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M. Schreiner, M. Strli(eds.):

Handbook on the Use of Lasers in Conservation and Conservation Science, COST G7 (2007)

2.1 Principles of Laser Cleaning in Conservation

Salvatore Siano

2.1.1 Introduction

2.1.2 Historical Note

2.1.3 Laser Systems and Parameters

2.1.4 Linear Laser-Material Interaction

2.1.4.1 Absorption and Scattering

2.1.4.2 Laser Heating

2.1.4.3 Photoacoustic Effects

2.1.5 Non-linear Interaction Effects

2.1.6 Note on Laser induced Plasma

2.1.7 Ablation Channels

2.1.7.1 Ablation Rate

2.1.8 Conclusive Note2.1.9 Supporting Information

2.1.9.1 References

Istituto di Fisica Applicata Nello Carrara, Consiglio Nazionale delle Ricerche, Sesto Fiorentino,

[email protected] http://www.ifac.cnr.it
mailto:[email protected]://www.ifac.cnr.it/http://www.ifac.cnr.it/mailto:[email protected]


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S. Siano: Principles of Laser Cleaning in Conservation

Handbook on the Use of Lasers in Conservation and Conservation Science. COST G7 (2007)

2.1.1 INTRODUCTION

The properties of monochromaticity, collimation, and coherence of laser light and the associatedinteraction features have favoured the development of a variety of applications in several fields,

such as for examples industrial [1-3], biomedical [4-6], and cultural heritage [7-9]. A general

distinction between applications not involving relevant and permanent physical changes to the

irradiated material and the ones producing irreversible modifications can be done. The former are

usually aimed at characterisation purposes, whereas the latter include both diagnostic and

processing techniques.

Laser ablation is one of the most important irreversible irradiation effect (see for example [10],

[11], and the other Proceedings of the COLA Conferences), which can be induced on opticallyabsorbing materials or in their close proximity. Laser cleaning is a particular case of laser ablation

where a specific material layer or substrate is uncovered through the removal of undesired layers or

incoherent particle distributions. Laser cleaning processes are exploited in different fields. Thus for

examples, besides the conservation of artworks discussed in the following, it is being used in a

number of industrial needs, such as semiconductors cleaning in microelectronics, die cleaning in

plastic pressure casting, paint stripping in the aircraft maintenance, and other. Even some laser

surgery treatments, such as for example the removal of tattoos, can be classified as a cleaning appli-

cation.

The first observation of the laser ablation and cleaning phenomena dates back to the origins of laser

technology [12, 16]. Significant advancements on the understanding of the physical mechanisms

through systematic phenomenological studies and the diagnostics of the laser-material interaction

were achieved starting from beginning eighties. The main results were provided by researches

related with medical surgery and microelectronic industry, which resulted of fundamental impor-

tance also in other fields of application including the present one.

After a short historical note, the following paragraphs report a brief review of the basic mechanisms

involved in the laser cleaning process in the restoration of art and historical artefacts. It is based on

the suitable framing of the general achievements mentioned above within the present application

domain and on the specific insights related with important conservation problems that were

approached by laser during the last decade.

2.1.2 HISTORICAL NOTE

The application of the laser cleaning in the conservation of artworks was proposed by J.F. Asmussince the beginning of seventies [17-19] through a set of practical tests carried out in Venice on

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encrusted stone artefacts. As reported by the involved conservation scientists and restorers [20-23],

the novel approach did not overcome the experimental stage for several years mainly because of the

technological limits of the pulsed laser sources available at that time: Ruby and Nd:YAG laserswith low pulse repetition rate, absence of versatile beam delivery systems, very low reliability for

long time operations, and high costs.

During the eighties the technological level of the laser devices increased significantly but the costs

still were out of scale for the specific field of application, even more whether considering the

relatively low productivity, as compared with traditional chemical and mechanical cleaning

techniques. The surviving of the novel conservation approach for more than a couple of decades,

against unfavourable performances along with the caution, scepticism, or indifference of the most

of the conservation community, has to be completely attributed to the perseverance of Asmus andhis co-workers [24-30].

