Study of the effects of low-fluence laser irradiation on wall paintings: Test measurements on fresco model samples

Applied Surface Science 284 (2013) 184–194

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Study of the effects of low-fluence laser irradiation on wall paintings: Test measurements on fresco model samples Valentina Raimondi a,∗ , Costanza Cucci a , Oana Cuzman b , Cristina Fornacelli a , Monica Galeotti c , Ioana Gomoiu d , David Lognoli a , Dan Mohanu d , Lorenzo Palombi a , Marcello Picollo a , Piero Tiano b a

‘Nello Carrara’Applied Physics Institute-National Research Council of Italy (CNR-IFAC), Firenze, Italy Institute for the Conservation and Promotion of Cultural Heritage-National Research Council (CNR-ICVBC), Firenze, Italy c Opificio delle Pietre Dure (OPD), Firenze, Italy d National University of Art, Bucharest, Romania b

a r t i c l e

i n f o

Article history: Received 2 May 2013 Received in revised form 21 June 2013 Accepted 4 July 2013 Available online 22 July 2013 Keywords: Laser irradiation UV effects Pigments Fresco wall paintings Laser induced fluorescence Cultural heritage

a b s t r a c t Laser-induced fluorescence is widely applied in several fields as a diagnostic tool to characterise organic and inorganic materials and could be also exploited for non-invasive remote investigation of wall paintings using the fluorescence lidar technique. The latter relies on the use of a low-fluence pulsed UV laser and a telescope to carry out remote spectroscopy on a given target. A first step to investigate the applicability of this technique is to assess the effects of low-fluence laser radiation on wall paintings. This paper presents a study devoted to investigate the effects of pulsed UV laser radiation on a set of fresco model samples prepared using different pigments. To irradiate the samples we used a tripled-frequency Q-switched Nd:YAG laser (emission wavelength: 355 nm; pulse width: 5 ns). We varied the laser fluence from 0.1 mJ/cm2 to 1 mJ/cm2 and the number of laser pulses from 1 to 500 shots. We characterised the investigated materials using several diagnostic and analytical techniques (colorimetry, optical microscopy, fibre optical reflectance spectroscopy and ATR-FT-IR microscopy) to compare the surface texture and their composition before and after laser irradiation. Results open good prospects for a non-invasive investigation of wall paintings using the fluorescence lidar technique. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The laser-induced fluorescence (LIF) technique is widely used in several scientific fields as a diagnostic tool for the characterisation of a given sample on the basis of its fluorescence properties [1]. The LIF technique has also extensively applied to the diagnostics of artwork materials and relevant deterioration processes [2–8]. It is based on the investigation of typical fluorescence emission bands that occur in the visible region of the electromagnetic spectrum when the sample is excited using laser radiation at a proper wavelength. The same principle is also exploited to carry out LIF spectroscopy on remote targets by means of the fluorescence lidar technique [9,10]. The latter is a remote sensing technique which relies on the use of a pulsed laser source, usually in the UV, a telescope and a suitable detection and acquisition system to carry out fluorescence measurements at a distance from the

∗ Corresponding author at: ‘Nello Carrara’ Institute of Applied Physics-National Research Council of Italy (CNR-IFAC), Via Madonna del Piano, 10, I-50019 Sesto Fiorentino, Firenze, Italy. Tel.: +39 055 5226379; fax: +39 055 5226328. E-mail address: [email protected] (V. Raimondi). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.07.077

investigated target. This technique was initially developed for the remote sensing of the marine environment, specifically for applications concerning the detection and classification of coloured dissolved organic matter, phytoplankton, oils and other pollutants in the water column [9,11–13]. Later on, its application was extended to the agro-forestry field for eco-physiological studies on vegetation by exploiting the typical Chl a fluorescence peak in the red region of the electromagnetic spectrum [14–16]. In the last decade it has also been applied to the diagnostics and documentation of the outdoor stone cultural heritage in the course of several field case studies, providing helpful data for the characterisation of various masonry materials (stones, mortars, consolidants, etc.) and for the detection and classification of biodeteriogens on the basis of their fluorescence features [17–23]. The technique is particularly suitable for on-site operation on monumental surfaces, since it does not require the use of scaffolds and can be used regardless of many external conditions, such as daylight. From this point of view, it can offer several advantages also for non-invasive diagnostics on wall paintings, since this often requires dealing with difficult-toreach large surfaces. Presently, the fluorescence features of fresco wall paintings are often examined with the aid of UV lamps and CCD cameras. These can provide very high spatial resolution images

