

5 DETERIORATION 6

6 METHODS OF MEASUREMENT 7

Direct methods 7

Petrography microscope analysis 7

Scanning electron microscopyanalysis (SEM) 7

Indirect methods 8

Mercury porosimetry 8

Nitrogen adsorption 8

Simple methods 9

Water absorption by total immersion 9

Water desorption 9

Water absorption by capillarity 9

Water vapour permeability 9

7 PRACTICALS 10

1 Measuring apparent volume and open pore volume 10

2 Measuring water absorption by total immersion 12

3 Measuring water desorption 15

8 CASE STUDY 17

9 BIBLIOGRAPHY 20

Footnotes11 Bibliographic references

CONTENTS

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

International Centre for the study

of the preservation and restoration

of cultural property

Via di San Michele 13I-00153 Rome RM Italy

e-mail iccromiccromorgwwwiccromorg

ISBN

92-9077-157-7

Porosity

Ernesto Borre l l i

ICCROMUNESCO

WHC

PORO

SITY

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

LA

BO

RAT

OR

Y H

AN

DB

OO

K

2VOLUME

99

AR

C

Scientific CommitteeGiacomo Chiari (Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic Sciences University of Turin)Marisa Laurenzi Tabasso (Assistant to the Director-General ICCROM) Jeanne Marie Teutonico (Senior Architectural ConservatorEnglish Heritage) Giorgio Torraca (Chemist University of Rome Faculty of Engineering)Ex officioErnesto Borrelli (ICCROM Laboratory Coordinator)Andrea Urland (ICCROM Architectural Conservation Project Manager)

ICCROM Project Team Andrea Urland Ernesto BorrelliCollaboration Beatrice MuscatelloAdvice Rocco Mazzeo Paolo Saturno Photographs E Borrelli A Ortolan

Cover M Alonso Campoy E Borrelli MT Jaquinta R Lujan A Ortolan

ISBN 92-9077-157-7

copy ICCROM 1999International Centre for the Study of the Preservationand Restoration of Cultural PropertyVia di San Michele 13 00153 Rome Italy

Printed in Italy by ATEL SpA ~ RomaGraphic Design wwwocomit ~ RomaEditing Christopher McDowallCover Design Andrea Urland

This publication has received financial support from the World Heritage Fund

The ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field of conserva-tion of architectural heritage and historic structures It has been prepared mainly for architects and engineersbut may also be relevant for conservator-restorers or archaeologistsIt aims to

- offer an overview of each problem area combined with laboratory practicals and casestudies

- describe some of the most widely used practices and illustrate the various approaches tothe analysis of materials and their deterioration

- facilitate interdisciplinary teamwork among scientists and other professionals involved inthe conservation process

The Handbook has evolved from lecture and laboratory handouts that have been developed for the ICCROMtraining programmes It has been devised within the framework of the current courses principally theInternational Refresher Course on Conservation of Architectural Heritage and Historic Structures (ARC)

The general layout of each volume is as follows introductory information explanations of scientific termi-nology the most common problems met types of analysis laboratory tests case studies and bibliography

The concept behind the Handbook is modular and it has been purposely structured as a series ofindependent volumes to allow

- authors to periodically update the texts- users to work selectively with the volume relating to the particular problem they are facing- new volumes to be gradually added in line with developing needs

1998 - 99 volumes(1) Introduction (2) Porosity (3) Salts (4) Binders (5) Colour specification and measurement

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

ARCLaboratory Handbook

Porosity

Ernesto Borrel l i

R o m e 1 9 9 9

1 INTRODUCTION 3

2 CLASSIFICATION OF PORES 3

Typology 3

Geometry 3

Size 4

3 BASE TERMS 5

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS 6

In liquid form 6

In vapour form 6

5 DETERIORATION 6

6 METHODS OF MEASUREMENT 7

Direct methods 7

Petrography microscope analysis 7

Scanning electron microscopyanalysis (SEM) 7

Indirect methods 8

Mercury porosimetry 8

Nitrogen adsorption 8

Simple methods 9

Water absorption by total immersion 9

Water desorption 9

Water absorption by capillarity 9

Water vapour permeability 9

7 PRACTICALS 10

1 Measuring apparent volume and open pore volume 10

2 Measuring water absorption by total immersion 12

3 Measuring water desorption 15

8 CASE STUDY 17

9 BIBLIOGRAPHY 20

Footnotes11 Bibliographic references

CONTENTS

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

Porosity

Ernesto Borre l l i

ICCROMUNESCO

WHC

PORO

SITY

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

LA

BO

RAT

OR

Y H

AN

DB

OO

K

2VOLUME

99

AR

C

Scientific CommitteeGiacomo Chiari (Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic Sciences University of Turin)Marisa Laurenzi Tabasso (Assistant to the Director-General ICCROM) Jeanne Marie Teutonico (Senior Architectural ConservatorEnglish Heritage) Giorgio Torraca (Chemist University of Rome Faculty of Engineering)Ex officioErnesto Borrelli (ICCROM Laboratory Coordinator)Andrea Urland (ICCROM Architectural Conservation Project Manager)

ICCROM Project Team Andrea Urland Ernesto BorrelliCollaboration Beatrice MuscatelloAdvice Rocco Mazzeo Paolo Saturno Photographs E Borrelli A Ortolan

Cover M Alonso Campoy E Borrelli MT Jaquinta R Lujan A Ortolan

ISBN 92-9077-157-7

copy ICCROM 1999International Centre for the Study of the Preservationand Restoration of Cultural PropertyVia di San Michele 13 00153 Rome Italy

Printed in Italy by ATEL SpA ~ RomaGraphic Design wwwocomit ~ RomaEditing Christopher McDowallCover Design Andrea Urland

This publication has received financial support from the World Heritage Fund

The ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field of conserva-tion of architectural heritage and historic structures It has been prepared mainly for architects and engineersbut may also be relevant for conservator-restorers or archaeologistsIt aims to

- offer an overview of each problem area combined with laboratory practicals and casestudies

- describe some of the most widely used practices and illustrate the various approaches tothe analysis of materials and their deterioration

- facilitate interdisciplinary teamwork among scientists and other professionals involved inthe conservation process

The Handbook has evolved from lecture and laboratory handouts that have been developed for the ICCROMtraining programmes It has been devised within the framework of the current courses principally theInternational Refresher Course on Conservation of Architectural Heritage and Historic Structures (ARC)

The general layout of each volume is as follows introductory information explanations of scientific termi-nology the most common problems met types of analysis laboratory tests case studies and bibliography

The concept behind the Handbook is modular and it has been purposely structured as a series ofindependent volumes to allow

- authors to periodically update the texts- users to work selectively with the volume relating to the particular problem they are facing- new volumes to be gradually added in line with developing needs

1998 - 99 volumes(1) Introduction (2) Porosity (3) Salts (4) Binders (5) Colour specification and measurement

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

ARCLaboratory Handbook

Porosity

Ernesto Borrel l i

R o m e 1 9 9 9

1 INTRODUCTION 3

2 CLASSIFICATION OF PORES 3

Typology 3

Geometry 3

Size 4

3 BASE TERMS 5

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS 6

In liquid form 6

In vapour form 6

5 DETERIORATION 6

6 METHODS OF MEASUREMENT 7

Direct methods 7

Petrography microscope analysis 7

Scanning electron microscopyanalysis (SEM) 7

Indirect methods 8

Mercury porosimetry 8

Nitrogen adsorption 8

Simple methods 9

Water absorption by total immersion 9

Water desorption 9

Water absorption by capillarity 9

Water vapour permeability 9

7 PRACTICALS 10

1 Measuring apparent volume and open pore volume 10

2 Measuring water absorption by total immersion 12

3 Measuring water desorption 15

8 CASE STUDY 17

9 BIBLIOGRAPHY 20

Footnotes11 Bibliographic references

CONTENTS

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

Scientific CommitteeGiacomo Chiari (Associate Professor of Applied Mineralogy Department of Mineralogical and Petrologic Sciences University of Turin)Marisa Laurenzi Tabasso (Assistant to the Director-General ICCROM) Jeanne Marie Teutonico (Senior Architectural ConservatorEnglish Heritage) Giorgio Torraca (Chemist University of Rome Faculty of Engineering)Ex officioErnesto Borrelli (ICCROM Laboratory Coordinator)Andrea Urland (ICCROM Architectural Conservation Project Manager)