Mainly thanks to the stimulus provided to the research of innovative technologies dedicated to the

study and safeguard of cultural heritage by European Framework Programmes and various National

Innovation Programs, the situation drastically changed during the nineties. Several research centres,

conservation institutions, and restoration enterprises initiated constructive interactions aimed at

developing laser systems and methodologies dedicated to different classes of materials and deterio-

ration problems. Up to 1995 the scientific results were reported in some conservation meetings,

disciplinary congresses, and journals without the possibility of real interdisciplinary debates and

experience exchanges on the topic [18-49]. The institution of the international conference

LACONA (Laser in the Conservation of Artworks) [50] held for first time in Crete (1995) has

represented a very important step for consolidating and refining the positive results already

achieved on encrusted stone and extending the application to other conservation problems by

increasing the scientific level and improving interdisciplinary approach. The conference has become

a fundamental reference for the development and dissemination of the laser techniques in the

conservation field.

Physical investigations on the laser cleaning processes of stones, metals, paintings, paper, parch-

ment etc., and thorough evaluations of the irradiation effects were reported throughout the six

editions of the LACONA conference [50-54] held up to now. In recent years, two more congresses

were born within the disciplinary context of the applied optics dedicated to the application of laser

techniques in conservation (SPIE conference Laser Techniques and Systems in Art Conservation

Munich 2001-05 [55-57] CLEO 2005 Symposium on: Laser technology for the preservation of

cultural heritage). Furthermore, specific sessions on the topic started to be included in congresses on

laser technology and applications (International workshop on: New trends in laser cleaning 2001-

05, COLA 2003 [10], and other). Some contributions can be also found in the proceedings of inter-disciplinary conferences such as Science and Technology for the Safeguard of the Cultural Heritage

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in the Mediterranean Basin (since 1995 [58]), International Congress on Deterioration and Conser-

vation of Stone, and other. The gradual acceptance within the scientific and conservation communi-

ties of what can be defined as a kind of revolution in the conservation procedures is alsodocumented by the increasing number of studies published in various applied physics, applied

chemistry, and interdisciplinary journals.

Anyway, as a result of this collective effort, which involved the scientific community of many

countries, the most important advancement is represented by the transition of laser techniques from

the laboratory experimentation to the everyday practice of important conservation institutions and

restoration enterprises. Along the last decade Italy is having a leading role in this acceptance and

dissemination process. Laser techniques are being used in the Opificio delle Pietre Dure (OPD), the

Istituto Centrale del Restauro (ICR) and other important conservation institutions, as well as inprivate organisations, and by some tens of restoration enterprises on a large variety of conservation

problems. Thus for example, eight different laser systems are presently used in the laboratories of

OPD, Florence, where a number of problems encountered in the restoration of Renaissance master-

pieces have been solved by optimised laser treatments. From this exploitation standpoint, important

results were achieved during the last fifteen years in France, Austria, Spain, and Portugal, for the

restoration of stone sculptural elements in historical faades and monuments, in England for

museum collections, in Holland for the first applications on paintings, and more recently in Greece

on ancient marble masterpieces and wall paintings. Furthermore, promising developments are

registered in Croatia and Slovenia where conservation institutions and some restoration enterprises,

in collaboration with research centres, are starting to use laser methodologies in their restoration

works.

There are no special recipes to speedup the acceptance and dissemination process. The only effec-

tive ways are the ones based on rigorous approaches to the material characterisation, physical

evaluation, and validation associated with specific conservation objectives, whose definition pass

through a strict interaction of the scientific component with the historical, artistic, and management

ones.

The present section aims at providing a first introductory picture of the main physical phenomena,

which should be taken into account for optimising laser cleaning treatments, while the state of the

art for the various materials and conservation problems is summarised in some details in the next

sessions of this chapter.

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2.1.3 LASER SYSTEMS AND PARAMETERS

Only pulsed lasers are used in the cleaning of art and historical objects. Up to a few year ago, themost employed systems in stone cleaning were based on Q-Switching (QS) Nd:YAG lasers emitting

at the fundamental harmonic (1064 nm) pulses of typical duration of 8-20 ns and energies between

0.1-1 J/pulse. Usually, also the Free Running (FR) regime is available on these commercial laser

systems providing pulse durations of 200-500 s and higher pulse energies up to 2 J but, as

explained in the following, this range of pulsewidth is not effective for the most of the cleaning

problems. A novel class of Intermediate Pulse Duration (IPD) Nd:YAG lasers was proposed and

commercialised in the last years. These systems are based on Short Free Running (SFR) and Long

Q-Switching (LQS) regimes providing pulse duration between 50 ns-3 s and 20-120 s, respec-

tively, with similar energies as FR and QS lasers [59-60].