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which supply useful data to identify varnishes, retouches, but also fluorescent dyes and pigments [24–26]. The evaluation of the fluorescent properties, however, is often obtained by the only visual inspection or, at most, on the use of a limited number of spectral channels [26,27]. Other methods rely on the use of portable fluorimeters for in situ point measurements by means of optical fibres to be put in direct contact with the investigated surface [28,29]. Generally speaking, however, these methods are well suited for movable works of art or to investigate limited portions of a wall painting from a short distance and without the presence of daylight. As a consequence, an alternative technique able to provide remote high-spectral resolution data on areas that are difficult to reach, although with a limited spatial resolution, could be interesting for a quick survey of extended areas and to address further specific analytical investigation. On the other hand, a preliminary step for the application of the fluorescence lidar technique to the non-invasive diagnostics of wall paintings is to address the effects of low-fluence laser radiation on wall paintings. Presently, there is an extensive amount of data in the literature concerning the effects of laser radiation on paintings [30–34]. These studies, however, are focused on laser cleaning applications and consequently explore ranges of laser fluences and wavelengths which are not suitable for methods aimed at the non-invasive diagnostics of the cultural heritage. In the latter case, actually, the laser source used to excite LIF is typically an UV laser using fluences at least two order of magnitude lower than those used for laser cleaning applications. On the other hand, there are several studies concerning the effects of continuous UV exposure for artworks in museums, thus mostly addressing ageing process and dosimetric issues [35–39]. In conclusion, at present there are not systematic data available in the literature on the effects of low-fluence laser radiation on wall paintings. The present study aims at investigating the effects of lowfluence pulsed UV laser radiation, as that typically used in fluorescence lidar applications, on a set of model samples of fresco wall paintings. The samples were prepared with the fresco technique and using three pigments: lime white, yellow ochre and ultramarine blue. A set of diagnostic techniques – colorimetry, optical microscopy, fibre optical reflectance spectroscopy, attenuated total reflectance Fourier-transform infrared microscopy – were used to compare the morphological and chemo-physical properties of the model samples before and after laser irradiation, so as to assess sustainable parameter settings for non-invasive diagnostics by means of the fluorescence lidar technique. 2. Experimental The study essentially consisted of three phases: (i) a set of test areas were delimited on each model sample and then

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characterised by means of several diagnostic techniques; (ii) each test area was exposed to laser radiation using different levels of exposition according to a given protocol, which mainly varied the laser fluence and the number of pulses applied to the surface; (iii) the irradiated test areas were characterised again using the techniques already applied during the first phase of the experiment. Finally, the properties of the samples, measured before and after laser irradiation, were compared to infer possible effects of the exposure to laser radiation. In the following subsections we describe: (1) the model samples, (2) the instrumentation and protocols used to irradiate the samples, and (3) the instrumentation and protocols used to characterise them before and after laser irradiation.

2.1. Model samples The model samples were prepared with the fresco technique. The dimensions of the model samples were 10 cm × 10 cm with a thickness of about 0.6 cm. Three different types of model samples were used for the experiment (Fig. 1). SG model samples – These were prepared with a paint layer of white lime (also called: bianco di San Giovanni [40]). The latter is a natural inorganic pigment which is obtained from lime. Its use dates back to mediaeval times and is still amongst the most used white pigments in fresco wall paintings, reason for which it was selected for this study. GO model samples – These were prepared with a paint layer of yellow ochre, which is a natural pigment, particularly suitable for fresco wall paintings. It comes from natural earths and also contains hydrated iron oxide. It is used since prehistoric times and is resistant to atmospheric and chemical agents. This pigment was selected for the study since it is widely used in fresco wall paintings and also because contains iron oxides that are present in several other pigments used in fresco wall paintings, such as: sienna earths, umber earths, red ochre, etc. [41]. BW model samples – These were prepared with a paint layer of ultramarine blue. The latter is a synthetic pigment featuring an intense colour and used in paintings since the mid 19th century. This pigment was selected for the experiment since it was a synthetic one and widely used in modern and contemporary wall paintings. For each different pigment two samples were prepared so that, in the whole, there were six model samples available for the experiment. On each model sample nine test areas (2-cm diameter each) were delimited so that; in the whole, there were eighteen test areas available for each type of pigment. The main characteristics of the model samples used for the experiment are summarised in Table 1.

Fig. 1. Model samples prepared with the fresco technique and different types of paint layer (a) lime white (SG), (b) yellow ochre (GO), (c) ultramarine blue (BW). Model samples dimensions approximately were 10 cm × 10 cm and thickness was about 0.6 cm.