ICCROM Project Team Andrea Urland Ernesto BorrelliCollaboration Beatrice MuscatelloAdvice Rocco Mazzeo Paolo Saturno Photographs E Borrelli A Ortolan

Cover M Alonso Campoy E Borrelli MT Jaquinta R Lujan A Ortolan

ISBN 92-9077-157-7

copy ICCROM 1999International Centre for the Study of the Preservationand Restoration of Cultural PropertyVia di San Michele 13 00153 Rome Italy

Printed in Italy by ATEL SpA ~ RomaGraphic Design wwwocomit ~ RomaEditing Christopher McDowallCover Design Andrea Urland

This publication has received financial support from the World Heritage Fund

The ICCROM ARC Laboratory Handbook is intended to assist professionals working in the field of conserva-tion of architectural heritage and historic structures It has been prepared mainly for architects and engineersbut may also be relevant for conservator-restorers or archaeologistsIt aims to

- offer an overview of each problem area combined with laboratory practicals and casestudies

- describe some of the most widely used practices and illustrate the various approaches tothe analysis of materials and their deterioration

- facilitate interdisciplinary teamwork among scientists and other professionals involved inthe conservation process

The Handbook has evolved from lecture and laboratory handouts that have been developed for the ICCROMtraining programmes It has been devised within the framework of the current courses principally theInternational Refresher Course on Conservation of Architectural Heritage and Historic Structures (ARC)

The general layout of each volume is as follows introductory information explanations of scientific termi-nology the most common problems met types of analysis laboratory tests case studies and bibliography

The concept behind the Handbook is modular and it has been purposely structured as a series ofindependent volumes to allow

- authors to periodically update the texts- users to work selectively with the volume relating to the particular problem they are facing- new volumes to be gradually added in line with developing needs

1998 - 99 volumes(1) Introduction (2) Porosity (3) Salts (4) Binders (5) Colour specification and measurement

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

ARCLaboratory Handbook

Porosity

Ernesto Borrel l i

R o m e 1 9 9 9

1 INTRODUCTION 3

2 CLASSIFICATION OF PORES 3

Typology 3

Geometry 3

Size 4

3 BASE TERMS 5

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS 6

In liquid form 6

In vapour form 6

5 DETERIORATION 6

6 METHODS OF MEASUREMENT 7

Direct methods 7

Petrography microscope analysis 7

Scanning electron microscopyanalysis (SEM) 7

Indirect methods 8

Mercury porosimetry 8

Nitrogen adsorption 8

Simple methods 9

Water absorption by total immersion 9

Water desorption 9

Water absorption by capillarity 9

Water vapour permeability 9

7 PRACTICALS 10

1 Measuring apparent volume and open pore volume 10

2 Measuring water absorption by total immersion 12

3 Measuring water desorption 15

8 CASE STUDY 17

9 BIBLIOGRAPHY 20

Footnotes11 Bibliographic references

CONTENTS

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

CONSERVATION OF ARCHITECTURAL HERITAGE HISTORIC STRUCTURES AND MATERIALS

ARCLaboratory Handbook

Porosity

Ernesto Borrel l i

R o m e 1 9 9 9

1 INTRODUCTION 3

2 CLASSIFICATION OF PORES 3

Typology 3

Geometry 3

Size 4

3 BASE TERMS 5

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS 6

In liquid form 6

In vapour form 6

5 DETERIORATION 6

6 METHODS OF MEASUREMENT 7

Direct methods 7

Petrography microscope analysis 7

Scanning electron microscopyanalysis (SEM) 7

Indirect methods 8

Mercury porosimetry 8

Nitrogen adsorption 8

Simple methods 9

Water absorption by total immersion 9

Water desorption 9

Water absorption by capillarity 9

Water vapour permeability 9

7 PRACTICALS 10

1 Measuring apparent volume and open pore volume 10

2 Measuring water absorption by total immersion 12

3 Measuring water desorption 15

8 CASE STUDY 17

9 BIBLIOGRAPHY 20

Footnotes11 Bibliographic references

CONTENTS

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

1 INTRODUCTION 3

2 CLASSIFICATION OF PORES 3

Typology 3

Geometry 3

Size 4

3 BASE TERMS 5

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS 6

In liquid form 6

In vapour form 6

5 DETERIORATION 6

6 METHODS OF MEASUREMENT 7

Direct methods 7

Petrography microscope analysis 7

Scanning electron microscopyanalysis (SEM) 7

Indirect methods 8

Mercury porosimetry 8

Nitrogen adsorption 8

Simple methods 9

Water absorption by total immersion 9

Water desorption 9

Water absorption by capillarity 9

Water vapour permeability 9

7 PRACTICALS 10

1 Measuring apparent volume and open pore volume 10

2 Measuring water absorption by total immersion 12

3 Measuring water desorption 15

8 CASE STUDY 17

9 BIBLIOGRAPHY 20

Footnotes11 Bibliographic references

CONTENTS

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

POROSITY

POROSITY

1 INTRODUCTION

Many building materials both natural (stones) and artificial (brickslime and cement mortar) contain a certain volume of empty spaceThis is distributed within the solid mass in the form of pores cavitiesand cracks of various shapes and sizesThe total sum of these emptyspaces is called porosity a fundamental characteristic of buildingmaterial that affects its physical properties (durability mechanicalstrength etc)The characteristics of pores in rocks depend mainly on their gen-esis (ie igneous sedimentary metamorphic etc) whereasthe porosity in man-made building materials depends on theirmanufacture 111The knowledge of the pore structure is an important parameter forcharacterizing materials predicting their behaviour under weatheringconditions evaluating the degree of decay and establishing the effec-tiveness of conservation treatments

2 CLASSIFICATION OF PORES 112

Pores can essentially be classified according to their typolo-gy geometry and size

Typology bull CLOSED PORES pores completely isolated from the exter-

nal surface not allowing the access of water in either liq-uid or vapour phase They influence neither permeabilitynor the transport of liquids in materials but do affect theirdensity and mechanical and thermal properties 113

bull OPEN PORES pores connected with the external surface ofthe material and therefore accessible to water have adirect bearing on deterioration phenomena Open porespermit the passage of fluids and retain wetting liquids by capillaryactionThey can be further divided into dead-end or interconnectedpores (Fig 1) 114

GeometryPores can also be classified according to their shapebull SPHERICAL PORES CYLINDRICAL PORES AND ELONGATED PORES

or according to their genesis (Fig2 photos 1-4 on inside back cover)bull BASIC PORES pores inherent to the process of rock formationbull DISSOLUTION PORES pores deriving from the chemical dissolution of car-

bonates (see transformation of carbonates into soluble bicarbon-ates) sulphates and organic materials (by transformation into CO2)

bull FRACTURE PORES pores and microcracks deriving from intro and inter-crystalline mechanical fracture linked to the tectonic deformation ofrocks and due to stress caused by applied loads

[ 3 ]

Fig 1 - Pore models (taken from Fitzner)

Rocks that have solidifiedfrom molten rock material bothbelow and on the earthrsquos surface(eg granite gabbro basalt tuffs)

