Journal of Molecular Structure 924–926 (2009) 420–426

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Journal of Molecular Structure

journal homepage: www.elsevier .com/locate /molstruc

Versatile pulsed laser setup for depth profiling analysis of multilayered samplesin the field of cultural heritage

N.F.C. Mendes a,b,*, I. Osticioli a, J. Striova a, A. Sansonetti c, M. Becucci a,b, E. Castellucci a,b

a Dipartimento di Chimica, Università di Firenze, Polo Scientifico, Via della Lastruccia 3, 50019 Sesto Fiorentino, Firenze, Italyb European Laboratory for Non-linear Spectroscopy (LENS), Università di Firenze, Polo Scientifico, Via N. Carrara 1, 50019 Sesto Fiorentino, Firenze, Italyc ICVBC-CNR Sezione di Milano ‘‘G.Bozza”, Via Cozzi 53, Milano, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 October 2008Received in revised form 21 January 2009Accepted 23 January 2009Available online 3 February 2009

Keywords:Pulsed Raman spectroscopyLIBS spectroscopyNd:YAG laser etchingLead whiteMural paintingsRabbit glue

0022-2860/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.molstruc.2009.01.047

* Corresponding author. Address: Dipartimento di CPolo Scientifico, Via della Lastruccia 3, 50019 Sesto F+393897804087.

E-mail address: nuno@lens.unifi.it (N.F.C. Mendes)

The present study considers the use of a nanosecond pulsed laser setup capable of performing laserinduced breakdown spectroscopy (LIBS) and pulsed Raman spectroscopy for the study of multilayeredobjects in the field of cultural heritage. Controlled etching using the 4th harmonic 266 nm emission ofa Nd:YAG laser source with a 8 ns pulse duration was performed on organic films and mineral stratameant to simulate different sequence of layers usually found in art objects such as in easel and muralpaintings. The process of micro ablation coupled with powerful spectroscopic techniques operating withthe same laser source, constitutes an interesting alternative to mechanical sampling especially whendealing with artworks such as ceramics and metal works which are problematic due to their hardnessand brittleness. Another case is that of valuable pieces where sampling is not an option and the materialsto analyse lie behind the surface. The capabilities and limitations of such instrumentation were assessedthrough several tests in order to characterize the trend of the laser ablation on different materials. Mon-itored ablation was performed on commercial sheets of polyethylene terephthalate (PET), a standardmaterial of known thickness and mechanical stability, and rabbit glue, an adhesive often used in worksof art. Measurements were finally carried out on a specimen with a stratigraphy similar to those found inreal mural paintings.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

A common problem when analysing artistic or archaeologicalobjects arises from the fact that most of these pieces are structuredin different layers, either from the manufacturing process or due todifferent degradation phenomena (e.g. formation of oxalates, ni-trates or sulphates) [1] and human interventions in the form ofconservation treatments or other additions (such as retouchingor the application of protective coatings) [2]. Superficial layersare often responsible for concealing or rendering more difficultthe observation of others resting deeper in the bulk of the piece.The heterogeneity of the objects under study can also result inerroneous assessments concerning their composition, manufactur-ing technique or state of conservation. Even supposedly uniformobjects such as marble or bronze statues can prove otherwiseand reveal complex layer composition [3]. This is a particularlyserious matter since some of the most used analytical techniquesfor the analysis of works of art which do not require sampling,

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himica, Università di Firenze,iorentino, Firenze, Italy. Tel.:

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are in their essence superficial techniques; Raman spectroscopy[4,5], X-ray fluorescence (XRF) [6,7], attenuated total reflectioninfrared spectroscopy (ATR) [8], all these lack the capability of pe-netrating deep into the matter. The problem is usually solved bytaking stratigraphic samples and cross-sectioning them so as to ex-pose the different layers to be studied; however, this approach can-not always be used since many pieces can see their integritycompromised, as with ceramics, or present hard surfaces difficultto cut such as with metal works. In other cases, because of the va-lue or frailty of the works of art, sampling is actually forbidden orundesired.