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Table 1 Main characteristics of the model samples prepared with the fresco technique. Label

Substrate

Pigment type

Pigment composition

Dimensions

SG1 SG2 GO1 GO2 BW1BW2

Plaster (lime mortar and hemp fibres) Plaster (lime mortar and hemp fibres) Plaster (lime mortar and hemp fibres)

Lime white Yellow ochre Ultramarine blue

Calcium carbonate Hydrated iron oxide Polysulphide sodium aluminosilicate

10 cm × 10 cm (9 test areas, 2-cm diameter each) 10 cm × 10 cm (9 test areas, 2-cm diameter each) 10 cm × 10 cm (9 test areas, 2-cm diameter each)

2.2. Instrumentation and protocols for model sample irradiation To irradiate the samples we used a commercial UV pulsed laser since this kind of laser source is typically used in fluorescence lidar systems for remote sensing applications to the cultural heritage [42,43]. The use of UV radiation in fluorescence lidar remote sensing, in fact, is suitable to examine the target in terms of different types of fluorescent materials emitting in the visible range. In the case of applications to the cultural heritage, for example, its use provides information not only about possible biodeteriogens present on the surface of the monument, such as algae and cyanobacteria that emits in the red region of the spectrum, but also about other possible fluorescent compounds that absorb in the UV and emit fluorescence in the visible. For this reason, we generally preferred the use of an UV laser source, although we also did a limited number of tests with different CW laser sources emitting in the visible (see irradiation protocols in Table 2). The UV laser source we used to irradiate the samples was a Qswitched tripled-frequency Nd:YAG laser (Continuum, Minilite II). The laser had output energy of 8 mJ at 355 nm and typical pulse duration of 5 ns. The pulse repetition rate was set to 10 Hz. Beam divergence was 3 mrad with an output beam diameter of 3 mm. To finely adjust the laser fluence impinging on the sample we used a dielectric-layer variable attenuator (Laseroptik, VA-355 nm) placed at the laser output. The experimental set-up was arranged so as to irradiate an area slightly larger than a 2-cm diameter spot on the target. In order to avoid the irradiation of contiguous test areas we used a 2-cm diameter mask to expose only one test area at a time. Each test area of the model sample was irradiated with different laser fluences and/or a different number of laser pulses. Before irradiating the model samples we measured the laser fluence on the target by using an energy detector (Gentec, SOLO-2). Laser fluence values were varied between 0.1 mJ/cm2 and 1 mJ/cm2 , whereas the number of pulses ranged between 1 and 500 pulses (Table 2). The range chosen for laser fluence comprehends typical values used for fluorescence lidar remote sensing applied to the detection of biodeteriogens [23]. In addition, we irradiated one test area for each type of pigment using a laser fluence about two-order of

magnitude higher than in the previous set-up and a very high number of pulses (1000). The aim was to induce visible damage at least in one test area. In this case the area that was irradiated on the sample was smaller (3 mm × 2 mm). Furthermore, two test areas were irradiated, respectively, with two different CW laser sources emitting in the visible: the first one was an He–Ne laser (Hughes 3224H-PC) emitting at 633 nm with an output power of 4 mW and a laser spot with a diameter of 0.8 mm at the output; the second one was a diode laser (Roithner LaserTechnik RLDD532-5-3) emitting at 532 nm with an output power of 5 mW and a laser spot with a diameter of 0.8 mm at the output. Each model sample had also one test area that was not irradiated and was used as a control (in this way there were two control areas for type of pigment). The irradiation protocols applied on each test area are summarised in Table 2. 2.3. Instrumentation and protocols for model sample characterisation To detect possible modifications in the morphological, chromatic and chemo-physical properties of the samples due to the exposition to laser irradiation we used the following diagnostic techniques: • • • •

optical microscopy (OM), colorimetry, fibre optical reflectance spectroscopy (FORS), attenuated total reflectance Fourier-transform infrared (ATR FTIR) microscopy.

2.3.1. OM and colorimetry OM observations of the model samples surface were carried out using an optical microscope (NIKON, Eclipse E600). The surface of the model samples was observed before and after laser irradiation, in reflected light mode, using 1× and 4× objectives. The images were recorded using a Nikon DXM1200F digital CCD camera. Colorimetric values of each area were measured using a Minolta Chroma Meters CR-200, a compact tristimulus colour analyser. The

Table 2 Irradiation protocols for the different test areas. Model sample label

# Area test

Wavelength (nm)

Fluence (mJ/cm2 )

Number of pulses

Irradiated area

SG1/GO1/BW1 SG1/GO1/BW1 SG1/GO1/BW1 SG1/GO1/BW1 SG1/GO1/BW1 SG1/GO1/BW1 SG1/GO1/BW1 SG1/GO1/BW1 SG1/GO1/BW1 SG2/GO2/BW2 SG2/GO2/BW2 SG2/GO2/BW2 SG2/GO2/BW2 SG2/GO2/BW2 SG2/GO2/BW2 SG2/GO2/BW2 SG2/GO2/BW2 SG2/GO2/BW2

