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The Gate of Paradise: Physical Optimization of the Laser Cleaning Approach

Author(s): S. Siano and R. SalimbeniSource: Studies in Conservation, Vol. 46, No. 4 (2001), pp. 269-281Published by: International Institute for Conservation of Historic and Artistic WorksStable URL: http://www.jstor.org/stable/1506776

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THE GATE OF PARADISE: PHYSICAL OPTIMIZATION OF THELASER CLEANING APPROACHS. Siano and R. SalimbeniSummary-A methodology for the use of lasers in the restoration of the Gate of Paradise, a gilded bronzeartwork by L. Ghiberti, was evaluated theoretically and experimentally. Preliminary characterizationof thematerials to be laser-cleaned made it possible to estimate the thermalregimes involved and to model the abla-tion process. Calculations aimed at controllingunwantedheating of the gold film were also developed beforelaser-cleaning tests on large areas were carried out. Finally, the cleaning tests were critically evaluated bymeans of instrumentalinspection. The practicability of the laser approach, either as a preliminary to gentlechemical cleaning involvinglower concentrationsof chemical agents and shorter exposure times or as a stand-alone treatment, was demonstrated.IntroductionThe Gate of Paradise by Lorenzo Ghiberti, as wellas the other two doors of the Baptistery ofFlorence, have undergone systematic investigationssince 1966, the year of the flood. The studies wereaimed at assessing the state of preservation, under-standing the deterioration mechanisms, and definingcriteria for the future conservation of these gildedbronze artworks [1-5]. The urgent need to makechoices also stimulated new historical researches onGhiberti's works [6, 7]. The large number of multi-disciplinary investigations performed on the Gate ofParadise up until the beginning of the 1980s, deal-ing with the various aspects of the conservationproblem, were briefly summarized in a report pub-lished by Baldini [8], then Superintendent of theOpificio delle Pietre Dure (OPD), the institutionresponsible for the restoration work.Basic data about the composition of the copperalloy, gold film, corrosion products and encrustationwere obtained through detailed analyses. The mainoutcome with respect to the conservation treatmentwas the establishment of a chemical cleaning proto-col, which was formulated following several opti-mization phases at the beginning of the 1980s [9,10]. After being dismounted from the single castingbronze framework, each sculptural element under-goes a specific sequence of chemical treatments inxylene-acetone (dimethylbenzene-propanone), dis-tilled water, and sodium potassium tartrate solutionbaths (for details see [8]). This last treatment elimi-nates dangerous copper corrosion products from thegilded surface and, to some extent, underneath it.The dismounting of the sculptural elements fromthe framework represents the most critical phase ofsuch a procedure. On the other hand, this step isnecessary to avoid the penetration of the chemicalReceived May 2000

agents and encrustation products inside the frame-work cavities behind the sculptures, which wouldcause serious corrosion problems. As six of the 10sculptured panels (79.5 x 79.5cm) were displaceddue to the banging of the door caused by the vio-lent flow of water during the flood of 1966 and wereafterwards fixed with screws, their removal for theconservation treatment was relatively simple. Upuntil 1985, four of these seriously damaged panelshad been dismounted and three of them restored,while the door was still in use at the Baptistery [11].The last two panels were restored after Ghiberti'sartwork was replaced by a copy in 1990, when theoriginal door was brought to the OPD laboratories.This first group of restored sculptures is exhibitedtoday in a controlled environment at the Opera delDuomo museum in Florence.The problem of dismounting the remaining ele-ments is not yet completely solved. So far, two morepanels and eight smaller sculptures surrounding thescenes of the Old Testament have been dismounted.Thus, the last two panels and 40 smaller elementswould still have to be removed before being sub-jected to the cleaning procedure described above.The mechanical dismounting takes a long timeand appears dangerous for the integrity of thesculptures because of several macroscopic fracturesand the very fragile internal structure of the bronzethat was revealed in gamma-ray images [8]. Thisproblem, besides the many questions about the pos-sible consequences of an overall chemical cleaningfollowing known treatments as well as previous,undocumented ones, led the OPD to investigatealternative treatment procedures. The main criteriawere that the treatment should:1 be non-invasive2 allow in situ cleaning of the remainingsculptures

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S. Siano and R. Salimbeni3 achieve a degree of cleaning comparable withthat obtained with the present protocol4 produce results aesthetically similar to thoseobtained for the recently restored panel5 minimize risks of future deterioration triggeredby cleaning agents

After introducing the problem and stating therestoration goal [12], the OPD requested us toinvestigate, from the point of view of physics, thelaser cleaning approach. As is well known, thelaser technique has undergone significant develop-ments during the last decade in terms of basicstudies aimed at understanding the specific physicaldynamics involved in the cleaning process, and interms of technological improvements made to lasersystems dedicated to conservation [13-17]. Thus,the situation was considered ideal for an in-depthdiscussion of the possible application of this newapproach to an important artwork such as the Gateof Paradise.In what follows we present a preliminary evalua-tion of the possibility of meeting the specificrequirements by a suitable integration of the lasercleaning technique in the restoration procedure.The experimentation was carried out on the panelentitled 'The Story of Noah' that was recently dis-mounted from the door. At the present stage, ourcleaning tests were aimed at optimizing the laserprocedure, then at evaluating the potential of thistechnique with respect to the specific restorationproblem. The work included the following steps:1 characterization of deterioration2 selection of a suitable laser source3 preliminary tests to estimate the averagereflectance of encrustation and gilded surface,cleaning thresholds, laser cleaning effectiveness,possible side-effects as a function of laser flu-ence and irradiation conditions4 physical modelling to estimate in a general waythe laser-material interaction effects in orderbetter to control them5 cleaning tests of different surface morpholo-gies: flat surface, bas-relief, and projectingsculptural elements6 evaluation of the cleaning results