Laser profiling may be considered as a valid alternative whenanalyses must be performed in the innermost layers of the objectwhile causing minimum damage to its structure and having virtu-ally no visual impact on its structure or appearance. Pulsed laserswith controlled fluences allow a slow removal of the materialswith simultaneous spectroscopic monitoring using techniquessuch as laser induced breakdown spectroscopy (LIBS) and time re-solved Raman spectroscopy. [9,10].

LIBS is a spectroscopic technique based on the formation of aplasma by an energetic laser pulse focused on the sample surfaceand the subsequent measurement of the atomic lines and someemissions provenient of molecular fragments as the plasma cools

N.F.C. Mendes et al. / Journal of Molecular Structure 924–926 (2009) 420–426 421

down. These lines can be spectrally and temporally resolved byusing a monochromator and an ICCD respectively. In this process,a small amount of material is ablated (typically less than 1 lg/pulse) [11] so that applying a series of laser pulses in the samespot, the radiation penetrates deeper in the material. This resultsin depth profiling measurements capable of distinguishing layerswith different elemental composition [12,13].

Raman spectroscopy is a well-known technique of molecularanalysis capable of identifying a wide range of organic and inor-ganic materials of use in the field of cultural heritage. [14–16]The fact that it is a non destructive and in some conditions noninvasive technique makes it particularly interesting when dealingwith unique artistic or archaeological artefacts.

A setup using a Nd:YAG pulsed laser capable of performing bothLIBS and Raman spectroscopy requiring only minimum changes toswitch from one to the other technique, has been assembled.

A study of the laser etching process was carried out using poly-ethylene terephthalate (PET) sheets of known thicknesses. PETsheets were chosen because they present a standard homogenousmaterial with good mechanical resistance and a constant thick-ness. Furthermore, being a polymer, they present LIBS spectra sim-ilar to other organic materials that can be found in the context ofcultural heritage. The etching of the sheets was performed withcontemporary acquisition of LIBS spectra which served as a markerfor the completion of the process.

Studies were then carried out on rabbit glue, which is com-monly used as an adhesive or sizing in several techniques of artproduction, such as the preparation of grounds in easel paintingsfor conservation procedures and in some restoration techniques.Moreover, the use of organic substances such as rabbit glue is re-quired in the process of detaching mural paintings from the wall[17], some cases have been reported where historical mural paint-ings were found with residual layers of up to 3 mm of rabbit glue[18]. That is why it may be of an utmost interest to provide analyt-ical tools that can estimate the thickness of organic coatings and beable to reach the inner layers so as to probe the local stratigraph-ical composition. To this purpose, samples simulating a muralpainting were prepared with a layer of rabbit glue applied to itssurface. Depth profiling was performed in this sample while con-ducting LIBS and Raman analysis for the characterization of the dif-ferent layers. The Laser etching and LIBS measurements werecarried out with 266 nm laser radiation since this proved to beeffective for both materials. Raman measurements where per-formed using the 532 nm laser line in order to avoid major fluores-cence contributions in the spectra. Remains of rabbit glue after theetching have made impossible the acquisition of Raman spectrausing the pulsed laser setup, this was due to the large diameterof the beam and the rise of fluorescence bands when 532 nm radi-ation is used. Using a micro-Raman system with a 785 nm diode la-ser these problems were solved and Raman acquisition possible.

Fig. 1. Pulsed laser setup used in the experiments.