Rocks formed by the accu-mulation and consolidation ofsediment at relatively low tem-perature and pressure (sand-stone limestone travertine)

Rocks that are the result ofthe structural transformation inthe solid state of pre-existingrocks under conditions of hightemperature and pressure (mar-ble gneiss schist)

See Laboratory HandbookSalts p 7

POROSITYPOROSITY

Basic pores Dissolution pores

Fracture pores Shrinkage pores

Fig 2 - Pore types (taken from Fitzner)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

[ 4 ]

bull SHRINKAGE PORES pores deriving from the contraction of the various com-ponents of masonry materials mainly artificial ones eg the shrinkage ofmortars due to rapid water loss

SizeThere is considerable variability in pore sizes they vary from a fewangstrom(Aring) to several millimetres Pores of greater dimensions are definedas cavities rather than pores and do not contribute to capillary action Poreswith radii less than 10 angstrom are not considered permeableThere are conflicting views concerning pore size classification In practicewhen conservation scientists speak of porosity they are not generally refer-ring to the values defined below but to a dividing line of lt 25 microm and gt 25microm between microporosity and macroporosity which is more realistic whendealing with building material 116International standards (IUPAC) 115 classify pores according to their radius as

MICROPORES radius lt 0001 microm (lt 10 Aring)MESOPORES radius between 0001 microm and 0025 microm (10 Aring and 250 Aring)MACROPORES radius gt 0025 microm (gt 250 Aring)

The percentage distribution of pores of differing radius within the material is an extremely impor-tant parameter for the evaluation of its behaviour in contact with water and therefore for the fore-cast of freeze-thaw cycles chemical reactivity etcThere is obviously a great variation in porosity from one material to another Igneous (eg granitebasalt) and metamorphic (eg marble gneiss) rocks are not very porous with maximum porositybetween 1 and 2 Unless they are fractured these low-porosity rocks are scarcely permeable Alot of sedimentary rocks however and particularly calcarenites have high porosity with maximum val-ues even reaching 45

The porosity and pore types of some common rocks are summarized in the following table

1 Aring = 10-10mor 1microm = 104 Aring

At a first approxima-tion all pores of any shapeare considered equivalentto round pores of equalnature The radius of theequivalent pores can bemeasured by severalmethods (see section 6)

These values are pro-posed by Russell (1927)De Quervain (1967)Ashurst amp Dimes (1977)Zehnder (1982) and Ve-niale amp Zezza (1987)

Rock type Genesis Geological formation porosity Predominantpressure temperature (average value) pore type

basalt igneous low very high cong 1 - 3 macrogranite igneous high very high cong 1 - 4 micro

tuff igneous low high cong 20 - 30 micro

gneiss metamorphic high high cong 04 - 2 micromarble metamorphic high high cong 02 - 03 microslate metamorphic high medium-high cong 01 - 1 micro

coral stone sedimentary low low cong 40 - 50 macrolimestone sedimentary low low cong 15 - 20 micromacro

plusmn equalsandstone sedimentary low low cong 10 - 15 macro

Table 1 - Porosity and pore type of some common rocks

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

POROSITY

[ 5 ]

3 BASE TERMS

bull PORE VOLUME (Vp)is the fraction of the total volume of a solid occupied by the pores (ie the empty space of a solid)

bull APPARENT VOLUME (Va)is the volume of a solid including the space occupied by pores(a piece of stone measuring 5 cm each side has an apparent volume of 5x5x5 = 125 cm3)

bull TOTAL POROSITY (P)is defined as the ratio between the volume of the pores (Vp) and the apparent volume (Va)expressed as a percentage

P = 100 x (Vp Va)

(a piece of limestone measuring 5 cm each side with a total porosity of 22 contains 275 cm3 of empty space)

bull OPEN PORE VOLUME (Vop) is the volume occupied by open pores

bull OPEN POROSITY

is less than or equal to the total porosity and is defined as the ratio between the volume of theopen pores (Vop) and the apparent volume (Va) expressed as a percentage

P = 100 x (Vop Va)

It is also known as effective porosity percentage of interconnected pore space (a piece of limestone measuring 5 cm each side with a total porosity of 22 that contains 20 cm3 of open poreshas an open porosity of 16 )

bull REAL VOLUME (Vr)is the difference between apparent volume (Va) and pore volume (Vp)

Vr = Va ndash Vp

(a piece of stone measuring 5 cm each side with a total porosity of 22 has a real volume of 975 cm3)

bull SPECIFIC SURFACE

(Ss) is the surface (m2) of the walls of the open pores and is expressed per unit volume of thematerial (m2m3)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

POROSITY

POROSITY

4 MOVEMENT OF WATER IN POROUS BUILDING MATERIALS

The study of porosity is fundamental for understanding phenomena of water transport within porestructure and interactions between materials and water

Water can penetrate a solid because there are interconnected channels (pores) inside the solid thatfacilitate its transportation Stone material can absorb moisture from the environment in vapourform depending on the relative humidity and in liquid form when exposed to the direct action ofwater (rainfall rising damp from the soil and water vapour condensation from the air) 117

IN LIQUID FORM

a) By capillary suction when an initially dry porous material comes into contact with water it getsprogressively wetter First it fills up the smaller pores and then creates a liquid film on the surfaceof the larger pores eventually filling these too

b) By diffusion due to the passage of water from a higher to a lower water content area

c) By osmosis when salts are present in water they are dissociated into electrically charged particles(ions) that attract water through electric force

IN VAPOUR FORM

a) By diffusion as vapour from pores with a high water vapour content to poreswith a lower one

b) By hygroscopic absorption which can occur even at temperatures above dewpointThis phenomenon is accentuated in the presence of soluble salts thatare hygroscopic and can absorb water also under average conditions of rela-tive humidity

c) By condensation when the temperature of the material is less than dew pointwater vapour then condenses within the pores In small pores condensationcan take place before the temperature reaches dew point

5 DETERIORATION

The size of the pores their distribution and geometry are fundamental factors in determining theproperties of materials and their suitability for building applications

The degree of porosity in different materials can be a positive characteristic for their use in some appli-cations (eg very porous plaster allows for water vapour transmission) but may have adverse effectson their performance in others (eg very porous stone generally deteriorates more easily) 118

One of the main causes of stone decay is the interaction between water and the porous structure

Water adsorption can induce weathering on stone materials in several ways

a) by chemical reaction (eg aggressive pollutants)

b) by a physical mechanism through mechanical stress due to freezethaw cycles

c) by acting as a transport medium for salts in dissolution and recrystallization processes within thepore space

d) by providing an essential substrate for biological growth 119

[ 6 ]

The temperatureat which the watervapour in the air issaturated As thetemperature falls thedew point is thepoint at which thevapour begins tocondense as dropletsof water

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

POROSITY

[ 7 ]

6 METHODS OF MEASUREMENT

The three structural properties that are fundamental in describing porous materials are porositypore size distribution and specific surface pore shape is also significant but less easily quantifiable

As these properties are geometrical they can be evaluated by direct observation Other methodsof assessment are termed indirect as they are obtained from calculations based on other parame-ters 1110

Direct methods

Those methods that make it possible to directly observe the porous structure using either a pet-rography microscope or scanning electron microscopy (SEM)

A) PETROGRAPHY MICROSCOPE ANALYSIS

Direct observation by microscope of thin sections of porous materialsmakes it possible to evaluate total porosity which includesclosed poresThis is a traditional method of studying theporosity on thin sections of material enabling the cal-culation of the area occupied by pores as a percentage ofthe total surface area under examination and at the sametime recording their size distribution (Fig 3)