1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

355 355 355 355 – 355 355 355 355 355 355 355 633 – 355 532 355 355

0.1 0.4 1 88 0 0.1 1 1 0.4 0.1 0.4 1 0.79 mW/cm2 0 0.1 2.1 mW/cm2 1 0.4

5 5 5 1000 0 125 500 125 125 1 1 1 160 s 0 25 60 s 25 25

2-cm diameter area 2-cm diameter area 2-cm diameter area 3-mm × 2-mm area Control 2-cm diameter area 2-cm diameter area 2-cm diameter area 2-cm diameter area 2-cm diameter area 2-cm diameter area 2-cm diameter area 2-cm diameter area Control 2-cm diameter area 2-cm diameter area 2-cm diameter area 2-cm diameter area

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Table 3 Variations in the surface morphology of the model samples observed using OM and colorimetry before and after laser irradiation. Model sample

SG1 SG2 GO1 GO2 BW1 BW2

Surface changes on test areas (before/after irradiation) 1

2

3

4a

5 (control)

6

7

8

9

¤/× ¤/× ¤/× ¤/× ¤/× ¤/×

¤/¤ — /¤  — ¤ ¤/× ¤/× ¤/× — /¤ ¤

¤/× ¤/× o/× ¤/× o/× ¤/×

a

¤/¤ — /× ¤ o/× ¤/× — /¤  — ¤ — ¤/¤ 

¤/× — /× ¤ ¤/¤ — /¤  — ¤ ¤/¤ —• ¤/ 

¤/× ¤/× ¤/¤ ¤/¤ ¤/× o/×

¤/× ¤/× o/¤ ¤/× o/× —• ¤/¤ 

¤/× ¤/× — /× ¤ ¤/× ¤/× —• ¤/¤ 

¤/× a

¤/¤ a

¤/×

— – detachment; • – colour change E > 1. Legend: o – no surface damage; × – no other damage; ¤ – micro crack;  a Colour change E for the test area SG1/GO1/BW1-#4 are not considered since the irradiated area was very smaller (3 mm × 2 mm) than the area measured by the colorimeter (8-mm diameter area).

sensor head had a diameter of 8 mm so that the measured area on the sample was about 50 mm2 . The illumination-observation geometry was d/0◦ (diffuse illumination/0◦ observation angle). The illumination source was a pulsed xenon lamp. The colorimetric value was calculated as the mean value of three different measurements carried out by rotating the sensor head of 90◦ after each measurement. Three main parameters (L*, a* and b*) were recorded for each measurement according to the colour parameters adopted by the CIEL*a*b* 1976 (CIELAB 1976, Commission Internationale de l’Eclairage) and EN15886 (Conservation of Cultural Property – Test Method – Colour Measurements of Surfaces, 2010), where L* represents the lightness/darkness coordinate, a* corresponds to the red/green coordinate (where

+a* indicates the red and −a* indicates the green), while b* stands for the yellow/blue coordinate (where +b* indicates the yellow and −b* indicates the blue). The total change in colour E* was calculated for each measurement according to formula (1), the colorimetric value being calculated as the mean value of three different measurements carried out in the same spot: E =



2

2

(L∗ ) + (a∗ ) + (b∗ )

2

(1)

2.3.2. FORS FORS measurements were carried out in the UV–vis–NIR spectral range by using two spectroanalysers (Zeiss, MCS 601 UV–vis

Fig. 2. OM observations before and after laser irradiation. (a–d) Test areas SG1-#5 (control) and SG1-#7 (fluence: 1 mJ/cm2 at 355 nm, 500 pulses) before and after laser irradiation. (e–h) Test area GO1-#5 (control) and GO1-#4 (88 mJ/cm2 at 355 nm, 1000 pulses) before and after laser irradiation (i–n) Test area BW2-#5 (control) and BW2-#8 (fluence: 1 mJ/cm2 at 355 nm, 25 pulses) before and after laser irradiation. The circled areas mark some areas in which micro-fissures and paint detachments were observed. The dashed-line circle in (h) points out an area in which we recorded a slight modification of colour after laser irradiation at the highest fluence (88 mJ/cm2 at 355 nm, 1000 pulses). (For interpretation of the references to colour in this figure citation in text, the reader is referred to the web version of this article.)

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Fig. 3. Colorimetric measurements E on the SG, GO and BW samples after irradiation at different fluences. (a) Test areas irradiated using a fluence of 0.1 mJ/cm2 at 355 nm and a number of pulses varied between 1 and 125. (b) Test areas irradiated using a fluence of 0.4 mJ/cm2 at 355 nm and a number of pulses varied between 1 and 125. (c) Test areas irradiated using a fluence of 1 mJ/cm2 at 355 nm and a number of pulses varied between 1 and 500. (d) Test areas irradiated using CW laser radiation in the visible: 2.1 mW/cm2 at 532 nm and 0.79 mW/cm2 at 633 nm for 60 s and 160 s, respectively.