IPD laser systems are always fibre-coupled (1 mm typical core diameter) and equipped with versa-

tile handpieces providing very homogeneous and a finely controllable irradiation spot. The market

also offers QS lasers coupled into relatively thick fibres (1.5 mm core diameter). The output energy

from a single fibre is limited to around 200 mJ because of damage risks due to the very high inten-

sity and associated non-linear absorption phenomena (optical breakdown). Only a few of these

systems are presently in use, whereas the most of commercialised QS laser systems are coupled in

an articulated arm, which allows to propagate higher pulse energies (up to 1 J) but with a low beam

quality because of possible hot spots and fringe structures usually associated with Nd:YAG laser

beams. The handpieces have a lower spot control and versatility with respect to fibre optical beam

deliveries. The success of this class of systems is due to the relatively high cleaning efficiency,

which favoured their employ for cleaning intervention on relatively large areas.

During the last years the mentioned limits along with the problem of yellow appearance associated

with the cleaning of whitish substrates [61-63] have significantly slowed the commercial spread and

hence the use of QS systems, which provided room to a relevant penetration of the market by the

SFR lasers, which was favoured by a number of basic studies and successful example applications

[47, 63-71].

Multi-wavelength QS Nd:YAG laser systems were also proposed for overcoming the problem of

the yellow appearance and to approach the cleaning of wall paintings. In particular, positive results

were documented for the second harmonic (532 nm) [71-74] and the combination of first and third

harmonics [75]. Despite the higher technological complexity and costs of the multi-wavelength

lasers, which are big obstacle to a wide spread, it is worth nothing that the double wavelength

solution was successfully applied to clean sculptural elements of the frieze of the Parthenon [76].

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Two more class of lasers were proposed for the cleaning of paintings through extensive laboratory

tests and some practical applications. They are KrF excimer laser (248 nm, around 30 ns) and FR

Er:YAG (2.94 m, 250 s) [77-80]. Different order of problems, concerning side effects and for theformer also costs and lack of portability, are preventing the acceptance and widespread of cleaning

treatment based on these laser systems. The present perspective appears hence not so promising.

Even less whether considering the positive results recently achieved by Nd:YAG laser systems [74,

81-85] that already dominate the present field of application.

2.1.4 LINEAR LASER-MATERIAL INTERACTION

Laser ablation of a material stratification is a strongly non-linear process occurring when the irra-diation fluence (pulse energy per unit area: F0=E/A) or in some cases intensity (peak power per unit

area:I0=P/A) overcomes a critical threshold, which is an intrinsic property of the material structures

under irradiation. In the domain of interest fluence and intensity are usually expressed in mJ/cm2

(m=10-3) or J/cm2and MW/cm2or GW/cm2(M=106, G=109), respectively.

The dynamical development of the laser ablation involves optical, photothermal and photo-

mechanical phenomena depending on the laser parameters and material properties. In order to

understand the different ways in which pulsed laser irradiation can produce material removal, it is

useful to introduce separately these different phenomena starting from their linear regimes

occurring at relatively low fluences, i.e. significantly lower than the critical ones for inducing any

irreversible effect to the irradiated material.

2.1.4.1 ABSORPTION AND SCATTERING

The incidence of a laser beam on a material (here and elsewhere if not specified differently the

material is assumed to be homogeneous) is accompanied by absorption and scattering phenomena

producing attenuation and spatial redistribution (diffusion) of the beam energy. In the case of a

layer of dielectric material (as for examples black crust, whitewashes etc.), it is useful distin-

guishing among back scattered, absorbed, and forward-scattered radiation and to introduce the

reflectance (R=Er/E), absorbance (A=Ea/E), and transmittance (T=Et/E) parameters of the material

layer (Fig. 2.1.1):

. (1).1+AR T=+

When irradiating thick or very absorbing materials T0, but in practical cases of adjacent material

layers, often the transmittance must be taken into account in the energy balance (Fig. 2.1.1c).

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The flux of energy, which propagates into the material, F, undergoes to a typical exponential

attenuation law along the optical axis,z:

(2)

where Fa=(1-R)F0 and is the effective absorption coefficient, whose reciprocal represents the

optical penetration depth =1/ ,also named optical extinction length. It is the length of the optical

path along thez-axis, which produces an attenuation of the energy flux at the material surface Faof

a factor 1/e.