The advantage of this technique is that it permits thedirect quantification of what is visibleA further importantcharacteristic is that specific data such as the size distri-bution of larger pores can only be obtained with this method 1111

It is limited in that a large number of thin sections from different layers and angles of the samplemust be examined in order to obtain a statistically viable result from these measurements

Although this analysis is suitable for larger pores pores with radii smaller than 4 microm are not meas-urableThis depends on the resolution of the optical microscope which limits the measurementto pores with radii ranging between 4 and 500 microm 1112

When combined with the digital analysis of the images this technique automatically calculates thearea occupied by pores comparing it to that occupied by the solid For this reason it is possibleto carry out a large number of measurements and therefore obtain statistically valid data

B) SCANNING ELECTRON MICROSCOPY ANALYSIS (SEM)

This is an effective technique for analysing materials that have a large num-ber of micropores It can be combined with the digital analysis of the imagesand computer-aided techniques to reconstruct three-dimensional images(3D-modelling) as opposed to the two-dimensional system in petrographymicroscopy This makes it possible to delineate the empty space occupiedby the pores and obtain direct information on their shape size and three-dimensional distribution

The advantage of this method is that it does not rely on fictitious pore mod-els which are generally assumed to be cylindrical (see Indirect methods p8)but provides a true description of the pore structure It is again necessaryto examine a large number of thin slices of the material for the result tobe representative 1113

These are obtainedby embedding the sam-ple in a synthetic resinand cutting across itsouter surface The sec-tions are then polisheduntil their thickness isless than 30 mm Theyprovide information onmineralogical composi-tion and microstructuralcharacteristics such asporosity

dimensions 10mm x10mm x 2mm

3 Pore size distribution

pores

Oslash microm

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

[ 8 ]

The problem of the representativity of the sample is the same as in the preceding case This is ahighly sophisticated method and is only used in specific areas of research 1114

Indirect methods

Indirect methods measure certain derived properties such as density permeability to fluids (liquid or gas)liquid imbibition rates adsorptive capacity and so on in order to evaluate the porous structure 1115

Mercury porosimetry and nitrogen adsorption measurement are the two most common indirectmethods in which porosity is essentially correlated to the pressure necessary to introduce a fluid intothe pores of the material

A) MERCURY POROSIMETRY MEASUREMENT 1116

This technique makes it possible to measure the distribution of pore sizes inside the materialThemercury is forced inside by applying steadily increasing pressure

The principle of measurement is based on Washburnrsquos equation

r = 2σcosθP

whereP = pressure exercisedσ = surface tension of mercuryθ = contact angle between the mercury and the solidr = pore radius

The distribution of the pores as well as the total porosity values the real and apparent densityand the volume of intrusion can be obtained from the proportionality between pressure neces-sary for penetration and the dimension of the poresThe theory upon which the Washburn equa-tion is based assumes that all pores are cylindrical In the case of ink bottle pores for exampletheir true dimensions are unobtainable as the measurement only refers to the radius of the poreentrance

Current instruments allow the pressure to reach 4000 bar (400 MPa)which permits them to fill macro and micropores However this methodcannot be recommended for very fragile materials

The amount of sample necessary for the analysis ranges from 05 to 1 g andthe current cost is around 150 US dollars per test

B) MEASUREMENT OF NITROGEN ADSORPTION (Fig4) 1117

This procedure is based on the quantity of gas adsorbed by aporous material at constant temperature and at increasing levelsof pressure A curve is obtained called the isotherm of adsorp-tion which is correlated to the distribution of pore sizes withinthe solid Various fluids can be used depending on the dimensionsof the pores to be measured but nitrogen has given the bestresults making it possible to determine micropores

A gram of sample is necessary for the analysis 4 Gas adsorption

Mercury is usedbecause of its non-wetting properties

1 bar = 09678 atm

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

[ 9 ]

Simple methods

All the previously mentioned methods both direct and indirect have the advantage of requiring onlya small quantity of sample for the analysis but the initial cost of the instruments is extremely high

Other indirect methods to study porosity can be used based on the derived properties of the mate-rial by measuring for example water absorption by total immersion water desorption water absorptionby capillarity and water vapour permeabilityThese tests which are easy to carry out make it possibleto observe the behaviour of building materials in contact with water 1115Although simple they arelimited by the necessity to work on samples of a precise geometrical shape and size (eg cubes cylin-ders etc) It is therefore seldom possible to take samples of this nature from a historic structureFurthermore several samples must be analysed to obtain a statistically viable result

A) WATER ABSORPTION TEST BY TOTAL IMMERSION

This test measures the water absorption rate and the maximum water absorption capacityThetotal quantity of water absorbed is related to the total open porosity while the kinetics of theprocess depend principally on the distribution of the pore sizes

B) WATER DESORPTION TEST

This measures the evaporation rate of saturated samples at room temperature and pressureThisis an extremely useful test that indicates the drying properties of the materials (ie whether theywill dry quickly or remain wet for a long time)The presence of ink bottle pores for example hasan adverse effect on the drying process due to their particular geometry

C) WATER ABSORPTION BY CAPILLARITY

This test measures the capillary rise of water the mostcommon form of liquid water migration in buildingmaterials It is inversely proportional to the diameterof the pores the smaller the diameter the greater thecapillary absorption Certain building materialsbecause of their low capillary absorption are selectedfor specific uses for example as a barrier wheremasonry is in contact with the soil or as a base forwood fixtures to protect the structure from risingdamp (photo 5)

All the above tests may be correlated to the behaviour of masonry incontact with liquid water

D) WATER VAPOUR PERMEABILITY

The permeability test is very important to predict the water vapour transmission capacity ofadded materials especially plasters It measures the quantity of water vapour that passes througha given thickness of material limited by parallel surfaces as a result of the partial difference in pres-sure of the water vapour between the two sidesThe test is also a useful method to evaluate thesuitability of paints as finishing layers which provide protection without reducing water vapourtransmission

Although all these measurements provide information relating to porosity they are often used tomake comparisons between quarried weathered or treated stone materials

5 Inadequate rising damp barrier

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

POROSITY

[ 10 ]

MEASURING APPARENT VOLUME AND OPEN PORE VOLUME OF A STONE SAMPLE

AimThe aim is to become familiar with a measuring procedure to obtain open porosity usingbasic equipment

Calculation Apparent volume Va (cm3) corresponds to the observed increase in the volume of watermeasured on the graduated cylinder

7 PRACTICALS

PRACTICAL 1

A cylinder of suitable sizeshould be chosen for the samplewith a scale that makes it easy toread the change in volume

Procedure

A) APPARENT VOLUME

- If the sample has a regular form (ie cube cylinder) it is suf-ficient to measure the size and calculate the geometricalvolume which in this case corresponds to the apparentvolume of the sample

In the case of small samples of irregular shapes and sizes - Wash the sample in deionized water before beginning this

test to eliminate powdered material from the surface- Dry the sample in the oven for 24 hours at 60degC and then

place it in a desiccator with dry silica gel to cool off- Weigh the sample Then repeat the drying process until

the mass of the sample is constant (ie until the differencebetween 2 successive measurements at an interval of 24hours is not more than 01 of the mass of the sample)

- Once the sample has been completely dried and the con-stant mass recorded (mc) place it in a container or beakeron a base of glass rods and slowly cover with deionizedwater until the sample is totally immersed with about 2cm of water above it

- Take the sample out of the container 8 hours later blot itquickly with a damp cloth to remove surface water andrecord its weight

- Re-immerse the sample in the water and repeat themeasurement until the difference in weight between 2successive measurements at 24-hour intervals is less than1 of the amount of water absorbed

- Record the mass of the wet samples (ms ) and the time ofmeasurement on the data sheet

- Put the saturated sample in a graduated cylinder filledwith deionized water and measure the increase in volumeindicated on the cylinder