Fig. 4. FORS spectra before (solid line) and after (dashed line) laser irradiation using a fluence of 1 mJ/cm2 at 355 nm and 500 pulses. The spectra were acquired with a 0◦ /45◦ /45◦ illumination-observation geometry. (a) Reflectance spectra on the SG sample. (b) Reflectance spectra on the GO sample. (c) Reflectance spectra on the BW sample.

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and MCS 611 NIR 2.2 WR) mounted on the same chassis and equipped with optical fibres [44,45]. The sensor of the MCS 601 spectroanalyser was a 1024 Si photodiode array with 190–1025 nm sensitivity range and nominal spectral resolution of 0.8 nm/pixel. The other spectroanalyser had a 256 InGaAs photodiode array detector working in the 910–2200 nm range with nominal spectral resolution of approximately 5.0 nm/pixel. The radiation is given by a tungsten–halogen lamp (Zeiss, model CLH600). The probe-head with a 0◦ /45◦ /45◦ geometry was a dark hemisphere, 2.5 cm in diameter, terminating in a flat base. The probe-head had three apertures on the dome for the passing of the light sent to (0◦ ) and the light backscattered from (45◦ ) the investigated surface. The reflectance spectra were recorded on a small area of about 0.3 cm2 , using 99% Spectralon® diffuse reflectance standard. Each measurement was obtained by averaging three acquisitions. In some cases different illumination-observation geometry (8◦ /8◦ geometry) was used. With this geometry the illumination was sent almost perpendicularly to the surface (8◦ ) and the back scattered radiation was collected at the same direction of the illumination, so as to reduce the area effectively measured on the surface to a spot of about 1-mm diameter. This configuration was used to analyse those test areas irradiated with the 88-mJ/cm2 fluence (test areas BW1/SG1/GO1-#4 in Table 2) since they had smaller dimensions (3 mm × 2 mm) than the other irradiated test areas. Three spectra were recorded for each test area by repositioning the probe-head each time on the investigated surface. 2.3.3. ATR FT-IR spectroscopy ATR FT-IR measurements were carried out using a FT-IR spectrophotometer (THERMO Nicolet, Nexus) coupled to an IR microscope (Continuum, TM IR) equipped with a liquid-nitrogencooled MCT detector, and an ATR objective incorporating a Si internal reflection element. The contact area with the sample was circular with an approximate diameter of 100 ␮m. The spectra were collected using the maximum signal available from the ATR objective, i.e. without using any aperture. ATR FT-IR spectra were recorded for each test area of the model samples before and after laser irradiation. Each spectrum was obtained by acquiring 128 scans in the spectral range 4000–650 cm−1 with a spectral resolution of 4 cm−1 . Background spectra were obtained through the ATR element when it was not in contact with the sample. ATR FT-IR spectra were smoothed with the Savitsky–Golay algorithm. For the sake of clarity, in some cases the peak due to CO2 was removed. 3. Results 3.1. Microscopy and colorimetric measurements The results of the comparison of the images acquired before and after laser irradiation are reported in Table 3. Almost all model samples showed the presence of micro-fissures also before irradiation. These phenomena remained mostly unchanged (as in the case of test areas #1 and #3 of all model samples), but in many areas led to new micro-fissures over time (e.g. SG1#4, GO1#6, BW2#2) and/or micro-detachments (e.g. SG2#2, BW2#8, BW2#9). New micro-fissures and micro-detachments arose over time even on some control areas (SG1#5, BW1#5; BW2#5 in Table 3). These micro-damages seem to be generally independent on the exposure to laser radiation. Fig. 2 shows some test areas in which minimal changes were recorded. Some of them, including the control ones, showed a very few areas with flaking off phenomena of the blue paint layer (Fig. 2l

Fig. 5. FORS spectra referring to the test area GO1-#4 (GO sample) before (dashed and dotted lines) and after (solid and dash-dotted lines) laser irradiation using a fluence of 88 mJ/cm2 at 355 nm and 1000 pulses. These spectra were acquired with a 8◦ /8◦ illumination-observation geometry. (a) Reflectance spectra in the 350–2200 nm range. (b) Detail of the 350–950 nm range where the spectral variations are more evident.