For very absorbing material =1/a, where a is absorption coefficient, whereas at the opposite

limit, when the propagation is dominated by the scattering, the diffusion approximation provides the

following expression:

(3)

where s is the scattering coefficient, g the anisotropy parameter, which represents the integral

average of coson the scatteringphase function. These are fundamental parameters of the material

depending on its composition and microstructure, as well as on the laser wavelength . The pene-tration parameter enables to estimate the irradiated (or absorption)volume,V=d, where dis thelaser spot diameter. For visible and near infrared wavelength the typical values of range from

several nanometres of a metal, to 10-100 m of a fairly homogeneous black crust or a brown

patination, up to several millimetres of calcite or gypsum. When the thickness of the irradiatedmaterial layer, l, is lower than it could be useful to consider =l.

Fig. 2.1.1: Representation of energy redistributions in laser-material interaction: absorbing material (a),diffusing material (b), and the adjacent absorbing and diffusing materials layers (c). Er, Ea=reflected andabsorbed energies.

Reflection

lobe

Laser beam (E,F0)

Er

Ea

Er

Laser beam (E, F0)

Ea

Fa/e

Laser beam (E, F0)

Er

Er1

Et

Ea2

Ea1 l

Fa

a) b) c)

( )[ ]{ } 21

13 + asa g

a ez ( ) zFF

=

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Fig. 2.1.1 displays a qualitative representation of the energy re-distribution in homogeneous mate-

rial layers in the case of absorption (Fig. 2.1.1a) and scattering limits (Fig. 2.1.1b), along with acomposite situation of an absorbing layer on a diffusing substrate (Fig. 2.1.1c). Anyway, in cases of

practical interest the irradiated material layers are strongly inhomogeneous. Thus besides the men-

tioned approximation limits also their superposition within the same layer must be taken into

account as possible description of the optical propagation regime. Fig. 2.1.2 shows qualitative

examples of optical propagation into real stratigraphies where the outer layer is absorbing

(Fig. 2.1.2a), diffusing (Fig. 2.1.2b) or both (Fig. 2.1.2c).

Fig. 2.1.2:Examples of material layers with different optical properties: a) absorbing, b) diffusing, c) inter-mediate situation. a) bronze-like organic binder patination on a mineralised bronze surface. b) whitewash

layer on a paint azurite layer. c) Typical gypsum-matrix black crust on Ca-oxalates film and white marblesubstrate. The represented optical distributions are purely qualitative.

Laser beamLaser beam Laser beama b c

Despite the strong variability that can be encountered from zone to zone of the same artefact or also

within the depth of a relatively thick stratification and the difficulty to determine the real energy

distribution, as well as to measure a, s, and phase function, useful estimations of the irradiation

effects can be derived from relatively simple reflectance and transmittance measurements

(Fig. 2.1.3). These allow measuringEA, the absorption volume, and hence the energy density inside

the material (=EA/V expressed in J/cm3), which is the fundamental parameter for any irreversible

phenomenon associated with the laser irradiation.

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Fig. 2.1.3:Experimental setup for reflectance (a, b) and transmittance (c) measurements.

Tab. 2.1.1 reports the results of the reflectance measurements in dry and wet conditions for two

stone typologies: white Siena marble and a yellow-brown Pliocene sandstone. The wet condition is

of great interest in laser cleaning of art and historical stone artefacts, since for the most of the cases

the water assists, usually performed using common atomisers, and provides clear advantages in

terms of effectiveness and efficiency of the treatment. The two stones are representative of

reflective and absorbing stones, respectively.

SAMPLEBLACKCRUST

CLEANEDSURFACE

Rdry,Rwet[%]

STONESUBSTRATE

Rdry,Rwet[%]

OP 334, black crust on

white Siena marble

11, 8 Ca-oxalates film

48, 29 69, 49

OP 205, black crust on

white Siena marble

20, 14 Ca-oxalate film

39,32 69, 49

Quarry sample of

Siena marble

-- --

64, 40

ER151, black crust on

Pliocene sandstone

21, 13 Sulphated surface:

46, 30 38, 25

Quarry samples of

Pliocene sandstone

-- --

40, 27

Tab. 2.1.1:Examples of reflectance measurements of stone quarry samples and

fragments from historical facades of Siena: P334) Logge del Papa, OP 205)

Baptistery, ER 151) Palazzo Spannocchi.