Equipment and chemicals

Oven

Technical balance

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

[ 11 ]

Calculationa) The open pore volume Vop (cm3) corresponds to the volume of water absorbed by the

sample Since the density of water is 1 gcm3 at 4 degC the difference in weight (g) ofthe sample before and after being saturated corresponds to the open pore volume

Vop = ms _ mc

wherems = the mass of the saturated sample (g)mc = the dry mass of the sample (g)Vop = open pore volume (cm3)

b) To calculate the open porosity use the following formula

open porosity = 100 x (VopVa)

whereVop = calculated open pore volume (cm3)Va = calculated apparent volume (cm3)

ProcedureB) OPEN PORE VOLUME

- Use the values recorded (mc and ms) to calculate open pore volume and openporosity

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

[ 12 ]

PRACTICAL 2

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the case of cubes theside should neither be less than 3 cm nor greater than 5 cm so that the value of the ratio SV(total surface to apparent volume) is between 2 and 12 cmndash1The number of samples required depends on the heterogeneity of the material being testedIn general a series of at least three samples is recommendedThese should be as similar aspossible in terms of physical properties and condition- Wash the samples in the deionized water before beginning this test in order to eliminate

powdered material from the surface- Dry the samples in the oven for 24 hours at 60degC (this relatively low drying temperature

will prevent the deterioration of organic substances in the case of treated samples)Thenplace the samples in a desiccator with dry silica gel to cool off

- Weigh the samples Repeat the drying process until the mass of the each sample is constantthat is until the difference between 2 successive measurements at an interval of 24 hoursis no more than 01 of the mass of the sample

- Once the samples have been completely dried and the constant mass recorded (mo) placethem in a container or beaker on a base of glass rods and slowly cover with deionized wateruntil they are totally immersed with about 2 cm of water above them

- At programmed intervals of time take each sample out of the container blot it quicklywith a damp cloth to remove surface water then record the mass of the wet samples (mi)and the time of measurement on the data sheet

- Re-immerse the samples in water and continue measuring until the difference in weight between2 successive measurements at 24-hour intervals is less than 1 of the amount of waterabsorbed

- At this point take the samples out of the water and dry them again in an oven at 60degC untilthey have reached constant mass (as above) Record this value (md) on the data sheetProceed with the calculations

WATER ABSORPTION BY TOTAL IMMERSION

DefinitionsWATER ABSORPTION BY TOTAL IMMERSION the quantity of waterabsorbed by a material immersed in deionized water at roomtemperature and pressure at successive time intervals (ie therate of water absorption) expressed as a percentage of the drymass of the sampleWATER ABSORPTION CAPACITY the maximum quantity of waterabsorbed by a material at room temperature and pressure underconditions of saturation again expressed as a percentage of thedry mass of the sample

AimThe measurement of water absorption is a useful laboratory testto characterize porous building materials evaluate the degree ofdeterioration and monitor the effects of conservation treatmentsHere is a simple method for such measurement that gives reli-able results without the use of sophisticated equipment

Equipment and chemicalsOven

Technical balance

Chronometer

Desiccator

Beakers or plastic containers

Soft cloth

Glass rods

Silica gel

Deionized water

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

[ 13 ]

The length of the intervalsduring the first 24 hours de-pends on the absorption charac-teristics of the materials

a) Stone and brick should beweighed after the first 5 minutesof immersion and then everyhour for the first 3 hours

b) Mortar samples should beweighed a few minutes after im-mersion and then at increasingintervals (15 min 30 min 1hour etc) for the first 3 hours

All samples should then beweighed 8 hours after the begin-ning of the test and then at 24-hour intervals until the quantityof water absorbed in two suc-cessive measurements is notmore than 1 of the total mass

0

2

4

6

8

10

12

14

0 10 20 30 40 50 60 70 80 90

time (min)

Mi

Calculationa) At each interval the quantity of water absorbed with respect to the mass of the dry

sample is expressed as

Mi = 100 x (mi _ mo)mo

where

mi = weight (g) of the wet sample at time timo = weight (g) of the dry sample

b) Record these values on a data sheet and on a graph as a function of time

c) Again using the figures from the data sheet calculate the water absorption capacity(WAC) with the following formula

WAC = 100 x (mmax _ md)md

where

mmax = the mass (g) of the sample at maximum water absorption

md = the mass (g) of the sample after re-drying at the end of the test

As an example a generic series of samples are recorded in Fig1 and Table 1

Fig 1 - Water absorption by total immersion

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

[ 14 ]

ti (min)

04812162030456090

14402880

MEASUREMENTINTERVALS

mi (g) Mi ()

12470 00013828 108814097 130514108 131414112 131614119 132214121 132414123 132514125 132714127 132914135 133514195 1383

SAMPLE 3mo = 12203 g

mi (g) Mi ()

11388 00012778 122112897 132512912 133812915 134112922 134712921 134612929 135312930 135412935 135912942 136512974 1393

Mi ()

00011901311132013231329133013341336134013451382

MEANVALUES

mi (g) Mi ()

12203 00013744 126213793 130313801 130913806 131313811 131813815 132113819 132413823 132713829 133213831 133413875 1370

SAMPLE 1mo = 12470 g

where

mo = weight of dry sample

mi = weight of the wet sample at time tiMi = 100 x (mi

_ mo)mo calculated for each interval time and for each sample

Table 1 - Water absorption by total immersion

SAMPLE 2mo = 11388 g

6 Measuring equipment

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

POROSITY

[ 15 ]

PRACTICAL 3

ProcedureSamples should be of a regular shape (cubes cylinders or prisms) In the caseof cubes the side should neither be less than 3 cm nor greater than 5 cm sothat the value of the ratio SV (total surface to apparent volume) is between2 and 12 cmndash1The number of samples required depends on the heterogeneity of the mate-rial being tested In general a series of at least three samples is recommend-edThese should be as similar as possible in terms of physical properties andcondition- Soak the samples in water to the point of saturation (see Practical 1) Blot them

with a damp cloth to remove surface water and weigh them (mo)- Place each sample inside the desiccator containing anhydrous silica gel with a

cobalt chloride indicator Store at a constant room temperature of 20 plusmn 1degCThe size of the desiccator and the number of samples for each desiccatormust be determined through preliminary tests by the nature of the mate-rial so that the relative humidity corresponding to the equilibrium of thesilica gel is maintained within the desiccator during the whole testing phaseCheck the relative humidity by placing a cobalt chloride strip indicator onthe wall of the desiccator The indicator must always remain blue through-out the test If not quickly replace the silica gel at the bottom of the des-iccator

- Remove samples periodically from the desiccator and weigh them Duringthe first 24 hours the length of interval will depend on the evaporationcharacteristics of the material which is determined by a preliminary test toidentify the initial evaporation rate

WATER DESORPTION

DefinitionThe variation in water content of the material expressed as apercentage of the dry mass of the sample at a constant tem-perature and under fixed conditions of humidity is measuredover a period of time

AimThe aim is to indicate the drying capacity of porous materials

Equipment and chemicalsOven

Balance

Chronometer

Desiccator

Soft cloth

Glass rods

Silica gel

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

CalculationThe residual water content Qi of the sample at time ti is calculated with the following formula

Qi = 100 x (mi ndash mof) mof

where Qi = water content at ti expressed as a percentage of the final dry massmi = mass (g) of sample at ti (hrs)mof = mass (g) of the desiccated sample at the end of the drying test

Record the values of Qi in a graph versus time ti and draw the relative curve

[ 16 ]

Water content versus time

000

200

400

600

800

1000

1200

1400

1600

1800

2000

0 2 4 6 8 10 12 14 16 18 20 22 24

ti (hours)

Qi (

)