'The Story of Noah' panelTo the naked eye, the panel appears covered with ablackish-green encrustation which still allows theunderlying gilding on most of the surface to be

indistinctly seen, with the exception of projectingfigures and the floor of the scene, where the encrus-tation is significantly thicker (Figure 1). The mostexposed gilding appears veiled by a dark greenishfilm, which alters the natural colour of the goldalloy. Furthermore, several brown areas, not show-ing any trace of gilding, can be clearly observed inthe sky. Since the flood did not detach the presentpanel, these brown zones seem to be the result ofprevious aggressive cleaning treatments. The shapeof these zones is fairly circular or rectangular,sometimes presenting straight lines like scratches.Finally, a diffuse distribution of a large number ofreddish-brown spots (not distinguishable in Figure1) was also observed.The encrustation distribution is similar to that onthe previously restored panels and it was reportedin the literature together with the specific deteriora-tion dynamics driven by the copper-gold galvaniccouple [2, 10, 18].Chemical analysisBesides the general importance of characterizingtheoriginal materials and understanding their deterio-ration, it should be underlined that knowledge ofthe composition of the materials that are going to

pct3 pct4 A (pct2) pct 1

B(pct5) C(pct 6)Figure 1 'The Story of Noah' panel. 'pet' indicatesthe preliminary test sites, while areas A, B and Cwere laser cleaned after optimization of the irradia-tion conditions andparameters.

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The Gate of Paradise:physical optimizationof the laser cleaning approachTable I Analysis of the copper alloyMetal* Technique Concentration(%)Cu Absorption spectrophotometry by flame in air/acetylene 90.9Sn Emission spectrophotometry by flame in nitrogen/acetylene oxide 6-6Zn Absorption spectrophotometry by flame in air/acetylene 1-2Pb Absorption spectrophotometry in graphite oven 1.1*Other lementsdo not exceed0-2%.be laser cleaned helps in choosing suitable laserparameters and in understanding the specific physi-cal dynamics involved in the removal processes. Inparticular, it allows realistic assumptions aboutthermal and mechanical properties and estimationof behaviour under different irradiation conditions.These concerns are very important in situationswhere it is difficult or impossible to perform directmeasurements of the main physical parameters suchas temperature and pressure associated withlaser-material interaction.

Figure 2 Backscattering SEM images of the exter-nal (a) and internal (b) surfaces of the gold film ontwo different gilding fragments.

The metalsAs reported, he door was madewith a quaternarycopper alloy and gilded by the mercury-amalgammethod[5, 19].In order to verifythe alloy compo-sition employedfor the 'Storyof Noah' casting,asmallbronze ragment 10-lmg)was removedat thebottom edge of the back of the panel, near theright edge. The concentrationof copper, tin, zincand lead was measuredby atomic spectrophotome-try (Table 1). The copperconcentrationwas foundto be similar o that of the 'Storyof Joseph'panel[8], whereas the total amount of alloying metalswas slightlyhigher.Severalmicroscopic ragmentsof gold film werecollected rom the waterused for wettingandwash-ing, to analysethe gilding.These fragmentscamefrom exfoliationmicrosites hat are distributedonthe panel surface at relief edges and corrosionmicroblisters.The smallest fragments, hardly separablefromthe encrustationproducts,were analysedby scan-ningelectronmicroscopy SEM)and energydisper-sive X-ray spectrometry (EDX). Backscatteringimagesof fragments howingboth sidesof the goldfilm are shown in Figure 2. The externalsurfaceappears smooth and presents straight marks.Conversely,the internal surface is irregularwithprotuberancesprobably caused by the amalgamflowingover the texturedsurfaceof the underlyingbronzesubstrate.The observationof verticalfrag-mentsand of the brokenedge shown in Figure2aallowedestimationof the averagethicknessof thegold film as about 61im.However, it should benoted that the fragmentsoriginatingfrom micro-blistersare expectedto be thinnerwith respecttothe undeformed ilm.Microprobeanalysisrevealeda residualmercurycontent (Figure 3), higheron the internalsurfacethan on the external surface. The relativeconcen-trationsof gold, mercuryand lead weredeterminedby atomic spectrophotometry nalysisof the frag-ments that were separatedfrom the encrustationproducts:98-3%Au, 1.3%Hg and 0.4% Pb. Thecopper was not measured because it would havebeen difficult o determine ts origin,since it could