As it can be seen, the samples from historical faades exhibit a significant increase of reflectancewhile stepping from the black crust, to the underlying layer (Ca-oxalates film or surface sulphates).

P

I S

To the scope

Optical fibre

FLens

From the laser

Integrating sphere

Sample PhotodiodeP

I S

To the scope

PanelOptical fibre FLens

From the laser

Filter

Integrating sphere

Photodiode

Sample

a b c

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This optical feature is very useful in laser cleaning treatments since it favours a self-terminating

behaviour of the laser ablation within relatively wide operative fluence ranges. Anyway, as stated

above, the most important parameter for any induced effect is energy density realised into the irra-diated materials, . Thus, beside high R, the self-termination is also favoured by high . For the

white marble of Tab. 1 both conditions are realised, being dry=3.30.5 mm and wet=5.80.5 mm,

as estimated by calculating aand sfrom the Kulbelka-Munk parameters and direct measurementof the scattering phase function [66]. Conversely, for Pliocene sandstone direct transmission

measurements (Fig. 3c) provided dry=150 m and wet=180 m. Hence, the cleaning of this stone

will be relatively more critical, as verified by irradiation tests [65].

As shown by reflectance and penetration measurements, the wetting of the irradiated surface

produces and increase of both A and parameters. For the present examples the two effects arebalanced, the wetting does not produces a relevant variation of the energy density within the irradi-

ated volumes (i.e. wetdry). On the other hand water assists plays an important thermal role.

2.1.4.2 LASER HEATING

In the most of the cases the absorbed energy Eais dissipated through the thermal channel, whereas

only at high irradiation intensities or short VUV wavelengths also the direct ionisation and

molecular photodissociation can play an important role. Hence, the main direct effect of the laser

irradiation is a temperature rise within and in proximity of the irradiated volume.

Theoretical estimations of the thermal distributions induced in homogeneous materials can be

derived through the heat conduction equation [86] under the assumption of constant optical and

thermal parameters. This hypothesis holds whenever the temperature peak is lower than the critical

ones for any irreversible phenomenon (discoloration, carbonisation, vaporisation etc.).

Let us consider a homogeneous laser beam of intensity I(t) incidents on the surface of a semi-

infinite conductive material and a surface photothermal conversion of the absorbed energy (=0).

The one-dimensional solution of the conduction equation allows achieving the temperature rise Tinduced by the intensityIa(t)=(1-R)I0(t):

(4)( )= dtID

z'

(

t Dt

z

at

ett

KtT

0

4

''

'),

2

where KandDare the thermal conductivityand diffusivityof the material, respectively.D=K/Cp,

with and Cpdensityand specific heatof the material.

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Besides the material parameters the use of eq. 4 requires the knowledge of the temporal profile of

the laser pulse. As a first approximation it can be often assumed as Gaussian in the nanosecondsrange and top-hat for longer pulse durations. For this latter case the expression of the surface

temperature assumes the following well-known form:

(5),22

1

=

D

K

FT asurf

where is the laser pulse duration (FWHM). As a general behaviour this equation states that the

surface temperature increases when the pulse duration decreases, which is particularly important in

the cleaning of metal artefacts. As an example Fig. 2.1.4 reports the temperature rise at gold-air

interface provided by equations 4 for Gaussian profiles and Fa=0.15 J/cm2

. As it can be seen, thetemperature peak decrease from 454 C to 148 C when the pulse duration increases from 6 ns to

100 ns, which corresponds to a scaling law around -0.4, only slightly different from the --1/2

dependence associated with top-hat pulses (eq. 5).

The parameter representing the heat propagation into the material is the thermal diffusion length:

(6).2 Dzth=

For top-hat laser pulses, it is the propagation distance of the thermal wave producing an attenuationof the peak temperature to about 0.1 of the maximum surface value. Fig. 2.1.5 displays the thermal

diffusion length of copper, sandstone, limestone, and water. Considering for example =200 s (FR

lasers) eq. 6 provideszth=305, 38, 30 and 11 m, respectively.

0

100

200

300

400

500

0 100 200 300 400

t [ ns ]

T[C

]

1E-6 1E-5 1E-4 1E-30

10

20

30

40

50

60

70

C Carbonatica

D Arenaria

B Acqua

zter

[m

]

tL

[ s]10-6 10-5 10-4 10

-3

Sandstone

Limestone

Water

Copper

10

20

0

30

40

50

60

70

Fig. 2.1.4:Examples of temperature transients Fig. 2.1.5: Thermal diffusion length as a function ofassociated with Gaussian pulses of different duration. the pulse duration.