2. Experimental

2.1. Pulsed laser setup

A compact (84 � 94 � 32 cm) nanosecond Q-switched Nd:YAGlaser (model CFR 200-GRM, Big Sky Lasers, 8 ns, 20 Hz, 0.005–115 mJ/pulse for the 532 nm emission) has been used as an irradi-ation source. The laser has a top-hat profile with a spot of constantenergy density. The emitted light from the sample is collectedthrough the same lens used to focus the laser beam, and subse-quently is focused into a 0.30 m imaging spectrograph (modelSP-2358i, Acton Research) equipped with three interchangeablediffraction gratings with 150, 300 and 1200 grooves/mm. The ICCDis a 1024 � 256 imaging pixels detector (model P.I. Max-1024/RB-

PTG, Princeton Instruments) with a minimum gate speed of 1 ns.The instrument allows time-resolved acquisition which has beenused mainly to discriminate Raman signal from background radia-tion or luminescence contributions (of particular interest for mea-surements done in outdoor environments), and to acquire LIBSspectra with a suitable delay and gate time; in the present workLIBS measurements were done using delay times between 800and 900 ns and a gate width of 900 ns.

For Raman spectroscopy analysis, in order to minimize interfer-ence in the Raman spectra from fluorescence, the green secondharmonic emission at 532 nm of the Nd:YAG laser was used. Thelight back-scattered from the sample is filtered by a notch filterthat is placed before the focusing lens in front of the slit of thespectrograph.

For the etching tests a UV emission at 266 nm, obtained by fre-quency doubling the 532 nm radiation of the Nd:Yag laser in a KDPcrystal placed after the aperture was used. The focusing lens has afocal length of 25 mm and a diameter of 12.7 mm. The focusedbeam on the sample forms an elliptical beam spot of30 lm � 19 lm. A general scheme of the instrumentation is shownin Fig. 1.

2.2. Micro-Raman

For Micro-Raman experiments a Renishaw RM 2000 single grat-ing (1200 grooves/mm) spectrometer coupled to an Argon ion lasersource (514.5 nm) and a diode laser (785 nm) equipped with a 50�objective was used for analysis. The spectral resolution wasapproximately 4 cm�1. Acquisition times were on the order of10 min, depending on signal intensity, and the power at the surfaceof the sample was in the order of 2 mW.

2.3. Samples

PET sheets, supplied by Toray Industries Inc, with 12, 16, 25 and50 lm thicknesses were used for the etch rate and depth profiling.

Rabbit glue was left swelling overnight in distilled water,heated afterwards on a steam bath not exceeding 40 �C resultingin a 20 (wt%) aqueous solution. This solution was applied with apipette on a quartz disk and dried slowly under the laboratory con-ditions so as to produce a layer as homogeneous as possible.

A specimen simulating a mural painting was prepared using aporous brick as a support. A mortar (arriccio) composed of slakedlime and sand (0.25–0.4 mm fraction) in a 1:2 volume ratio was ap-plied over the support as a 1 cm thick layer. The samples were keptin contact within humid environment and complete carbonatation

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was checked by XRD analysis. After 1 month of curing, a secondlayer of the mortar (intonaco) with finer sand granulometry(0.16–0.25 mm fraction) was applied as a 0.5 cm thick layer overthe arriccio following the same curing procedure. The pictoriallayer, composed of rabbit glue water solution (11 wt%) combinedwith white lead pigment (Riedel de Haen) in a 1:2 ratio(binder:pigment), was applied by brush on the completely curedsupport damped with diluted water solution (5 wt%) of rabbit glue.After 3 months, the sample was treated with the rabbit glue watersolution (20 wt%) to produce a thick layer (ca. 400 lm).