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S. Siano and R. Salimbeni2

1CuKa IgLa

I8.00 9.00 10.00 11.00 12.00

298 CNT 9.92KeV 10 eV/ch B EDAXFigure 3 Evidence of residual mercury content byEDX analysis.have been related to copper compounds attached tothe gold film.From the previous data one could expect thermalproperties of the gilding film close to those of puregold, in particular if the heat pulse is released onthe external surface as happens during laser clean-ing.Encrustation productsEncrustation samples were removed with a smallstick and analysed by ionic chromatography andFourier transform infrared (FTIR) spectroscopy.The average soluble salt concentrations are reportedin Table 2, revealing a predominance of sulphatesand a lower quantity of chlorides. The insolublefraction contains mostly carbon compounds and sil-icates. FTIR spectra were also dominated by sul-phate absorption bands (with main peaks at 3408,1621 and 1118cm-'), with traces of nitrates.This analysis provided the reference data for theones performed on the ablated material in order toassess any chemical changes caused by laser heat-ing, in particular on the copper compounds.Table 2 Soluble saltchromatography

concentrations (%?)by ionic

Sample Sulphates ChloridesEoEbEdEf

25-0522-3625-5625-12

6-854-055.893-85

Nitrates

3-015-7*Eorepresents n encrustationampletaken with a smallstick. Eb, Ed and Ef are laser-ablated products removed atthe beginning, during, and in the last phase of the clean-ing process, respectively.

0 10 20 40 60 80 100

t [ns]Figure 4 Laser intensityILas a function of time, asmeasured by means of a fast photodiode. The curvewasfitted with an arbitrary exponentialfunction f(t)(completely superimposed)that was employed or thethermalestimations.

Choice of a suitable laser sourceIn the near infrared spectral region the gold reflec-tivity is quite high (up to 99% for polished surfaces[20]), whereas encrustation products such as carbonblack particles and copper compounds exhibit highoptical absorption. Thus, in principle, Nd:YAGlasers emitting at 1064nm could represent a goodchoice in order to realize the auto-termination ofthe removal process at the gold film surface, and tolimit heating of the film. However, it was importantto measure the reflectance of the encrustation andgilded surfaces in order to achieve realistic esti-mates of the heating effects.The pulse duration of a suitable Nd:YAG lasershould be short enough to allow a fast encrustationremoval, thus preventing conduction heating of theunderlying gold film. On the other hand, the tran-sient heating of the gilding by direct irradiation isexpected to be higher for short pulses compared tolong ones. In the present case, we chose a commer-cial Q-switched Nd:YAG laser with a pulse dura-tion TL= 28ns (full width at half maximum, Figure4). As will be shown in the next section, it repre-sents a good compromise and allows the optimiza-tion of the operating regime.The output beam was coupled to an optical fibre(core diameter 15mm) terminated by a home-madeoptical manipulator designed for easy handling andchanging the size of the irradiation spot duringcleaning.Reflectance measurementsFigure 5 shows the experimental set-up employed to


AuLa E

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The Gate of Paradise:physical optimization of the laser cleaningapproach

ISL F %

Fromthe laser To theFigure 5 Experimental set-up for the mof the optical reflectance. OF: optical fibF: neutral optical filter, IS. integratingphotodiode.measure the optical reflectance, Rop,of thtion and gilding film. Low energyfocused onto the surface through a lensintegrating sphere (IS). The reflected beaiirradiated area is reflected several timinternal wall of the sphere, achievingneous energy distribution inside theobtain the absolute reflectance of the sutested, the signal amplitude measured byode (P) is compared with the one obtastandard reflectance sample (see for examWe measured the variation of R forent encrustation typologies describeAabcthe gold film. In this last case the reflemeasured on very localized cleaningaverage values reported in Table 3 for e;typology correspond to independent mon different sites. The error on the sing]ment (

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S. Siano and R. Salimbeniwards 'erased' by smoothing the surface with asmall stick.In the best operating conditions the ablatedmaterial, including a significant fraction of atom-ized water, was projected 10-20cm away from theirradiated surface. In order to obtain informationabout the ablation dynamics we performed chemi-cal and structural analysis of the ablated material.The samples were taken by irradiating the surfacethrough a glass slide, then collecting the ablatedmaterial on it. Three samples were taken: at thebeginning (Eb), during (Ed) and at the end of thesurface cleaning (E). The ionic chromatographydata are reported in Table 2 together with those ofthe encrustation samples taken mechanically (EO).The sulphate and chloride concentrations were simi-lar, whereas in samples Edand Ef nitrates were alsodetected that are therefore localized in close prox-imity to the gold film. These data, along with opti-cal and SEM microscopy of the ablated material,which did not reveal structural changes, melting orvaporization effects, demonstrated the absence ofthermal and chemical transformations of the coppercompounds and atmospheric particulates generatedby the laser heating driving the ablation process.The analysis also suggests that the photomechanicalcontribution plays a significant role in the materialremoval process.Cleaning tests on red spots revealed interestingfeatures. Most of them looked like soft wax andwere easily atomized or vaporized by laser irradia-tion. However, in a number of cases, they werehard and the laser irradiation caused flaking, aftera few pulses, of large pieces, up to about 0.5cm2and some hundreds of tm thick. This material, itsdistribution and stratigraphy are interesting froman historical point of view and are now underinvestigation.Finally, the cleaning tests of the brown spotsrevealed even more interesting features. In fact,while, as expected, the small spot (pct 3 in Figure1) did not show gold underneath, the larger one(pct 4) covered an area of gilding that was not sus-pected. In spite of a significant gold content, con-firmed by absorption spectrophotometry, this areabehaved differently when submitted to the lasercleaning operation. Besides a lower cleaning thresh-old (about 300mJ.cm-2), we also observed theoccurrence of gilding atomization at a fluence aslow as 600mJ.cm-2. After cleaning, the reflectanceat the laser wavelength was observed to be about55%, i.e., significantly lower than the typical valuesmeasured for the gilding (Table 3). All these pecu-liarities suggested that the large brown areas corre-spond to old integration treatments (for examplewith gold leaf) to repair damages, in agreementwith the hypothesis formulated by Bearzi [2].