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Laser irradiation is indicated as thermally confinedwheneverzth


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2.1.4.3 PHOTOACOUSTIC EFFECTS

Pulsed irradiation can generate acoustical transients, which propagates into the irradiated materialsstructures along distances much larger than and zth. The basic mechanisms can be very different

depending on the physical properties and laser parameters. For solid absorbing materials laser inten-

sities of order of 106-108 W/cm2 the photoacoustic effect is usually originated by the

thermoelasticity.

All the materials in different extents exhibit a volume variation when heated, which is reversible

within specific temperature and pressure range, i.e. thermoelasticity domains. The parameter

characterising the effect is the thermal expansion coefficient,, representing the relative volume

variation produced by unit temperature variation.

Laser irradiation with pulse duration short enough to realise the thermal confinement condition

(


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(10)( ) = '()( dtIpr

+

)''2

tttC

t ap

(11)( )td

tdppf =

ct r

)(

where is a delayed time, the thermoelastic transfer function, and c the sound speed into the

material. These formulas show the strong dependence of the peak pressure on the laser pulse

duration, appearing both as inverse proportionality and through the intensity Ia. Fig. 2.1.6 displays

qualitatively the development of pressure transients for the two boundary conditions. As described

above, the compression phase is generated by thermal expansion whereas the development of a

rarefaction phase in free boundary condition is due to the reflection of the compression wave at the

solid-air interface. The acoustical reflection coefficientis defined as:

t

(12)ma

maac

ZZ

ZZR

+

=

where Zaw=a,mca,mare the acoustic impedances of air and solid material, respectively. Since it is

Zm >>Za eq. 11 provides Rac-1, that corresponds to a total reflection at the water-air interface,

which is accompanied by a phase inversion producing the rarefaction peak described by eq. 11.The transit time along the distance is named elastic relaxation time, ael:

(13),elc

=

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ablation plume. Before approaching the description of the different possible ablation channels let us

list some of the main non-linear interaction effects, which are involved at sufficiently high fluence

or intensity levels.

Variation of the macroscopic parameters

Discoloration, optical trapping, and atomic scale effects are responsible for significant varia-tions of the optical parameters.

For amorphous solids a temperature rise usually produces an increase of KandD. The thermal expansion coefficient depends on the temperature. Porosities, surface roughness, microfractures and other structural features can affect in

different extend the description of the laser induced effects based on macroscopic material

parameters. Laser induced structural modifications can produce strong non-linear effects. Material removal is an efficient cause of cooling of the material substrate. It produces strong

discontinuities in the optical, thermal and mechanical propagation phenomena.

Atomic and molecular scale

Multiphoton absorption Saturation of specific absorption bands Photodissociation

Photoionisation Plasma formation

2.1.6 NOTE ON LASER-INDUCED PLASMA

A plasma is a macroscopically neutral gaseous phase where a relevant fraction of particles (of order

of 10% or more) is ionised. Laser irradiation of absorbing materials at high intensity (above 108

W/cm2) can induce the onset of plasma in proximity of the surface through the so-called optical

breakdown phenomenon [90]. It develops from a number of initial electrons, mostly generated by

multiphoton ionisation of atoms and molecules. The energy of these free electrons is increased by

absorption of incident photons (inverse bremsstrahlung) up to the ionisation energy levels, thus

driving an avalanche multiplication. Depending on the intensity and temporal profile of the laser

pulse, after some or several nanoseconds the electron density reaches values of order of 1018-1020

cm-3and the electron temperaturesrises up to 104K [91].

The plasma plume is very opaque at long wavelength, such as the ones on CO2, Er:YAG and

Nd:YAG lasers, being the plasma optical extinction length p(1/)2

. Thus for example, ifTe=15000 K and ne=10

20cm-3 the analytical estimation at =1064 nm returns p=15 m, against

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typical plasma sizes 1 mm. Hence a dense plasma acts as an effective optical shielding while any

further effect to the irradiated material is mostly mediated by the complex fluid dynamic processes

associated with the plasma expansion.