3. Results and discussion

3.1. PET etching tests

In order to test the experimental setup PET sheets were used forthe etch rate and depth studies. The PET sheets were placed in abottom-free holder and positioned in the beam path at the focalpoint. The 266 nm laser radiation used was suitable as polyethyl-ene terephtalate has a strong absorption peak between 225 and275 nm due to the benzoate structure [19]. The laser fluence usedin the etching process was chosen to be as low as possible, so as toallow a low etch rate and at the same time high enough to providea clear LIBS signal throughout the experiment. The fluence valueused was of 5.0 J/cm2 corresponding to a pulse energy of 23.4 lJ.The number of pulses required to etch each PET film of a specificthickness was determined by monitoring the intensity of the CNband emission at 388 nm and the completion of the process waschecked by optical microscopy. The CN spectral feature is relatedto the molecular emission of the CN (B2R+–X2R+) violet systemthat appears during ablation of carbon-containing compounds innitrogen rich environment [20]. Its formation was proposed by Viv-ien et al [21] to take place in the periphery of the C2-containingplume through the reaction with N2 from the environment. Thisemission has been proved useful in ablation monitoring of variousorganic substances by Pouli [22], Gaspard [23], and others. Duringthe PET measurements it was observed a considerable intensity de-crease of this peak corresponding to the complete etching of thefilm. Fig. 2 shows a series of 100 LIBS spectra acquired on a50 lm-thick PET film. Molecular emissions of the CN group at359, 388 and 421 nm are observable until the 60th spectrum.

The fact that the CN band does not disappear completely as thefilm is completely etched may be due to the contribution of theedges of the hole. i.e., some plasma may be still created on the con-tact of the laser beam with the edges of the excavated hole. The

Fig. 2. Series of LIBS spectra acquired on a 50 lm-thick PET film.

number of pulses required to etch the films were counted on thebasis of the CN band decrease for four different film thicknessesand the craters were checked with the help of an optical micro-scope. The results of 10 measurements for each sheet thickness, to-gether with the corresponding standard deviations, are plotted inFig. 3, all the measurements were performed with a laser fluenceof 5.0 J/cm2 (23.4 lJ/pulse).

The linear dependence of the number of required pulses on thesheet thickness suggested a constant etch rate calculated as0.8 lm/pulse, obtained by dividing the number of pulses by thesheet thickness. The results showed that with this fluence valuethe CN band of the LIBS spectra could be used to estimate the abla-tion depth and could also serve as a marker of the organic andunderlying interface.

3.2. Rabbit glue measurements

The measurements on the PET films proved useful in testing theinstrumental setup. Another etching experiment was performedusing a rabbit glue film as a target. The suitability of the 266 nmlaser line for depth profiling studies of collagen containing materi-als was demonstrated by Gaspard et al [23]; the animal glues areprevalently composed of proteinaceous substances (in particularcollagen) and as a consequence, their etching may be favouredby the presence of aromatic amino acids that cause the absorptionof light in the 250–350 nm range. The difficulty of preparing a uni-form layer of rabbit glue with a constant thickness required an ap-proach different than the complete etching carried out on the PETfilms. Several sets of laser shots, with a five shot increment andusing a fluence of 84.2 J/cm2 were fired into the rabbit glue filmand the depth of each of these craters was measured using an opti-cal microscope. All the shots provided a similar LIBS spectrum, ascan be seen in Fig. 4, containing the same CN band as the one foundin the PET films. Additionally, other bands at 316, 318, 393, 397,423 and 431 nm ascribed to Ca and at 280 and 285 nm ascribedto Mg can be seen. Calcium is commonly found in rabbit gluemainly because of alkaline treatment of the animal tissues andbones from which is extracted [24]. The peak at 266 nm is probablydue to laser reflections from the sample surface towards themonochromator.

The plot of the crater depth with five pulse increments observedin the glue film is presented in Fig. 5.

Fig. 3. Etch depth measurements carried out on the standard PET sheet samples of12, 16, 25, 50 lm thicknesses for a fluence of 5.0 J/cm2 (23.4 lJ/pulse).

Fig. 4. LIBS spectrum of rabbit glue. Molecular emissions of the CN group andatomic emission bands of Ca and Mg are clearly observable in the spectrum.

Fig. 5. Etch depth in the rabbit glue film according to the number of pulses.