Laser-material interactionmodellingOn the basis of the materials characterization,reflectancemeasurements nd preliminary leaningtests, it was possibleto describe, rom the point ofview of physics, the cleaning process of theencrusted ildedsurface.According o the previousexperimental bserva-tions, at the beginningof the surfaceirradiationmostof the laserenergy s dissipated o heat andtoremovethe encrustation,whereasduringthe finish-ing phasethereis a non negligibledirectheatingofthe gold film.Most of the irradiated reais alreadycleanedduringthis last phase.Thus, it is usefultodiscuss separatelytwo interaction regimes: laserablationand direct aserheatingof the goldfilm.Ablation mechanismIn the presentcase, as well as in most conservationapplications,aserablationof the encrustationhasa thermalnature, i.e., it arises by conversionofphoton energy to heat inside the materialsunderirradiation.Dependingon the laserfluencesand onthe distributionof optical absorberswithin theencrustation,different material removal regimescouldbe involvedduring he cleaning reatment.Atthe cleaning threshold derived above, the mostimportant ontributionso the materialremovalare

Laserbeam

wEGFCOB

Figure 6 Schematization of the panel surface andencrustation and of the laser-inducedphotomechani-cal action in wet conditions. B: bronze substrate,CO: copper oxide layer, GF: gilding film, E: encrus-tation layer, W: water layer. The arrows indicate thepropagationof the pressure wave.


- i _

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The Gate of Paradise:physical optimizationof the laser cleaning approachfrom localized vaporization of water and fromphotoacoustic generation. To describe the dynam-ics, let us consider the stratigraphy shown in Figure6 where the encrustation products are completelyimmersed in a slightly thicker water layer.From an optics point of view, the water does notabsorb at the Nd:YAG laser wavelength, while theencrustation is an inhomogeneous absorber with adistribution of absorption centres containing vari-ous 'chromophores'. Among these, carbon blackparticles are very effective, providing a reflectanceof 15% and an absorption coefficient of 8-3 x104cm- [22]. This corresponds to an optical pene-tration depth of 120nm (i.e., the penetration lengthinside the material providing an attenuation of theincident fluence of l/e, with e = 2.73).Laser irradiation produces a temperature rise ofthe surface absorbing centres, driving their rapidexpansion (Figure 6) on a nanosecond time-scale.During the same time the generated heat spreads inwater along a characteristic distance given by thethermal diffusion length [23]:

l,w =2VDw' rL (1)where Dw is the thermal diffusivity of water.Therefore, the water temperatureincreases up to thevaporization limit in a very small depth (lthw127nm). Rapid expansion of the absorbing compo-nents and water vaporization both contribute togenerating a compression wave (arrows in Figure 6)that propagates in a multiphase medium (liquid,solid and gas), inducing structural weakening andlocal microfragmentation of the encrustation. Whenthe pressure transient reaches the water-air interfaceit is reflected back following the amplitude law [24]:

R PaCa PwCw (2)Pac a + Pwcw

where Rac is the acoustic reflection coefficient, Paand pware the densities of air and water, Caand care the acoustic speeds in these two media. SincePwCw > Paca, Rac -1, which means a total reflec-tion at the surface with a phase inversion. Thus,the compression wave becomes a rarefaction wavewhich returns back inside the encrustation, con-tributing to the cavitation expansion induced bywater vaporization [25, 26] but also generatingmore fragmentation. This contribution to the abla-tion dynamics is usually called 'cold spallation' (seefor example [27]). The gilding layer acts as a rigidboundary [24] by increasing the photoacousticpressure then the spallation phenomenon. In fact,the acoustic impedance of gold is much higherthan that of water so the pressure wave is mostly