and n) or the presence of micro-cracks (Fig. 2a and b and g and h), the latter alteration being already present on the surface before the exposure to laser radiation. Fig. 3 shows some graphs reporting colour variations E calculated from the data acquired on the irradiated test areas. Fig. 3a–c shows the colour variations referring to the test areas irradiated using UV fluences of 0.1 mJ/cm2 , 0.4 mJ/cm2 and 1 mJ/cm2 , respectively, reported as a function of the number of pulses applied. Fig. 3d shows the colour variations E calculated on the test areas irradiated using CW laser radiation. E of the control test areas (labelled as ‘Ref’ in the figure) are also reported in the graph for comparison. The E of the control test areas were calculated as an average of the values obtained on the control test areas of the two model samples available for each type of pigment. Errors on E values were obtained by propagating quadratically the standard deviations of the measurements of the L, a and b parameters. Colour variations for the test areas irradiated using the highest laser fluence at 355 nm (88 mJ/cm2 fluence, 1000 pulses) were not reported since they cannot be considered meaningful because of the small dimensions of the irradiated area (a 3-mm × 2-mm irradiated area against a 8-mm diameter area measured by the colorimeter).

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Fig. 6. ATR FT-IR spectra acquired on the (a) SG, (b) GO and (c) BW model samples after laser irradiation at different fluences. The number of laser pulses applied to the surface was 125 (1000 for the 88 mJ/cm2 fluence). The ATR FT-IR spectra acquired on the control areas are shown as well.

3.2. FORS measurements The FORS measurements did not detect any meaningful changes in the spectroscopic characteristics of the model samples after laser irradiation in the fluence range of 0.1–1 mJ/cm2 and using a number of pulses between 1 and 500. Modifications were not also recorded for the few test areas irradiated with CW laser radiation. As an example, Fig. 4 shows the reflectance spectra of the three samples before and after laser irradiation using a fluence of 1 mJ/cm2 at 355 nm and applying 500 pulses. Also the test areas irradiated with a fluence of 88 mJ/cm2 did not show any significant modification in their reflectance spectra, except for the GO sample where a change in the reflectance spectrum between 550 nm and 650 nm was detected (Fig. 5). This is clear in Fig. 5b which shows a detail of the reflectance spectrum of the irradiated GO sample and compares it with the reflectance measured before irradiation on the same test area. All the measurements on the test areas irradiated at the highest fluence (test areas BW1/SG1/GO1-#4) were acquired using illumination–observation geometry of 0◦ /0◦ in order to reduce the area effectively measured on the surface to a spot of about 1-mm diameter. In this way it was possible to match the actual area irradiated by the laser since this had smaller dimensions (3 mm × 2 mm) than the other test areas irradiated using low fluence (2-cm diameter). 3.3. ATR FT-IR measurements Fig. 6 shows the ATR FT-IR spectra acquired on the SG, GO and BW model samples irradiated using different fluences (0.1 mJ/cm2 , 0.4 mJ/cm2 and 1 mJ/cm2 ) and by applying the same number of laser pulses (125 laser pulses). For comparison, the ATR FT-IR spectra

obtained on the control areas and on the areas irradiated using the highest fluence (88 mJ/cm2 , 1000 pulses) are shown as well for comparison. Fig. 7 shows the FT-IR spectra acquired on the SG, GO and BW model samples irradiated using the same laser fluence (1 mJ/cm2 ) but with a different number of laser pulses (1, 5, 25, 125 and 500 pulses). The ATR FT-IR spectra acquired on the control areas are reported as well. Fig. 8 shows instead an evaluation of the variability of the FTIR measurements due to the measurement conditions, such as: pressure of the crystal on the surface, surface roughness, exact positioning of the measurement head on the investigated area, inhomogeneity of the surface composition at micrometric scale, etc. The spectra refer to a set of measurements acquired on the control test area of the GO1 model sample. The measurements were acquired after removing the measurement head from the surface each time and by repositioning it on the same spot measured before. In order to decrease the variability introduced by the measurement conditions, we considered the ATR FT-IR spectra obtained by averaging the spectra acquired on the test areas irradiated using the same laser fluence (0.1 mJ/cm2 , 0.4 mJ/cm2 and 1 mJ/cm2 ), but by applying a different number of laser pulses. These spectra are shown in Fig. 9 and are compared with the spectra acquired on the control areas (before and after laser irradiation of the other test areas) and the test areas irradiated with the highest laser fluence (88 mJ/cm2 , 1000 pulses). 4. Discussion In general, the OM observation of the test areas before and after laser irradiation (Fig. 2) did not reveal any changes of the

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Fig. 7. ATR FT-IR spectra acquired on the (a) SG, (b) GO and (c) BW model samples after laser irradiation at 1 mJ/cm2 using different number of laser pulses (1, 5, 25, 125 and 500 pulses). The ATR FT-IR spectra acquired on the control areas are shown as well.