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In comparison with the PET experiments, the etch depth curvefor the animal glue film exhibited a less linear character. Thismay be caused by the natural inhomogeneity of the animal glueand the presence of impurities within the film. An etch rate of8.2 lm/pulse was achieved for the first five pulses. This etch ratewas obtained by dividing the depth of the crater by five appliedpulses. In the interval of 10–80 applied pulses, the etch rate, de-rived from the linear fit curve of this interval using regression anal-ysis, decreased to yield a mean of 3.0 lm/pulse.

3.3. Mural painting simulation analysis

After the PET and rabbit glue calibration measurements, atten-tion was drawn to the mural painting simulation, particularly tothe possibility of achieving an effective depth profiling with con-temporary characterization of the different layers through theuse of LIBS and pulsed Raman spectroscopy. At higher fluences,the laser ablation process can be spectrally monitored observingthe signal intensity changes of the plasma atomic and molecularemissions during the application of a series of laser pulses on thesame spot. By studying the entire set of the LIBS spectra acquiredfrom each laser shot, it is possible to achieve a 3D-image of thevariations in the elemental composition of the sample from thesurface up to its inner part. Furthermore, the capability of giving

information about the chemical composition of the material ana-lysed together with the knowledge of the laser etch-rate ratio,could allow both to perform controlled depth-profiles and to havean immediate response regarding the structure and the thicknessof the layers.

In Fig. 6(a), a series of 150 LIBS spectra acquired on the samespot of the sample are shown. The most intense atomic emissionsvisible in the spectra are ascribed to the presence of calcium, lessintense but still observable are the bands of the CN of the gluearound 386–88 nm. The weak lines inside the circle correspondto the lead atomic emissions of the pigment. A magnified view(Fig. 6(b)) of the last 50 spectra of Fig. 6(a) shows the differenttrend of the Pb lines intensities respecting the bands of the CN ofthe glue. In particular, it is noteworthy to observe the intensitydecreasing of the CN in combination with a considerable rising ofthe atomic lines of Pb. This behaviour could be explained by a tran-sition from the glue layer to the pigment layer. For a better visualclarity, Fig. 7 reports both the intensity of the bands of the CN at386–88 nm and the line of Pb at 406 nm versus the number ofpulses.

For the first 115 pulses, the emission intensity of the CN ishigher than the emission of Pb; around the 120th pulse, a steepdecreasing of the CN intensity corresponds to a considerable ris-ing of the intensity of Pb. Both the emissions become almostcomparable after the 120th pulse and show a completely rever-sal trend after the 125th pulse when the glue layer has been re-moved and the pigment layer is reached. Fig. 8 summarizes theprocess by showing in detail four significant LIBS spectra of theseries.

Both the spectra 8a and 8b are dominated by intense atomicemissions of Ca (at 316, 318, 393, 397, 423 and 431 nm) of Mg(at 280 and 285 nm), CI (at 248 nm) and C II (at 388 nm). Themolecular emission of CN gave rise to a weak signal at 359 nm(B2R+–X2R+, Dm = +1) and a broad intense signal with the maxi-mum at 388 nm (B2R+–X2R+, Dm = +0). [25] A very weak atomicband of Pb at 406 nm is shown in both the spectra. In the spectrum8a, the peak at 266 nm is probably due to the fourth-order diffrac-tion of spontaneous fluorescence from the laser crystal.

In the spectrum 8c, the intensity of the atomic emission bandsof Pb (at 261, 266, 280, 283.3, 287, 357, 364, 368, 374 and 406 nm)are clearly observable. Furthermore, the intensity of the peak at406 nm is higher than the CN group, less intense but still clearlyvisible in the spectrum. In the spectrum 8d, the intensity of thesame emission line at 406 nm is even higher than the emissionsof Ca; whereas the CN peak is very weak. The energy used to ac-quire the spectra was of 176 lJ/pulse corresponding to a fluenceof 84.2 J/cm2.

The fluence value used for the experiment was again chosen tobe as low as possible to allow a low etch rate, but high enough toprovide a good LIBS signal to noise ratio during the measurements.In this condition, the removal of the uppermost layers is indeedmore controlled and less traumatic for the surface.