reflected at the gold surface (Rac 0.95) withoutphase change.The absence of visible vapour jets, the observa-tion of material projected far away from the irradi-ated surface, besides the analysis results of theablated material and the estimate provided byequation (1), suggest that cold spallation plays asignificant role in the present irradiation conditions.When the water thickness exceeds a certain limit,the first phase of the interaction process is similarto that in the previous case, but the increased dis-tance between the water-air interface and theencrustation can result in an independent spallationof water or a strong amplitude attenuation of thepressure wave by diffraction [24]. In such confinedconditions, the encrustation fragmentation can alsooccur by free cavitation bubble collapses [25],whereas fast material removal is inhibited bymechanical impedance of the water layer. This is inagreement with the experimental observations.During the following cleaning tests we used justenough water to achieve a cold spallation mediatedregime; thick water layers were found both ineffec-tive and dangerous, because of possible temperatureand cavitation damages.Heating of the gold filmIn the ablation regime occurring at the beginning ofthe laser cleaning, the encrustation shields the goldfilm to some extent, whereas during the finishingphase several pulses can reach it directly. It is there-fore necessary to evaluate the temperature distribu-tions due to the irradiation of the film.Since the thermal conductivity of gold (K312W.m-lK [28]) is much higher with respect to airor water (K = 0O03W.m-'Kand K = O-6W.m-'K[29]), heat may propagate inside the film during thelaser pulse. The thermal diffusion length, calculatedby replacing Dwwith the gold thermal diffusivity Dgin equation (1), is lthg = 3-7m. Thus, at least forareas showing a continuous gilding (i.e., withoutexposed substrate), heat propagation is confinedwithin the gold film during laser irradiation. Thatallows the use of the semi-infinitive solids approxi-mation where an ideal thermal contact betweengold and water is assumed [23]. The heat suppliedper unit area and unit time at the interface can bewritten as:

f(t) = AOP IL (t)where IL is the laser intensity (laser power per unitarea, usually measured in W.cm-2) and A = 1 -R is the optical absorbance which, basel on thenumbers in Table 3, ranges between 0-2 and 0.3,while f(t) follows the temporal shape of the laser


(3)

275

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S. Siano and R. Salimbeni250

0? 200U)

150

) 300 mJcm20 100 200 300

Time ns ]Figure 7 Thermal transient induced by laser pulse(dashed line) at the gold-water interface. The curvescorrespond to different laser fluences and gildingreflectances. From the bottom to the top. FL =300mJ. cm-2, R 0-8 and R 0 7, FL =500mJ.cm-2, R p 08 and R = 7.op oppulse (Figure 4). The theory provides exact solu-tions for the temperature rise distributions in goldfilm and water [23, 30].The interface temperature transients at the oper-ating fluences are reported in Figure 7. Maximumtemperature increases between 143 and 213?C areexpected at the cleaning threshold, which are wellbelow the critical value of the gilding film. Theexperimental damage threshold (2.3J.cm-2) corre-sponds to peak temperatures of 656-983?C (accord-ing to the different values of Ao). The simultaneousheating distributions inside the water layer are plot-ted in Figure 8. These have to be considered justdescriptive because after about 30ns (in the time--, 2500o , 200

150|- 100Q- 50E o

50 ns"k

scale given in Figures 4 and 7) the surface tempera-ture exceeded 100?C, generating nanoscale vapor-ization fluid dynamics.About the thermal wave propagation inside thegold film, the theory provides, in the worst condi-tion (Aop 0-3), a quick peak temperaturedecrease. By assuming a film thickness of 6itm, onecan expect a maximum temperature at the internalfilm surface not higher than several tens of degreesCelsius (Figure 9). This last evaluation holds whentaking into account also the finite size of the filmand the decrease in thermal conductivity of the cop-per oxide substrate (see Figure 6).In general, besides the single pulse effect, thecumulative heating should be considered wheneverthe time lapse between consecutive pulses is shorterthan that required for the complete thermal relax-ation of the irradiated surface to the ambient tem-perature. In the present case, the problem ofcumulative heating does not rise by optical absorp-tion of the gold layer because of its high thermalconductivity, whereas it is expected to be significantfor exposed copper oxide areas and residual hardencrustation. To give an idea, a pulse frequencyratef = 20Hz on a low thermal conductivity mater-ial could generate overheating of some hundreds ofdegrees [31], that is likely to occur in cases wherethe low porosity of the irradiated material does notallow an effective cooling by water.

Cleaningof large areasThe physics-based analysis discussed above alloweddefinition of a cleaning procedure based on a suit-able water-assisted laser ablation. It was accom-plished through two steps: rough cleaning by a laserfluence FL = 300mJ.cm-2 at a pulse frequency rate- 2500o- 200'

150I.*, 1001.a)E 50E

30 ns\

20 ns

0 0.1 0.2 0.3Distancefrom heinterface[ pm ]

Figure 8 Thermal distributions inside the waterlayer duringlaser irradiation.

0 2 4 6 8 10Distanceromheinterfacepm

Figure 9 Propagationof the thermal wave inside thegoldfilm.Studies in Conservation46 (2001) 269-281

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The Gate of Paradise: physical optimizationof the laser cleaningapproach

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f = 5Hz; finishing cleaning by FL = 500mJ.cm-2 andf = 2-5Hz. A final washing with deionized water forabout 30 minutes was also performed to removepossible encrustation microfragments and solublecompounds trapped in the surface texture, exfolia-tion microsites and exposed substrate.A flat surface (A), a bas-relief figure (B) and afigure including the previous features and a project-ing element (C) were selected to evaluate the qualityand feasibility of the optimized laser treatment.These areas are indicated in Figure 1 by solid linerectangles.The flat surface (6 x 6cm) was cleaned in about30 minutes (Figure 10). Most of the time was spentperforming the finishing cleaning, this phase beingslow and delicate because it was difficult to distin-guish with the naked eye between exposed micro-

,+.g,hi~~~~~~~~~~~~~~~~~~~~~~~~..:....... :: '7 ;i:: w....? ?:? ? .',.. w . ^. "^. S ..:.'