surface morphology that could be ascribed to laser irradiation when using UV laser fluences in the 0.1–1 mJ/cm2 range and a number of pulses between 1 and 500. The same behaviour was recorded for the CW-laser irradiated areas. Although the OM observation pointed out the expansion of some small cracks in some test areas (lightcolour circled in Fig. 2g and h), these modifications in the surface morphology could not be ascribed to laser irradiation since their presence did not show any correlation with the fluence values or the number of pulses applied to the relevant test area, as it can be inferred from the data reported in Table 3. In addition, these

Fig. 8. A series of ATR FT-IR spectra acquired on the same spot of the control test area of the GO1 model sample. Each time the ATR tip was removed from the surface and replaced on the same measurement spot.

modifications often occurred also in the control areas of the model samples, as it can be seen by comparing Fig. 2a and b. They can be due to the minor defects in execution technique of frescoes [46,47]. Another modification observed under the microscope was the presence of small detachments of the paint layer, especially on the BW samples (Fig. 2m and n). These detachments were present in several BW test areas (not shown), including the control ones (Fig. 2i and l). Consequently, these modifications of the paint layer morphology were rather attributed to the curing of the sample or its handling during the measurements and transportation to the different laboratories. They pointed out also that the BW paint layer was more fragile and susceptible to damage than the other paint layers. These small detachments of the paint layer have presumably also caused the high E values registered for several BW test areas, as reported in Fig. 3. In particular, the data reported in Table 3 show a correlation between the presence of paint detachment and E values >1, especially on the BW samples since in this case the detachment of the paint layer exposes the underlying white substrate (see, for example, Fig. 2n) and this, of course, deeply affect the colour measurement of the blue paint layer. On the contrary, in the case of the SG samples, which are white, the detachment of the paint layer did not significantly affect the colour measurement of the paint layer. The SG and BW samples did not reveal any meaningful changes under OM observation also in the test areas exposed to the highest laser fluence (88 mJ/cm2 at 355 nm, 1000 pulses; test areas BW1/SG1-#4), as it is clear from the data reported in Table 3. Instead, the GO test area exposed to the highest laser fluence (test area GO1-#4) showed a nearly imperceptible colour change of the paint layer (Fig. 2n; dashed-line circled area, on the bottom right), which could also be noticed by the naked eye. The effect of this colour change is evident in the FORS measurements,

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Fig. 9. The average ATR FT-IR spectra acquired on the (a) SG, (b) GO and (c) BW model samples after laser irradiation for different fluences. For each fluence, the average spectrum were obtained by averaging the measurements acquired after applying a different number of pulses at the same laser fluence. The ATR FT-IR spectra acquired on the control areas (measured before and after laser irradiation on the other test areas) and the spectra acquired after laser irradiation at the maximum fluence (88 mJ/cm2 , 1000 pulses) are shown as well.

which recorded a modification of the spectral behaviour of the reflectance spectrum of the irradiated GO1-#4 area (Fig. 1). The colorimetric measurements as well did not reveal any significant modifications of the samples after UV laser irradiation when using UV fluences in the 0.1–1 mJ/cm2 range and a number of pulses between 1 and 500. The same behaviour was recorded for the CWlaser irradiated areas. In general, colour variations E were less than 1 (Fig. 3) and in many cases of the same order of colorimetric measurement errors. The only exceptions were three test areas (BW2-#6, #8 and #9) of the BW2 sample (see also Table 3), which showed E values comprised between 1 and 2.4, as a consequence of the unveiling of the white preparation beneath the painted layer due to the detachment of small flakes of the blue layer as discussed above. For the other two types of pigments almost all E values were below 0.5, except for the GO sample that showed an increased colour variation increasing the number of laser pulses at a fluence of 1-mJ/cm2 (Fig. 3c). Also in this case, however, the E value was below 1. In general, on the basis of the graphs shown in Fig. 3a–c it can be concluded that there is not any meaningful correlation between an increase in colour change E and an increase in the UV laser fluence (from 0.1 mJ/cm2 to 1 mJ/cm2 ) or in the number of pulses (from 1 to 500) applied to the area. These results are also consistent with the outcomes of the FORS measurements: these did not show any changes in the spectral features of the model samples after laser irradiation at 355 nm varying the fluence in the 0.1–1 mJ/cm2 range and applying a number of pulses between 1 and 500. Similar results were obtained for CW-laser irradiation in the visible (2.1 mW/cm2 at 532 nm and 0.79 mW/cm2 at 633 nm for 60 s and 160 s, respectively). On the