The same fluence of 84.2 J/cm2 was used on the sample pre-pared in the laboratory in order to remove the rabbit glue layerand reveal the lead white pigment layer underneath. A small areaof the sample (5 � 5 mm) was subjected to a progressive series ofUV laser pulses: the number of applied shots was decided on thebase of the appearance in the LIBS spectra of the atomic emissionlines of Pb due to the presence of lead white in the pigment layerunderneath the glue coating. At lower fluences than 84.2 J/cm2 theacquisition of spectra becomes more problematic and noisier be-cause of the lower intensity of the elements atomic emission lines.

Immediately after cleaning, the sample area was analysedthrough Raman spectroscopy since the laboratory instrumentationis able to perform both Raman and LIBS measurements using a sin-

Fig. 6. (a) Series of 150 LIBS spectra acquired on the same spot. The more intense atomic and molecular emissions are ascribed to the rabbit glue. The weak bands, inside thecircle, correspond to the lead atomic emissions of the lead-white pigment. A magnified view (b) of the last 50 spectra of figure (a) shows the different trend of the intensity ofPb lines respect to that ones of the CN group of the glue.

Fig. 7. Intensity profile curves (CN at 386–388 nm; Pb at 406) from a series of 150emission spectra obtained by irradiating a rabbit glue coating cast on a quartz disc.

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gle setup. Fig. 9 shows four Raman spectra acquired on differentlayers of the sample.

Spectrum 9a was acquired focusing the laser at the surface ofthe glue layer; the broad bands observable in the spectrum aredue to organic fluorescence. The Raman spectrum acquired onthe area after cleaning (9b) shows the peaks of lead white at1050 cm�1 (symmetric stretching of CO2�

3 ion) together with twopeaks of Calcite at 280 and 1086 cm�1 that correspond to the lat-tice mode libration and the symmetric stretching of CO2�

3 ion,respectively. The presence of calcite is due to the contribution ofthe preparation layer underneath. Spectrum 9c shows a more in-tense peak of lead white at 1050 cm�1, this spectrum was acquiredon the area not covered by the glue coating. Spectrum 9d was ac-quired on the preparation layer and shows again the two intensebands of calcite and a band at 464 cm�1 due to the bending vibra-tional mode (Si–O) of quartz.

Raman spectra were acquired using the second harmonic emis-sion at 532 nm in order to avoid fluorescence emissions, whichtend to be very intense in the UV region. An energy of 6.4 lJ/pulsethat corresponds to a fluence value of 1.4 J/cm2 was used for theacquisition of Raman spectra.

The Raman signal was recorded using a temporal gate of 10 nsand a delay of 5 ns so that the contributions from the intense lumi-nescent background caused by the emissions of the neon lights ofthe laboratory were completely avoided. In the future, this instru-mental feature could be employed for performing multi-analyticalmeasurements (Raman-LIBS) in situ, for instance, in museums,archaeological sites, and churches.

Despite the controlled ablation process used for the removal ofthe glue coating, it was not possible to avoid partial blackening ofthe lead white pigment when irradiating with the laser. Samplescontaining lead white often alter in colour as a result of photother-mal effects experienced during laser ablation and the causes of thediscoloration have been studied by many researchers [25–27].

The capability of LIBS to monitor the elemental composition as afunction of depth can easily provide important information aboutthe inner part of an object without requiring sampling and thepreparation of cross-sections. To this respect, the capability of per-forming Raman analysis inside the crater formed during the abla-tion process could be very important in order to gather in situinformation on the elemental and molecular levels so as to charac-terize completely the chemical composition of the different layersin a very small area of the analysed object. Unfortunately, the pres-ence of even small amount of glue at the bottom or at the edges ofthe crater gives rise to very intense and broad fluorescence bandswhen irradiated at 532 nm, covering the weaker Raman bandsemissions. Choosing a proper laser wavelength (closer to the infra-red region in order to eliminate fluorescence emissions) and com-bining it with the use of a microscope objective lens for focusingthe beam, it may be possible to get Raman signal from the bottomof a crater following ablation.