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cFigure 10 Laser cleaning of the flat site A (6 X6cm): (a) before, (b) after, (c) detail (1-5 X 1cm).

Figure 11 Images extractedfrom the magnetic tapeon whichthe videomicroscopyobservation was stored:(a) microscale image (area A), showing the highquality of the laser cleaning without any side-effects;(b) detail including a microblister exhibiting radialfailures and gold loss at the apex, generated by nat-ural deterioration.Studies in Conservation46 (2001) 269-281

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S. Siano and R. Salimbeni

.M- ~ ~ .~Hi~r~iR..I.,.p"

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Figure 12 Detail of site B: before (top) and after(bottom) the laser cleaning test.sites and encrustation residues. For an overall lasertreatment, coupling of the laser beam with a stereo-microscope to perform the finishing cleaning wouldrepresent an improvement.

The observation of the cleaned area through thestereomicroscope showed a finely textured surfacewith scratches and microblisters (Figure lOc), hencethe high quality and selectivity of the laser removal.In some cases, thinning at the apex of the micro-blisters associated with radial failures and/or partialgilding loss by exfoliation were observed. These lastphenomena, already observed on chemically cleanedpanels, are attributable to natural deterioration andnot to cleaning, as was also underlined in [10].Before moving to laser cleaning of the relief fig-ures in the main scene (B, C), further inspection of

Figure 13 Site C after laser cleaning.the area A was performed by high magnificationoptical fibre video microscopy (Figure 11a), toanalyse possible residual encrustation and any dis-turbance of the surface texture. This technique waschosen because it allows easy inspection of sculp-tural elements by flexible optical fibre bundle andstorage of the surface scan on a magnetic tape thatis useful for further observation, comparison, moni-toring, etc. An optimum degree of cleaning wasconfirmed, as well as the absence of melting or tex-turing effects attributable to the laser action. Thisconclusion is also supported by the SEM analysisof microfragments discussed above.As expected, the time spent to clean site B (planeprojection about 2 x 35cm) was relatively longerthan required for site A: about 90 minutes. The fin-ishing cleaning of the chiselled lines, such as forexample those of the tresses and of the details of theface of the figure, required a careful action becauseof the larger holding surface of the hard encrusta-tion residues at the bottom of the groves. The detailof Figure 12 shows the good quality of the cleaning


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The Gate of Paradise:physical optimizationof the laser cleaningapproach

Figure 14 Details of different surface textures ofsite C after laser cleaning.treatment, confirmed by surface analysis performedas described for the previous test site (Figure 1 b).The problem of cleaning the inside of the project-ing elements, for example the outstretched arm ofthe figure in site C (Figure 13), was solved bydesigning a suitable optical manipulator. This wasrealized by arranging a small dielectric mirror(2.5mm in diameter) on a plastic support allowingthe orientation of the output laser beam from theoptical fibre. During the cleaning operation theirradiated area was observed through a reflectingfoil placed on the panel plane. This allowed clean-ing of the inside of the arm and could be employedfor other projecting sculptural elements.The cleaning of site C (plane projection about 10x 35cm) was accomplished in about eight hours.The results were also very good in proximity to thepanel fractures, on exposed substrate and on thevarious textural features of the gilding (Figure 14).

ConclusionsWe investigated a safe laser cleaning methodologyfor the restoration of a number of sculptural ele-ments of the Gate of Paradise. The optimization ofthe treatment, allowing avoidance of any mechani-cal or thermal damage, was achieved through pre-liminary irradiation tests and a detailed physicalinterpretation of the experimental observationsaided by chemical analysis. The cleaning tests per-formed on large areas demonstrated the applicabil-ity of the developed methodology on the differentsurfaces and encrustation features of the sculpturesto be restored. The laser treatment also increasedour understanding of the artwork under restorationbecause of the specific interaction of the laser withthe materials of which it is composed. However, wecannot rule out other peculiarities of the encrusta-tion distribution and composition, calling for addi-tional research to improve the technique and bettercharacterize the state of preservation and the histor-ical issues related to the art object.The main advantages of an optimized lasermethodology are that it makes it possible to cleanthe sculptures without the dangerous mechanicaldismounting from the framework, and that itavoids or at least limits the use of reactive chemicalagents. In collaboration with the team of expertsfrom the OPD we will perform further studiesaimed at comparing the results of the laser method-ology we have presented with those obtained usingan integrated approach combining laser ablationand the use of chemical poultices with lower con-centrations of active agents.

AcknowledgementsThis study was supported by the 'Cultural Heritage'Special Project of the Italian National ResearchCouncil. The authors also wish to thank Dr AnnaMaria Giusti and Dr Mauro Matteini of the OPDfor having entrusted them with the present study,and the group of conservators working on the Gateof Paradise: F. Burrini, S. Agnoletti, A. Brini, L.Nicolai, for their kind and passionate participation.Finally, thanks are extended to M. Miccio of theSoprintendenza Archeologica della Toscana, forvideomicroscopy analysis and helpful advice.