other hand, irradiation using an UV laser fluence of 88 mJ/cm2 , that is a fluence two order of magnitude higher than those previously mentioned, together with a very high number of pulses (1000), could induce a noticeable change in the colour of the GO sample (test area GO1-#4), clearly pointed out by a significant modification of the reflectance spectrum shape (Fig. 5) and also faintly perceived under OM (Fig. 2n). The reflectance spectra relevant to this area (Fig. 5b) are in fact characterised by an overall decrease in the reflectance intensity and a different spectral behaviour in the UV and blue–green spectral range with respect to the spectra acquired on the same area before laser irradiation, as well as to the spectra acquired on the control areas (not irradiated). The intense and broad absorption band at about 380 nm of the mineral goethite, which gives the yellow/light-brown colour in ochre and earth pigments, showed a bathochromic shift towards longer wavelengths (400–420 nm) determining a decrease in the reflectance of the relative reflection maximum and reflection shoulder at 460 nm and 600 nm, respectively. At the same time, the typical absorption band of goethite at about 950 nm was shifted to shorter wavelengths (930 nm). This change in the reflectance spectral shape is consistent with the change of colour occurred to this test area, that is from yellow to a brownish red hue. Such a change can be ascribed to a loss of water molecules in the goethite, ␣-FeO(OH), and to the subsequent formation of hematite, ␣-Fe2 O3 [48,49,50]. The latter is typically found in natural red-brown ochre and earth pigments, such as red and brown ochre and burnt Sienna earth [41]. In general, the FT-IR measurements did not show meaningful changes in the spectral characteristics of the samples after irradiation in the range of laser fluences between 0.1 mJ/cm2 and

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1 mJ/cm2 , regardless of the number of pulses applied (up to 500). From the data shown in Figs. 6 and 7, it can be inferred that there are no substantial differences in the shape of the spectra. The variations of intensity of the bands are likely due to poor reproducibility of the measurement conditions (e.g. pressure of the crystal on the surface, inhomogeneity of the sample surface, etc.) since there is not any consistent trend with increasing the applied fluence (Fig. 6) or, for a given fluence, with an increase in the number of pulses applied (Fig. 7). The fact that the intensity of the spectral bands is dominated by the conditions of measurement is well highlighted in Fig. 8: here the FT-IR spectra acquired in the same area show considerable variations in intensity while the spectral shape does not undergo substantial variations. To better assess any changes in the spectral shape and diminish the influence of the conditions of measurement, averaged spectra were calculated by averaging the measurements carried out in the different test areas irradiated with the same fluence but using a different number of pulses. A comparison of the averaged spectra with the measurements carried out at the maximum fluence (Fig. 9) shows that there are no significant variations of the spectral shape for the SG and BW samples. Also for the case of the GO sample, no meaningful variation in the spectral shape is observed. 5. Conclusions The properties of a set of model samples, prepared using the fresco technique and three pigments (lime white, yellow ochre, ultramarine blue), were studied before and after exposition to lowfluence UV laser radiation. On the basis of the results obtained by using OM, colorimetry, FORS and ATR FT-IR techniques to characterise the test areas before and after irradiation, the examined samples did not show any significant variations of their characteristics and morphology that could be ascribed to laser irradiation using fluences between 0.1 mJ/cm2 and 1 mJ/cm2 at 355 nm and a number of pulses between 1 and 500. The laser employed to irradiate the samples was a tripled-frequency Q-switched Nd:YAG laser. The pulse repetition rate used for the measurements was 10 Hz and pulse width was 5 ns. The investigated range of laser fluences, the excitation wavelength and, in general, the type of laser used for the experiment reflected the characteristics of laser sources typically employed in fluorescence lidar remote sensing for the diagnostics and documentation of the cultural heritage. A further test carried out increasing the laser fluence (88 mJ/cm2 at 355 nm), that is two order of magnitude higher than the investigated range of laser fluences, together with a very high number of laser pulses (1000 pulses), showed that such irradiation was able to induce a detectable colour change only on the yellow ochre (GO) sample, whereas the ultramarine blue (BW) and lime white (SG) samples still remained unchanged. The modification of the colour properties of the GO sample was attributed to a loss of water molecules in the iron oxy-hydroxide lattice, which is abundantly present in the yellow ochre. This interpretation is supported by the measurements carried out using the FORS technique: these showed that the reflectance spectrum of the GO sample, which was characterised before laser irradiation by the typical absorption bands of goethite (yellow ochre), changed its spectral shape showing the beginning of the transition from goethite to hematite, which is the main compound in red–brown ochre and/or earth pigments. Acknowledgements This study was carried out in the frame of the TDT-Bioart project – “Innovative operational protocols in the field of the cultural heritage for the diagnostics and treatment of biodeteriogens” – funded

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by Regione Toscana within the POR FESR 2007–2013 “Attivita’ 1.1 Linee d’intervento D” funding scheme.

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