In the practical cases of laser cleaning, plasma formation can occur when using short QS Nd:YAG

lasers (ex. 5 ns) at relatively high fluences and hence intensity (ex. 3 J/cm2, 6108W/cm2) to removehard or weakly absorbing stratifications. Anyway, these operative conditions are often ineffective

and harmful for the integrity of the substrate. Whereas, at the typical operative fluences (0.5-1

J/cm2) in water assisted conditions, the occurrence of a dense plasma phase is unlike, even though

some not significant ionisation phenomena, favoured by inhomogeneous beam and strongly

absorbing components (hot spots and black carbon particles), could accompany the ablation

dynamics [92].

Similarly, no dense plasma is expected for cleaning with LQS and excimer lasers because of the

low operative intensities. For example the maximal operative fluence in stone and metal cleaning

with LQS Nd:YAG laser pulses of 20-120 ns is around 3 J/cm2, which corresponds to 2.5-15 107

W/cm2, while excimer laser removal of varnishes is carried out around 0.5 J/cm2 (about 1.7107W/cm2).

SFR lasers at fluences as high as 20-30 J/cm2can produce the formation of rarefied plasma, which

is optically thinand hence does not influence significantly the ablation dynamics driven by direct

laser-material interaction [64].

2.1.7 ABLATION CHANNELS

A general scheme of the different laser ablation channels is reported in Fig. 2.1.7, for pulse duration

ranging between a few nanoseconds up to hundreds of microseconds. Let us distinguish ablation

occurring below and above the vaporisation thresholdof the irradiated material.

At fluences below the minimum one for vaporisation the only possible ablation mechanisms are of

photomechanical type, apart from the special case of VUV wavelengths, where the direct molecular

bond breaking can provide a relevant contribution. The two main channels are based on pressure

confinement(


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generated at the absorption centres, secondary and water mediated spallation. These latter two

mechanisms are frequent in cleaning applications. As schematically illustrated in Fig. 2.1.8, in

secondary spallation a relevant part of the radiation is absorbed underneath the outer layer of theirradiated stratification, which is removed by interface pressure development in thermoelastic or

vaporisation regimes. Instead the concept of water-mediated spallation aims at representing the

significant role of water assists in the mechanical coupling and propagation of the pressure wave.

Above the vaporisation threshold only fast thermal explosioninduced by laser pulses in the nano-

seconds range is the most properly called laser ablation, where the thermal confinement is usually

assumed as rigorously verified, but in the present concern it is useful to extend the concept to

plasma-mediated material removal, of importance in some specific high intensity treatment, and to

quasi-continuum vaporisationproduced by SFR and FR Nd:YAG lasers.

Fast thermal explosion and plasma-mediated ablation are usually characterised by fluid dynamic

regimes and strong recoil stresses released to the material surface [95-98]. For pulse duration in the

order of microseconds, pure vaporisation and plasma channels do not exhibit strong differences

because, as mentioned above, the plasma phase is expected to be very rarefied. For this reason in

Fig. 2.1.7 they were merged in a single channel. The front of the ablation plumeproceeds with a

quasi-sonic speed, so it cannot drive a shock wave and then a significant recoil stress [64]. Finally,

not thermally confined (>th) slow vaporisation is the ablation mechanism at pulses duration of

200-500 s, usually provided by commercial Nd:YAG laser systems.

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OPTICAL ABSORPTION

SLOW

VAPORIZATION

VaporizationTurbolent expansion

NOT CONFINEDHEATING

FASTVAPORIZATION

Quasi-sonicexpansion

Possibility of ararefied plasma

BELOW THE VAPORISATIONTHRESHOLD

ABOVE THE VAPORISATION THRESHOLD

10-100 ns 200-500 sF10-100 J/cm2

20-100 sF1-20 J/cm2I107-108W/cm2

PRESSURECONFINEMENT

Pressure gradientat the interface

5-10 nstI106-108W/cm2

DENSE PLASMA

Plasma expansionShock wave

PhotomechanicalStrong recoil

Optical breakdown

5-100 ns, F0.1-4 J/cm2

I107-109W/cm2

Explosive removalRecoil stress

FAST THERMALEXPLOSION

Thermoelasticphase

PRIMARYSPALLATION

Double phasepressure wave

THERMOELASTIC

EXPANSION

IMPULSIVEEJECTION

Fig. 2.1.7: Laser ablation channels for homogeneous absorbing materials. Pulse durationscorrespond to the one of the commercial laser systems. Fluences and intensities are roughlyestimated, they only indicate orders of magnitude.