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S. Siano and R. SalimbeniAtti e Notizie, Associazione Italiana diMetallurgia,Milan (1968) 3-79.2 DASSU, G., PIAZZESI,G., and ALESSANDRINI,G., 'Problemi di conservazione delle Porte delBattistero di Firenze' in Conservazione deiMonumenti. Atti del XXIX CongressoNazionale dell'Associazione TermotecnicaItaliana, Italian National Research Council,Milan (1976) 155-165.3 CESAREO,R., and MARABELLI,M., 'AnalisiXRF di antiche porte Italiane in leghe dirame' in Atti del Convegno dei Lincei,Accademia dei Lincei, Rome (1976) 409-420.4 ALESSANDRINI,G., DASSU, G., PEDEFERRI, P.,and RE, G., 'On the conservation of theBaptistery doors in Florence', Studies inConservation24 (1979) 108-124.5 LEONI,M., 'Studio metallografico della Portadel Paradiso di Lorenzo Ghiberti delBattistero di Firenze', La Fonderia Italiana 4(1981) 99-101.6 Lorenzo Ghiberti,materiae ragionamenti,exhibi-tion catalogue, Centro Di, Florence (1978).7 Lorenzo Ghiberti nel suo tempo, Atti delConvegnoInternazionaledi studi, L.S. OlschkiEditore, Florence (1980).8 'Lorenzo Ghiberti, Storie di Giuseppe e diBeniamino, Storie di Adamo ed Eva: basso-rilievi in bronzo della Porta del Paradiso' inMetodo e scienza operativitd e ricerca nelrestauro, ed. U. BALDINI,Sansoni Editore,Florence (1983) 168-206.9 MATTEINI,M., and MOLES,A., 'Kinetic controlof the reactivity of some formulation utilisedfor the cleaning of bronze works of art' inICOM Committee for Conservation 5thTriennalMeeting, Ottawa (1981) 23/4.10 FIORENTINO, ., MARABELLI,M., MATTEINI,M.,and MOLES,A., 'The condition of the Door ofParadise by L. Ghiberti. Tests and proposalsfor cleanings', Studies in Conservation 27(1982) 145-153.11 L'oro del Ghiberti. restauri alla Porta delParadiso, booklet of the exhibition, ed. G.LINARES,dizioni Panini, Modena (1985).12 GIUSTI,A., and MATTEINI,M., private commu-nication (2 May 1999).13 SALIMBENI,R., MAZZINGHI,P., PINI, R., SIANO,S., VANNINI, M., MATTEINI, M., andALDOVRANDI,., 'Laser restoration of paint-ings: issue and perspective' in Proceedings ofthe Congress on Science and Technology forthe Safeguard of Cultural Heritage in theMediterraneanBasin, Vol. 1, Italian NationalResearch Council, Palermo (1998) 811-815.14 SIANO,S., MARGHERI,., MAZZINGHI,., PINI,R., SALIMBENI, ., TocI, G., and VANNINI,

M., 'Laser ablation in artworks restoration:benefits and problems' in Proceedings of theConferenceon Laser '95, STS Press, McLeanVA (1996) 441-444.15 SIANO,S., and PINI, R., 'Analysis of the blastwave induced by QS Nd:YAG laser photo-disruption of absorbing targets', Optics Com-munications135 (1997) 279-284.16 SIANO, S., MARGHERI,F., MAZZINGHI,P., PINI,R., and SALIMBENI,R., 'Cleaning processes ofencrusted marbles by Nd:YAG lasers operat-ing in free-running and Q-switching regimes',Applied Optics36 (1997) 7073-7079.17 Proceedings of Lasers in the Conservation ofArtworks, LACONA III, Journal of CulturalHeritage 1 (Supplement 1) (2000).18 MAZZA, B., PEDEFERRI,P., RE, G., SINIGAGLIA,D., and CIGADA, A., 'Nuovo metodo divalutazione istantanea della velocita dicorrosione atmosferica dei bronzi dorati'in Conservazione dei Monumenti: Atti delXXIX Congresso Nazionale dell'Associ-azione TermotecnicaItaliana, Italian NationalResearch Council (1976) 155-165.19 GARBASSI, ., and MELLO,E., 'Surface spectro-metric studies on patinas of ancient metalobjects', Studies in Conservation 29 (1984)172-180.20 Handbook of Optics, ed. G. DRISCOLL,McGraw-Hill, New York (1978).21 BUDDE, W., Optical Radiation Measurements,Vol. 4, Academic Press, New York (1983).22 ESENALIEV, R.O., KARABUTOV, A.A.,PODYMOVA,N.B., and LETOKHOV,V.S.,'Laser ablation of aqueous solutions with spa-tially homogeneous and heterogeneousabsorption', Applied Physics B 59 (1994)73-81.

23 CARSLAW, H.S., and JAEGER, J.C., Conductionof Heat in Solids, Clarendon Press, Oxford(1978).24 ESENALIEV,R.O., OREAEVSKY,A.A., LETOKHOV,V.S., KARABUTOV, A.A., and MALINSKY,T.V., 'Studies of acoustical and shock wavesin the pulsed laser ablation of biotissue', Laserin Surgeryand Medicine13 (1993) 470-484.25 SIANO, S., PINI, R., SALIMBENI, S., andVANNINI, M., 'Imaging and analysis of photo-mechanical effects induced in water by highpower laser-target interaction', AppliedPhysics B 62 (1996) 503-510.26 YAVAS, O., SCHILLING, A., BISCHOF, J.,BONEBERG, J., and LEIDERER, P., 'Bubblenucleation and pressure generation duringlaser cleaning of surfaces', Applied Physics A64 (1997) 331-339.27 PALTAUF, G., and SCHMIDT-KLOIBER, H.,


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The Gate of Paradise:physical optimizationof the laser cleaning approach'Model study to investigate the contributionof spallation to pulsed laser ablation of tis-sue', Lasers in Surgeryand Medicine 16 (1995)277-287.