Procedure continued- Repeat weighing at 24-hour intervals until the following formula has been ver-

ified

10 ge (mo ndash mi-1) (mo ndash mi) ge 090

where m0 = mass (g) of sample at time t0 (hrs)mi-1 = mass (g) of sample at time ti ndash 1 (hrs)mi = mass (g) of sample at time ti (hrs)

- Proceed with the desiccation of the samples in an oven at 60 plusmn 5degC until constantmass is reached Mass is considered constant when the difference between two suc-cessive measurements at 24-hour intervals is less than or equal to 001 of themass of the dry sample

- Plot the experimental values to obtain a ldquodrying curverdquo (water content as a func-tion of time) (Fig 1)

The initial wa-ter content corre-sponds to the val-ue of the absorp-tion capacity WACdetermined duringthe process of totalimmersion

Fig 1 - Water content versus time

Qi(

)

ti (hours)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

POROSITY8 CASE STUDY

PRELIMINARY SURVEY FOR THE CONSERVATION OF THE NORTHFACcedilADE IN THE CHURCH OF S MARIA FORMOSA-VENICE( The author acknowledges the k ind contr ibut ion of W Schmid ICCROM)

Background This survey was organized in 1997 as part of the XII International ICCROMUNESCO Course onthe Technology of Stone Conservation (SC97) It was undertaken by a multidisciplinary group ofconservation architects conservator-restorers and engineers supervised by experts in the field(NB Only the part of the survey referring to the porosimetry analysis is reported here)

Aim The aim was to develop a proposal for conservation treatment by assessing the conditions of thenorth faccedilade integrating in situ observations historical data and scientific investigations and carryingout treatment trialsA program of laboratory analysis was carried out in order to- assess the faccediladersquos existing condition and possible decay mechanisms- identify altered materials- better understand the original material

DescriptionS Maria Formosa (Fig 1) is considered one ofthe earliest Venetian churches (7th century)However over the centuries it has under-gone so many modifications that it nowappears in typical Renaissance style It has aLatin cross plan with three aisles deep sidechapels a presbytery a semicircular apse anda central dome over the transeptThe chapelon the right-hand side borders the canalwhereas the north faccedilade to which the casestudy refers overlooks Campo S Maria FormosaThe faccedilade is divided into five parts by pilasters with four semicircular win-dows in the first order Three busts are located in the upper part of thefaccediladeThe central bust stands at the base of an oculum at the centre ofan open arch with volutes Five statues are symmetrically positioned abovethe upper cornice

Survey materials previous treatments and state of conservation of north faccedilade The faccedilade of the church is made up of Istrian limestone blocks the three busts of Carrara marblewhile the five statues may have been originally carved out of Vicenza limestoneAttention was mainly concentrated on the structural condition of the church although there was alsosignificant decay and physical change of the stone surface areas of black crusts rainwashing andpigeon excrement as well as biological growth Flaking and scaling was evident on all the stone sur-faces Disaggregation was apparent on the three marble busts and the upper right sculptures and blis-tering on the top central sculpture Erosion was present mainly below the cornices on the cornersnear the pilasters where there is rainwater washing

[ 17 ]

CASE STUDY

Fig 1 - S Maria Formosa

Porosity cong 02 - 05

Porosity cong 05 - 1

Porosity cong 25 - 28

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

Fig 2 shows sampling locations with indishy FIg 3 - Mercury porOSlmeuy analysis

cations of relevant analyses

POROSIMETRIC ANALYSIS

Using a mercury porosimeter the total g ISOO

porosity and open pore size distribution ~ was calculated for sample n I 10 from a i 1000

sculpture at the uppermost part of the faltade and on a quarry sample of unweathered Vicenza limestone

The total porosity of the quarried stone was 1824 compared to the 2729 of the statue (Fig 3) This sharp increase of Fig 4 - Quarried SlOne sample (Pore size dis[(ibulion)

about 50 in total porosity is due to the extreme weathered condition of the

2500

stone

The open pore size distribution of the two samples was determined because

g 1500it plays an important role in the chemshyical and physical behaviour of porous ~

~ 1000materials

A comparison of the pore size distribushy 00

tions of the two samples shows a marked difference (FIg 4 and 5) The quarried limeshy

~cIL=20=LPo~i===lmii ~ro~Po2iceLs~a LC

2000 1-----

-

P7gt

~ ~ ~ ~ bull00 DOS 010 bull lO 100 gt00 lt00 1000 150060stone shows a bell-shaped curve with a pore diameter (jlm)

maximum amount of pores in the range Fig 5 - Weathered sample rom the sculpture (Pore size diWibulion)

[18 J

005 010 020 040 060 080 100 200 400 1000 ISOO

pore diameter (jlm)

There were quite a few small and medium sized broken or loose pieces on corners and projecting architectural elements Cracks fisshysures and fractures were related either to difshyferential decay and bedding of the stone or to structural problems Furthermore many of the joints between the stone blocks were devoid of mortar especially on the right side of the fa~ade and in places where considershyable water erosion had taken place

Experimental

The scientific investigations focused on the essential problems of the faltade due to limited t ime and funds Apart from soluble ~

salts analysis biological identification and 5 petrography analysis research concentratshy

sect ed on the porosimetry of Vicenza limeshy ~

stone in order to assess the degree of ~ deterioration of the uppermost sculpshy ~ tures

Sampling

PTR PelrogiOphy onolyss XRD X-ray diffractio n PRD Po(osm~ry

FIg 2 - Samplrng plan

0 1

008

006 1---shy

01gtlt 1-------- shy

-- Wtiathered umple

001 01

012 r--- ==------- --o--+- Quamed stOlle

002 1---- shy

10 100

pore diameter (IJm)

[ 19 ]

of 04 - 08 micromThe weathered sample from the sculpture shows a bi-modal curve the first part ofthe pore size distribution curve is similar to that of the quarried sample (04 - 06 microm) while thesecond part of the curve indicates higher open porosity in the range of 4 -10 microm

ConclusionsIt must be noted that all the statues require some form of consolidation to re-establish the cohesionof the materialThe increase in the amount of larger open pores reflects the severe deterioration ofthe sculpture which may be due to several factors For example soluble salt crystallization cycleswould greatly affect the pores due to the expansion and pressure caused by salt crystals There arecement fills in the sculpture from where these salts probably originate Other sources of salts maybe from air pollutionThe severe weathering of these sculptures is visible to the eye but the analysis made it possible toqualify and quantify the deterioration

[ 20 ]

BIBLIOGRAPHIC REFERENCES1 FITZNER B Porosity properties and weathering behaviour of

natural stones-Methodology and examples in Papers collectionof the Second Course Stone material in monuments diagnosisand conservation Heraklion-Crete 24-30 May 1993 ScuolaUniversitaria CUM Bari (Italy) Conservazione deiMonumenti 1993 pp 43-53

2 ROTA ROSSI-DORIA P Il problema della porositagrave in rappor-to al degrado ed alla conservazione dei materiali lapidei inBollettino drsquoArte vol 1 suppl to n 41 Rome Ministero per iBeni Culturali e Ambientali 1987 pp 11-14

3 CARRIO-SCHAFFHAUSER E and GAVIGLIO P ldquoExemple detransformations des proprieacuteteacutes physiques drsquoun mateacuteriau cal-caire lieacutees agrave une variation de porositeacute dans le milieu naturelrdquoin MASO JC [ed] Pore structure and materials propertiesProceedings of the 1st International RILEM Congress Versailles(France) 7-11 Sept 1987 London Chapman and Hall 1987pp 277-284 MENGUY G EZBAKHE H and LEVEAU JldquoInfluence de la porositeacute sur les caracteacuteristiques thermiquesdes mateacuteriaux de constructionrdquo in MASO JC [ed] Pore struc-ture and materials properties Proceedings of the 1st InternationalRILEM Congress Versailles (France) 7-11 Sept 1987 LondonChapman and Hall 1987 pp 269-276