Fig. 10 shows two Raman spectra acquired at the bottom of aLIBS crater after ablation. Both the spectra were acquired by using

Fig. 9. Four Raman spectra acquired on different layers of the sample: (a) on the surface of the glue layer, (b) on the area after cleaning, (c) on the area not covered by the gluecoating, (d) on the preparation layer (+, quartzs Raman bands;�, calcite Raman bands; �, lead white Raman bands).

Fig. 8. Four LIBS spectra from a series of 150 laser shots: (a) 1st pulse, (b) 110th pulse, (c) 130th pulse, (d) 150th.

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Fig. 10. Raman spectra acquired at the bottom of a LIBS crater after ablation: (a)using the green light emission of a CW argon ion laser (514.5 nm); (b) using the redemission of a diode laser at 785 nm. In both the spectra a 50�microscope objectivelens has been used.

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a commercial micro-Raman Renishaw 2000 apparatus. Spectrum10a was acquired by using the green light emission of a CW argonion laser (514.5 nm), in this case it was impossible to observe anyRaman band because of the very intense fluorescence coming fromthe glue. On the other hand, by using the red emission of a diodelaser at 785 nm, the lower intensity of the fluorescence backgroundallows the appreciation of the Raman band of lead white at1050 cm�1in the spectrum 9b. The power used for micro-Ramanmeasurements was on the order of 2 mW for the green and red la-ser emissions.

4. Conclusion

The pulsed laser setup used for the different measurements hasproved itself to be a versatile instrument capable of performingelemental and molecular characterization of materials used inthe field of cultural heritage. Depth profiling capabilities have beenexplored and the results obtained show the possibility of studyingdeep underlying layers without the need of manual sampling whileguaranteeing minimum damage to the work of art.

The analysis of PET films was performed using the minimal flu-ence value (5 J/cm2) in order to render ablation less traumatic aspossible and, at the same time, to monitor the process by meansof the LIBS emissions of the elements. A linear etching rate of0.8 lm/pulse has been calculated. The study of the rabbit glue filmcast on a quartz disk showed a different trend respect to PET. In or-der to acquire well-resolved LIBS spectra a higher fluence value(84 J/cm2) was required. At this fluence, an etching rate of2.7 lm/pulse was calculated. The same value was reached for thesample that simulates a mural painting: a glue coating with athickness on the order of 400 lm was removed by applying 150 la-ser shots at a fluence of 84 J/cm2 (the same used for the rabbit gluefilm test).The reproducibility of this result highlights the possibil-ity of using this laser for controlled laser ablation applications.

Furthermore, the capability of the device for a multi-analyticalapproach is proven: Raman analysis, performed in the area aftercleaning, provides the identification of the chemical composition

of the pigmented layer at a molecular level. Unfortunately, as inthe case of the presence of lead white pigment, it was not possibleto avoid partial darkening of this pigment when irradiated by thelaser emission. This darkening is due to the ablation process ratherthan the Raman analysis.

By using a green laser wavelength, the high fluorescence emis-sion coming from the surface and the borders of the crater pro-duced after ablation, hinders the acquisition of Raman spectra atthe bottom of the hole. However, by choosing a proper wavelength(closer to the IR region) using a micro-Raman instrument, a clearRaman spectrum of lead white has been acquired.

A particularly interesting application of this instrument may bemonitoring of cleaning procedures, removing both organic andinorganic patinas while characterizing their composition. Other fu-ture developments of the instrument may include a transportableversion optimized for in situ measurements and the developmentof its optics so as to perform analysis at stand-off distances.

Acknowledgments

We would like to thank ‘‘Ente Cassa di Risparmio di Firenze” forthe financial support and ‘‘TORAY” company for providing the PETfilms used in the analysis.

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