28 INCROPERA, F.P., and DEWITT, D.P.,Fundamentalsof Heat and Mass Transfer,3rdedn, John Wiley and Sons, New York (1991).29 Handbook of Chemistry and Physics, 60th edi-tion, ed. R.C. WEAST, CRC Press, BocaRaton (1980).30 HASSAN,A.F., EL-NICKLAWY,M.M., EL-ADAWI,M.K., NASR, E.M., HEMIDA, A.A., and ABDEL-GHAFFAR, O.A., 'Heating effects inducedby a pulsed laser in the semi-infinitetarget inview of the theory of linear systems', Optics &Laser Technology28 (1996) 337-343.31 SIANO,S., FABIANI, ., PINI,R., SALIMBENI,.,GIAMELLO, M., and SABATINI, G.,'Determination of damage thresholds to pre-vent side effects in laser cleaning of pliocenesandstone of Siena' in Proceedings of Lasers inthe Conservationof Artworks, LACONA III,Journalof CulturalHeritage 1 (Supplement 1)(2000) S47-S53.

AuthorsSALVATOREIANO,born 1962, graduated in physicsfrom the University of Florence. As a researcherof the Quantum Electronics Institute of Florence(CNR) since 1994, he was involved in the fieldof the diagnostics and modelling of laserablation. In particular, he participated in thescientific activities of various Italian nationalresearch projects devoted to the developmentof laser technologies and methodologies forartwork restoration. Address: Istituto di ElettronicaQuantistica-CNR, Via Panciatichi 56/30, 50127Firenze, Italy.RENZO SALIMBENI, born 1948, graduated in physicsfrom the University of Florence. He has been thedirector of the Quantum Electronics Institute ofFlorence (CNR) since 1991. His scientific activityhas been dedicated to laser physics and technologyand their application in various fields. In particular,he has been in charge of several research projectsdevoted to the safeguard of cultural heritage.Address: as for Siano.

Resume-On a evalue, defacon theoriqueet pratique, une methodologiede nettoyage au laser pour la restau-ration de la Porte du Paradis, un bronze dore de L. Ghiberti.La caracterisationpreliminairedes materiaux anettoyer a permis d'estimer les regimes thermiquesmis en atuvreet de modeliser le processus d'ablation. Descalculs ayant pour but de controler le chauffage indesirablede la feuille d'or ont egalement ete faits avant lestests de nettoyage menes sur des surfaces importantes. Finalement, les tests de nettoyage ont ete evalues defacon critique par inspection instrumentale. On demontre la faisabilite du nettoyage au laser, soit commeetape preliminaire avant un nettoyage leger t base de produits chimiquespeu concentres et de courte dureed'action, soit comme traitementprincipal.Zusammenfassung-Eine Methode zur Verwendungvon Lasern bei der Restaurierungder Paradiestiir, einemvergoldeten Bronzeobjekt von L. Ghiberti, wurde theoretisch und praktisch beurteilt. Eine Charakterisierungdes mit dem Laser zu reinigendenMaterials erlaubte es, die vorherrschendeTemperaturabzuschdtzen und einModell far die Ableitung der Wdrme zu entwickeln. Berechnungen, die darauf abzielten, ein ungewolltesErhitzen der Goldauflage zu vermeiden, wurden ebenfalls entwickelt, bevor Tests der Laserreinigung aufgrofien Bereichen durchgefiihrt wurden. Abschliefiend wurden die Reinigungsversuchedurch instrumentelleUntersuchungenbewertet. Die Durchfiihrbarkeitder Laserreinigungkonnte nachgewiesen werden und zwarsowohl als alleinige Ma,iname, als auch als erster Schritt bei einer Reinigung mit Chemikalien, um so dieKonzentrationender Behandlungsmittelzu verringernund die Behandlungsdauer u verkirzen.Resumen-Una metodologia para el uso del laser en la restauraciodn e la Puerta del Paradiso, una obramaestra de bronce dorado realizada por Lorenzo Ghiberti,fue evaluada teoretica y experimentalmente.Lacaracterizaci6npreliminarde los materiales que debian ser limpiados por medio del klser hizo posible estimarlos niveles termalesque debian registrarse, asi como configurarel proceso de separaci6n de los dep6sitos. Loscdlculos se llevaron a cabo antes de las pruebas de limpieza con ldser con el fin de controlar los niveles decalor no deseadospara la pelicula de oro. Finalmente, las pruebas de limpieza se evaluaron criticamentepormedio de la inspecci6ninstrumental.La utilidad de la teoria de la limpieza con laser se demostr6, tanto comoun mediopreliminara la suave limpieza quimicapor medio de bajas concentraciones de agentes quimicos y encortas exposiciones, o como tratamientoindividualaislado.

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