4 FITZNER B op cit p 455 IUPAC Manual of Symbols and Terminology Appendix 2 Pt I

Colloid and Surface Chemistry Pure Applied Chemistry 31 5871972

6 RODRIacuteGUEZ NAVARRO C Teacutecnicas de anaacutelisis del sistemaporoso de un material peacutetreo ornamental in CuadernosTeacutecnicos Instituto Andaluz del Patrimonio Histoacuterico Seville Juntade Andaluciacutea 1996 pp 51-65

7 TORRACA G Porous building materials Rome ICCROM1988 pp 11-17

8 Ibidem pp 109-1139 MENG B Characterization of pore structure for the inter-

pretation of moisture transport in THIEL MJ [ed]Conservation of stone and other materials Proceedings of theInternational RILEMUNESCO Congress Paris 29 June-1 July1993 London E amp FN SPON 1993 pp 155-162

10 HAYNES JM Determination of pore properties of construc-tional and other materials General introduction and classifica-tion of methods in Mateacuteriaux et constructions vol 6 n 33Paris RILEM 1973 pp 169-174

11 Ibidem p 17012 FITZNER B op cit p 4813 QUENARD DA et alii Microstructure and transport proper-

ties of porous building materials in Mateacuteriaux et constructionsvol 31 n 209 Paris RILEM 1998 pp 317-324 MONTOTO MRODRIacuteGUEZ-REY A and FERNAacuteNDEZ MERAYO N ldquo3Dcharacterization of fissures in granites under confocal laserscanning microscopyrdquo in VICENTE MA [ed] Degradation andconservation of granitic rocks in monuments Proceedings of theEC workshop Santiago de Compostela (Spain) 28-30 Nov1994 pp 265-267 RODRIacuteGUEZ NAVARRO C op cit pp 54-55

14 FITZNER B op cit p 4815 HAYNES JM op cit p 17116 FITZNER B op cit p 48 RODRIacuteGUEZ NAVARRO C op cit

p 5417 RODRIacuteGUEZ NAVARRO C op cit p 5618 DE LA TORRE LOacutePEZ MJ Propiedades hiacutedricas de los materi-

ales lapiacutedeos Ensayos in Cuadernos Teacutecnicos Instituto Andaluz delPatrimonio Histoacuterico Seville Junta de Andaluciacutea 1996 pp 66-71

GENERAL BIBLIOGRAPHYASTM Annual Book of ASTM Standards Philadelphia (USA) ASTMCOMMISSIONE NORMAL Raccomandazioni NORMAL Rome

(Italy) CNR-ICRDEL REY BUENO F Porosimetriacutea de mercurio in Cuadernos

Teacutecnicos Instituto Andaluz del Patrimonio Histoacuterico Seville Juntade Andaluciacutea 1996 pp 46-50

GARCIacuteA PASCUA N SAacuteNCHEZ DE ROJAS MI and FRIacuteAS MStudy of porosity and physical properties as methods toestablish the effectiveness of treatments used in two different

Spanish stones limestone and sandstone in Proceedings of theInternational Colloquium on Methods of evaluating products forthe conservation of porous building materials in monumentsRome 19-21 June 1995 Rome ICCROM 1995 pp 147-162

HAYNES JM Porosity of materials permeability and trasport inDurabilite des betons et des pierres Seminaire organiseacute avec lacollaboration de lrsquoUNESCO par le College international des sci-ence de la construction Saint-Remy-les Cheuvreuse (France)17-19 Nov 1981 Paris Conseil international de la languefranccedilaise 1981 pp 81-94

IHALAINEN PE Changes in porosity of some plutonic buildingstones depending on the type of artificial weathering treat-ment in FASSINA V [ed] The Conservation of Monuments inthe Mediterranean Basin Proceedings of the 3rd InternationalSymposiumVenice 22-25 June 1994Venice Soprintendenza aiBeni Artistici e Storici di Venezia 1994 pp 109-114

RILEM 25 PEM Commission ldquoExperimental methodsrdquo inProceedings of the International RILEMUNESCO Symposium onDeterioration and Protection of Stone Monuments Paris 5-9 June1978

TEUTONICO JM A laboratory manual for architectural conserva-tors Rome ICCROM 1988

STANDARDS

ASTM standards (American Society for Testing and Materials)Designation C 121-90 Standard test method for waterabsorption of slateDesignation D 4404-84 (reappr 1992) Standard testmethod for determination of pore volume distribution of soiland rock by mercury intrusion porosimetryDesignation D 4959-89 Standard test method for determi-nation of water (moisture) content of soil by direct heatingmethodDesignation D 4525-90 Permeability of rocks by flowing airDesignation D 653-90Terminology relating to soil rock andcontained fluidsDesignation C 566-89Test method for total moisture con-tent of aggregate by dryingDesignation E 12-70 (reappr 1991) Density and specificgravity of solids liquids and gases

RILEM tests (Reacuteunion Internationale des Laboratoires drsquoEssais desMateacuteriaux)

Test n I1 Porosity accessible to waterTest n I2 Bulk and real densitiesTest n I3 Air-permeabilityTest n I4 Pore-size distribution (suction)Test n I5 Pore-size distribution (mercury porosimeter)Test n II1 Saturation coefficientTest n II2 Coefficient of water vapour conductivityTest n II3 Water absorption under low pressure (boxmethod)Test n II4 Water absorption under low pressure (pipemethod)Test n II5 Evaporation curveTest n II6Water absorption coefficient (capillarity)Test n II8aWater drop absorption

NORMAL documents (Commissione Normativa ManufattiLapidei)

NORMAL 4 80 Distribuzione del volume dei pori in fun-zione del loro diametroNORMAL 7 81 Assorbimento drsquoacqua per immersionetotale ndash Capacitagrave di ImbibizioneNORMAL 11 85 Assorbimento drsquoacqua per capillaritagrave ndashCoefficiente di assorbimento capillareNORMAL 21 85 Permeabilitagrave al vapor drsquoacquaNORMAL 29 88 Misura dellrsquoindice di asciugamento (Dryingindex)NORMAL 33 89 Misura dellrsquoangolo di contattoNORMAL 44 93 Assorbimento drsquoacqua a bassa pressione

1Coral stone

MICROPHOTOGRAPHS OF THIN SECTIONSExamples of macroporosity

2Slate

3Granite

4Organogenic limestone

POROSITYPOROSITY

International Centre for the study

of the preservation and restoration

of cultural property

Via di San Michele 13I-00153 Rome RM Italy

e-mail iccromiccromorgwwwiccromorg

ISBN

92-9077-157-7

[ 20 ]

BIBLIOGRAPHIC REFERENCES1 FITZNER B Porosity properties and weathering behaviour of

natural stones-Methodology and examples in Papers collectionof the Second Course Stone material in monuments diagnosisand conservation Heraklion-Crete 24-30 May 1993 ScuolaUniversitaria CUM Bari (Italy) Conservazione deiMonumenti 1993 pp 43-53

2 ROTA ROSSI-DORIA P Il problema della porositagrave in rappor-to al degrado ed alla conservazione dei materiali lapidei inBollettino drsquoArte vol 1 suppl to n 41 Rome Ministero per iBeni Culturali e Ambientali 1987 pp 11-14