Primary spallation

Secondary spallation

p(t)

t

p(t)

Fig. 2.1.8:Representation of possible spallation mechanisms. Primary spallation: produced byrarefaction peak. Secondary spallation: produced by laser heating and subsequent pressure

development in proximity of the interface between two adjacent layers. Water-mediated spallation:similar to primary spallation, where water plays an important role in photomechanical and pressurewave propagation. A relatively strong fragmentation effect is expected in this latter case.

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2.1.7.1 ABLATION RATE

The ablation raterepresents the single pulse efficiencyof the laser removal at a given fluence. It isusually measured in g/pulse or m/pulse, while the ablation efficiencyin g/Jcm-2/J or g/Jcm-2.The knowledge of the dependence of the ablation rate on laser fluence is of practical importance in

order to precisely control the removal of the stratification. The typical behaviour for ablation above

the vaporisation threshold is the one reported in Fig. 2.1.9a, where the fluence and rate scales were

arbitrarily assumed as the ones of the experimental data of Fig. 2.1.9b.

FsFth

Linear ablation domain Saturation domainBelowvaporisation

hreshold

Fm

Fig. 2.1.9:Ablation rate for vaporisation-mediated removal; left) general behaviour; right) experimental dataachieved for laser ablation of black crust standards using different pulse duration between 125-950 ns (SFR

Nd:YAG laser).

Laser ablation starts to be observed above the minimum fluence Fth named ablation threshold.

Above this value the removal is almost linear up to the saturation fluenceFs, indicating where the

efficiency is significantly reduced by dissipative phenomena as in particular ionisation and plasma

formation. The maximum efficiency is achieved at the intermediate fluence Fm. The maximum rate

in a pure vaporisation regime is equal to the optical penetration depth, .

The so-called blow-offmodel provides a theoretical estimation of the rate curve[99]. It is based on

the assumption the material is immediately removed when the laser heatingovercomes the critical

energy densityof the irradiated material, cr, which allows achieving the following estimation of the

ablation rate and threshold:

(13)

,ln 0

=

cr

abl

Fz

,crthF =

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The conditionzabl=allows determining the saturation fluence Fs=eFth3Fth, which can be

considered as a limit operative fluence.

Equations 13 are derived under the assumption of thermal confinement, i.e. from eq. 2 and eq. 8. It

is important to note that in the present approximation Fthdoes not depend on the laser pulse dura-

tion. Conversely, for long pulses (>th) eq. 5 (conduction limit) provides the following expression:

,2

2),0( 00max

D

KTFT

D

K

FTTTT cthc

c

==+=+= (14)

which shows a dependence on pulse duration of type 1/2. Thus for example if Fthis the ablation

threshold at 50 s, the threshold at 500 s will be 3.3 Fth. The strong reduction of efficiencyexplains why long pulses are rarely used in laser cleaning.

For real stratifications it is useful to introduce concepts like cleaning fluenceof cleaning threshold

Fcl, which is the minimum laser fluence providing the desired cleaning result in self-terminated

cleaning treatments, since Fthcan exhibit strong variations from point to point and within the strati-

fication depth. Anyway, their fluence and pulse duration dependence are still roughly estimated by

eq.s 13 and 14.

2.1.8 CONCLUSIVE NOTE

In this section we provided an introductory description of the main physical concerns involved in

the lasers cleaning of artworks. Several insights on the various topics and examples of application

aimed at optimising specific laser cleaning treatments can be found in the following sections and in

the reported literature. At the same time it is worth noting that further studies are needed in order to

provide thorough quantitative descriptions of the interaction dynamics occurring in real inhomo-

geneous multilayer stratifications.

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http://bookstore.spie.org/index.cfm?fuseaction=DetailVolume&productid=414764&CFID=961536&CFTOKEN=64965141http://bookstore.spie.org/index.cfm?fuseaction=DetailVolume&productid=477567&CFID=961536&CFTOKEN=64965141http://bookstore.spie.org/index.cfm?fuseaction=DetailVolume&productid=477567&CFID=961536&CFTOKEN=64965141http://bookstore.spie.org/index.cfm?fuseaction=DetailVolume&productid=414764&CFID=961536&CFTOKEN=64965141


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[57] Proceedings of Optical Methods for Arts and Archaeology, R. Salimbeni, L. Pezzati (ed.s)

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