López-Polín, L. et al. Láser removing carbonated matrices from fossils. 2008

8/7/2019 Lpez-Poln, L. et al. Lser removing carbonated matrices from fossils. 2008

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Lasers in the Conservation of Artworks - Castilleja et at. (eds) 2008 Taylor & Francis Group, London, ISBN 978-0-415-47596-9

Laser for removing remains of carbonated matrices fromPleistocene fossilsL. Lopez-Polin & x.oueInstitut Catala de Paleoecologia Humann t Evolucio Social- Area de Prehistoria, UniversitatRovira iVirgili Tarragona, SpainJ. Chamon & 1 . BarrioDepartamento de Prehistoria y Arqueologla, Universidad Autonoma, Madrid, Spain

ABSTRACT: This paper studies the feasibility of using lasers to remove carbonated sediment layers fromPleistocene bones. The laser facility is a portable Nd:YAG laser system with optical fibre. Laser cleaning iscompared with mechanical cleaning with a scalpel, a tool that is normally used to prepare fossil remains. Thiscomparison takes into account precision and the time needed to achieve similar results with both. laser .andscalpel. To assess precision, the changes on the.surface of the bones are monitored using Optical and ElectronicMicroscopy; Scanning Electron Microscopy (SEM) and Environmental Scanning Electron Microscopy (ESEM).The results show that laser cleaning is feasible and that, in some cases, it is more precise than mechanicalcleaning.

1 INTRODUCTIONThe study of fossil remains often depends on spec-imens which have been treated by conservators. Forfossils to be classified taxonomically and fully Ofpartially observed, they need to be cleaned.

Compacted and hardened sediments are common inmany sites, particularly in those in which the infiltra-tion of calcium carbonate is usual. Calcium carbonateleads to the presence of hard deposits on the surfaceof the fossils which are difficult to remove.

Traditionally the enclosing matrix has beenremoved by mechanical tools and chemicals, whichare also often used for final surface cleaning. Althoughlasers have been used in other fields of conservation formany decades, they have not been used (atleast to suchan extent) in the field of ancient bone remains. In fact,very little work has been done on the application oflasers to palaeontological specimens (Landucci et a1.1999,2000, 2003, Asmus 2000, Cornish & Jones 2003,Cornish et al, 2004) and there are very few papersthat deal with lasers being used to remove particles ofsediment (Landucci et al. 1999,2000,2003).

This paper presents the results of some tests onPleistocene bones with two specific aims: to deter-mine whether it is feasible to use lasers to removetheremains of a carbonated matrix; and to compare theeffectiveness of lasers and traditional manual tools.

In this study, effectiveness is considered to be acombination of speed and precision.In order to achieve these aims, separate test pits weremade.in some specimens with both laser and scalpel.Then, the effectiveness of each of the techniques wascompared by monitoring how long they wok and thefinal appearance of the fossils (particular attention was

. paid to the degree of cleaning and the respect for theoriginal surface of the bones).

MATERIALS MID fETHODS

The fossil sampleThe fossil sample consisted of macro mammal bonesfromtwo Pleistceene site : Gran Dolina (Sierra de Ata-p e iI" .Burgos, SpaID. and La Cansaladeta (La Riba,Tarragena, Spain l.

In rhi - smdy. twelve samples were treated. Ten boneswere from two different levels of the Gran Dolina site:TD6 c .&OO_OOOyearsago andTDIO (c. 350,000 yearsago) (Carbonell et al. 1999). One bone was from levelK of La Cansaladeta (c. 400000 years ago) (Angelucciet at 2004). All these bones had carbonated sedi-ment remains on their surface. Finally, a fragment ofbone from TO lOof little scientifi c value was used fordestructive assays.

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The concretions that had to be removed were madeup of carbonated sediment, mainly clay, but also siltand sand. The colour of the concretions (red) was dif-ferent enough from the colour of the bone (white) toallow laser treatment.2.2 The laser treatmentThe laser equipment was a portable Nd: YAG laser sys-tem with optical fiber. It emits near infrared radiation(l064 nm) in the Short Free Running mode (SFR).In some cases, prior to laser cleaning, th e concre-tions were reduced with mechanical tools to obtain alayer that was approximately Imrn thick.

Some initial tests were made on numerous samples(on more than the twelve selected for this experiment)to determine the- range of suitable parameters for safeand effective cleaning. As a result of these tests, thefollowing parameters were chosen: energy between0.7-1.1; a spot of 6 mm; fluences between 2.5 and3.5 J/cm 2; frequency between 4 and 10Hz.The laser cleaning was performed in water-assistedconditions: that is to say, the surface was either kept wetduring irradiation or the sample was totally immersed.2.3 Surface examination techniquesThe surface of the bones was examined after the treat-ment by means of optical and electronic microscopyto evaluate the precision of both the laser and scalpelcleaning techniques. The following equipment wasused: an Olympus SZJJ stereoscope (with a digi-tal photo system Infinity . Y ) , a JEOL 6400 Scan-ning Electron Microscope, and a FEI Quanta 600Environmental SEM.

3 RESULTS AND DISCUSSION3.1 Induced damageTo characterize the potential alterations caused by laserirradiation, the surface of one of the samples wasdeliberately damaged.

The laser was used to irradiate the clean surface ofa bone. A systematic sweep (making lines) was madeusing different parameters (Fig. 1). Half of the bonewas dry; while the other half was wet.

In the dry half, the sweep produced visible grey orblack lines. Furthermore, excessive exposure of boneto the laser produced small holes, craquelures, and amolten appearance. These surface modifications seemto have been caused by the heating of the surface underthe laser spot. 611 the other hand; the wet half of thebone was mostly unaltered using the same parameters.Thus, the damage threshold depends significantly onthe dampness of the surface. In this specific sample,the damage threshold for the dry substrate was estab-lished using the minimum parameters tested: energy of

Figure 1. Fossil showing damage induced by laser. The darkbands correspond to laser sweeps carried outusing differentparameters. The box indicates the area shown inFIgures 2a'and 2b.0 .2 J; spot of 1 0 nun; fluence of 0,.3 J/cm 2. The damage,threshold of the wetsubstrate, however, was consider-ably higher because a higher flu en ce o f 4 J/cm1(energyof 0.5 J and 4 mm of spot) only left a slight mark.

This test showed the considerable differencebetween the effects of the dry and the wet cleaning.As other authors have. pointed out (Landucci 2000,

López-Polín, L. et al. Láser removing carbonated matrices from fossils. 2008

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