3 CARRIO-SCHAFFHAUSER E and GAVIGLIO P ldquoExemple detransformations des proprieacuteteacutes physiques drsquoun mateacuteriau cal-caire lieacutees agrave une variation de porositeacute dans le milieu naturelrdquoin MASO JC [ed] Pore structure and materials propertiesProceedings of the 1st International RILEM Congress Versailles(France) 7-11 Sept 1987 London Chapman and Hall 1987pp 277-284 MENGUY G EZBAKHE H and LEVEAU JldquoInfluence de la porositeacute sur les caracteacuteristiques thermiquesdes mateacuteriaux de constructionrdquo in MASO JC [ed] Pore struc-ture and materials properties Proceedings of the 1st InternationalRILEM Congress Versailles (France) 7-11 Sept 1987 LondonChapman and Hall 1987 pp 269-276

4 FITZNER B op cit p 455 IUPAC Manual of Symbols and Terminology Appendix 2 Pt I

Colloid and Surface Chemistry Pure Applied Chemistry 31 5871972

6 RODRIacuteGUEZ NAVARRO C Teacutecnicas de anaacutelisis del sistemaporoso de un material peacutetreo ornamental in CuadernosTeacutecnicos Instituto Andaluz del Patrimonio Histoacuterico Seville Juntade Andaluciacutea 1996 pp 51-65

7 TORRACA G Porous building materials Rome ICCROM1988 pp 11-17

8 Ibidem pp 109-1139 MENG B Characterization of pore structure for the inter-

pretation of moisture transport in THIEL MJ [ed]Conservation of stone and other materials Proceedings of theInternational RILEMUNESCO Congress Paris 29 June-1 July1993 London E amp FN SPON 1993 pp 155-162

10 HAYNES JM Determination of pore properties of construc-tional and other materials General introduction and classifica-tion of methods in Mateacuteriaux et constructions vol 6 n 33Paris RILEM 1973 pp 169-174

11 Ibidem p 17012 FITZNER B op cit p 4813 QUENARD DA et alii Microstructure and transport proper-

ties of porous building materials in Mateacuteriaux et constructionsvol 31 n 209 Paris RILEM 1998 pp 317-324 MONTOTO MRODRIacuteGUEZ-REY A and FERNAacuteNDEZ MERAYO N ldquo3Dcharacterization of fissures in granites under confocal laserscanning microscopyrdquo in VICENTE MA [ed] Degradation andconservation of granitic rocks in monuments Proceedings of theEC workshop Santiago de Compostela (Spain) 28-30 Nov1994 pp 265-267 RODRIacuteGUEZ NAVARRO C op cit pp 54-55

14 FITZNER B op cit p 4815 HAYNES JM op cit p 17116 FITZNER B op cit p 48 RODRIacuteGUEZ NAVARRO C op cit

p 5417 RODRIacuteGUEZ NAVARRO C op cit p 5618 DE LA TORRE LOacutePEZ MJ Propiedades hiacutedricas de los materi-

ales lapiacutedeos Ensayos in Cuadernos Teacutecnicos Instituto Andaluz delPatrimonio Histoacuterico Seville Junta de Andaluciacutea 1996 pp 66-71

GENERAL BIBLIOGRAPHYASTM Annual Book of ASTM Standards Philadelphia (USA) ASTMCOMMISSIONE NORMAL Raccomandazioni NORMAL Rome

(Italy) CNR-ICRDEL REY BUENO F Porosimetriacutea de mercurio in Cuadernos

Teacutecnicos Instituto Andaluz del Patrimonio Histoacuterico Seville Juntade Andaluciacutea 1996 pp 46-50

GARCIacuteA PASCUA N SAacuteNCHEZ DE ROJAS MI and FRIacuteAS MStudy of porosity and physical properties as methods toestablish the effectiveness of treatments used in two different

Spanish stones limestone and sandstone in Proceedings of theInternational Colloquium on Methods of evaluating products forthe conservation of porous building materials in monumentsRome 19-21 June 1995 Rome ICCROM 1995 pp 147-162

HAYNES JM Porosity of materials permeability and trasport inDurabilite des betons et des pierres Seminaire organiseacute avec lacollaboration de lrsquoUNESCO par le College international des sci-ence de la construction Saint-Remy-les Cheuvreuse (France)17-19 Nov 1981 Paris Conseil international de la languefranccedilaise 1981 pp 81-94

IHALAINEN PE Changes in porosity of some plutonic buildingstones depending on the type of artificial weathering treat-ment in FASSINA V [ed] The Conservation of Monuments inthe Mediterranean Basin Proceedings of the 3rd InternationalSymposiumVenice 22-25 June 1994Venice Soprintendenza aiBeni Artistici e Storici di Venezia 1994 pp 109-114

RILEM 25 PEM Commission ldquoExperimental methodsrdquo inProceedings of the International RILEMUNESCO Symposium onDeterioration and Protection of Stone Monuments Paris 5-9 June1978

TEUTONICO JM A laboratory manual for architectural conserva-tors Rome ICCROM 1988

STANDARDS

ASTM standards (American Society for Testing and Materials)Designation C 121-90 Standard test method for waterabsorption of slateDesignation D 4404-84 (reappr 1992) Standard testmethod for determination of pore volume distribution of soiland rock by mercury intrusion porosimetryDesignation D 4959-89 Standard test method for determi-nation of water (moisture) content of soil by direct heatingmethodDesignation D 4525-90 Permeability of rocks by flowing airDesignation D 653-90Terminology relating to soil rock andcontained fluidsDesignation C 566-89Test method for total moisture con-tent of aggregate by dryingDesignation E 12-70 (reappr 1991) Density and specificgravity of solids liquids and gases

RILEM tests (Reacuteunion Internationale des Laboratoires drsquoEssais desMateacuteriaux)

Test n I1 Porosity accessible to waterTest n I2 Bulk and real densitiesTest n I3 Air-permeabilityTest n I4 Pore-size distribution (suction)Test n I5 Pore-size distribution (mercury porosimeter)Test n II1 Saturation coefficientTest n II2 Coefficient of water vapour conductivityTest n II3 Water absorption under low pressure (boxmethod)Test n II4 Water absorption under low pressure (pipemethod)Test n II5 Evaporation curveTest n II6Water absorption coefficient (capillarity)Test n II8aWater drop absorption

NORMAL documents (Commissione Normativa ManufattiLapidei)

NORMAL 4 80 Distribuzione del volume dei pori in fun-zione del loro diametroNORMAL 7 81 Assorbimento drsquoacqua per immersionetotale ndash Capacitagrave di ImbibizioneNORMAL 11 85 Assorbimento drsquoacqua per capillaritagrave ndashCoefficiente di assorbimento capillareNORMAL 21 85 Permeabilitagrave al vapor drsquoacquaNORMAL 29 88 Misura dellrsquoindice di asciugamento (Dryingindex)NORMAL 33 89 Misura dellrsquoangolo di contattoNORMAL 44 93 Assorbimento drsquoacqua a bassa pressione

1Coral stone

MICROPHOTOGRAPHS OF THIN SECTIONSExamples of macroporosity

2Slate

3Granite

4Organogenic limestone

POROSITYPOROSITY

International Centre for the study

of the preservation and restoration

of cultural property

Via di San Michele 13I-00153 Rome RM Italy

e-mail iccromiccromorgwwwiccromorg

ISBN

92-9077-157-7

1Coral stone

MICROPHOTOGRAPHS OF THIN SECTIONSExamples of macroporosity

2Slate

3Granite

4Organogenic limestone

POROSITYPOROSITY

International Centre for the study

of the preservation and restoration

of cultural property

Via di San Michele 13I-00153 Rome RM Italy

e-mail iccromiccromorgwwwiccromorg

ISBN

92-9077-157-7

International Centre for the study

of the preservation and restoration

of cultural property

Via di San Michele 13I-00153 Rome RM Italy

e-mail iccromiccromorgwwwiccromorg

ISBN

92-9077-157-7