What Is the Benefit of Using Raman Spcetrometer to Anaylze Art

Abstract

The street art murals 'The Big Mother' by Gola Hundun, the 'Big Sacral Bird' past Kenor, the 'Oriental Rug" by H101 and "The Economy Subdues You lot" by Zosen, belonging to the Cooperative Popular Houses of Mancasale and Coviolo in Reggio Emilia (Italy), were investigated by the use of various mobile Raman spectrometers coupled to different lasers and past micro-Raman spectroscopy on selected samples. The study was made necessary by the evident fading of many colours, despite the young age of the paintings, realized in 2010. The offset step of the investigation, realized by the on-site campaign, was the identification of the materials, and in item of the dyes. The master chromophores were identified as polycyclic, monoazo- and disazo- organic pigments, with inorganic compounds every bit bismuth vanadate (BiVO₄) together with the all-encompassing presence of rutile (TiO₂). The 2nd footstep was devoted to the study of the degradation mechanism affecting the colourful layers of the murals. Information technology required the use of laboratory micro-spectrometers and was carried out on a reduced set up of samples, selected during the in-situ campaign. This combination of on-site and laboratory Raman spectroscopy allowed the obtaining of the complete identification of the palette used by the different artists in a single solar day of measurements, in a complete non-destructive day. In addition, it was possible to minimize the number of samples required for the report of the degradation process.

Graphical abstract

Introduction

Outdoor artworks of gimmicky public art are usually realized with modern synthetic materials which suffer from lack of long-terminal stability, producing alteration effects and deterioration issues co-ordinate to their chemic composition. Furthermore, the exposure to severe and farthermost atmospheric condition conditions and ecology pollution are crucial problems to be considered in the conservation and protection of these artifacts.

For these reasons, a lot of attention has been recently paid to the investigation of the mechanical country of conservation of contemporary outdoor artworks and their aesthetic alterations [ane,2,3,4,5,half dozen,seven,8,9].

The iii-yr European project "Conservation of Art in Public Spaces" (CAPuS), involving xv partners among universities, academies, companies and research centres, is designed to report artworks of contemporary public art, including murals and outdoor sculptures. The CAPuS project, co-funded by the ERASMUS + Knowledge Alliances 2018–2021 program, aimed at contributing to the dissemination of knowledge in the field of public art conservation. In particular, the CAPuS projection has developed specific conservation guidelines for artworks in public spaces together with an innovative preparation module for higher education institutions likewise as an open east-learning module for professionals [10]. An illustrated multi-lingual glossary providing definitions in street art and conservation, and an open-access digital repository containing subpages of selected artworks and specific details (i.e. materials analysis, condition reports, and general archival documentation) have been also realized and constantly implemented during project lifespan and across [11]. The characterization of artworks materials and their deposition processes is of fundamental importance to the development of conservation treatment protocols. Within the project, the Italian partners focused on works of postal service muralism and street fine art to first study the compositional materials and their characteristic degradation processes and later develop specific conservation methods. The data acquired during in situ campaigns contributed to the formalization of conservation guidelines, as well as to the creation of formative modules for students and professionals.

This work describes the research carried out on murals belonging to the Cooperative Popular Houses of Mancasale and Coviolo in Reggio Emilia (Italy), realized on buildings synthetic during the late-1950s by the Italian-Spanish collective Proyecto Ritual in 2010. The analyses were carried out on the murals "The Large Female parent" past Gola Hundun, "Large Sacral Bird" past Kenor, "Oriental Carpet" past H101 and "The Economic system Subdues You" past Zosen [12, 13]. In these murals the importance of the man-nature human relationship is emphasized, being one of the principal characteristic themes of the undivided workers' houses since their foundation in 1909, which were designed every bit housing focused on a self-subsistence economy [14, fifteen].

Reverse to the relatively good mechanical conservation country of the murals, a clear chromatic deterioration occurred. The south-facing orientation of some of the murals without any protection from the lord's day is disquisitional for the degradation processes induced past the exposure to sunlight and weathering [1, five, 16, 17]. The chief effects consist of total or fractional color fading, a chromatic alteration manifested as the weakening of blush, resulting from chemical reactions or exposure to direct sunlight [11], and other colour shifts, causing the loss of original contrast and therefore the readability of details. The commercial formulations used by the artists were cheap and like shooting fish in a barrel to use paints that allow to pigment quickly and with groovy bear upon on large formats without an important interest in conservation over fourth dimension. These formulations are very complex mixtures of binding media, dyes, extenders, and additives mainly of organic nature at low resistance to degradation. Recurring alterations in some colours were besides observed [xviii,xix,twenty,21]. Some details are clearly visible in Fig. S1 (supplementary textile). Furthermore, nifty with flaking, losses with scissure pattern and efflorescence between the ground layer and the plaster occurred in some of the murals [20, 22].

Due to these chromatic alteration and deterioration effects, an in situ campaign with portable Raman spectrometers has been performed, to characterize the colorants involved in these phenomena. Raman spectroscopy was chosen as a fast, not-subversive technique, suitable for in situ apply [23,24,25,26]. Raman spectra allow to uniquely identify organic and inorganic dyes and inorganic pigments. The utilise of three different instruments, with different excitation wavelengths, is useful to reduce the negative effect of fluorescence on the identification [23]. Moreover, in some cases it allowed united states of america to accept advantage of possible resonance effects, promoting the identification of both the colour palette and the alteration products [nine, 17]. Thus, this study shed light on the deterioration mechanisms in the investigated outdoor artworks. In general, Raman spectroscopy had already proven to be a valuable tool for the investigation of synthetic organic pigments (SOPs) used in modern and contemporary art and on street art murals [nine, 16, 27,28,29,xxx,31,32,33,34].

Numerous strategies accept been employed regarding the identification of synthetic organic pigments. Brostoff et al. in 2009 [35] used micro-Raman spectroscopy, micro-X-ray diffraction (µXRD) and 10-ray diffraction (XRD) to report various forms of constructed organic pigments as powders (measured equally received past the manufacturer), stratified layers, and artists paints, in different media. Although XRD is underlined as a promising complementary technique for the identification of SOPs, the characterization of SOPS in oil paints was considered difficult. Quillen Lomax in 2010 [36] similarly ended that XRD is of little use when SOPs are present in oil or gum. Saverwyns in 2010 [37] employed micro-Raman spectroscopy on paintings attributed to Liubov Popova. The technique was proven a valuable tool specially for the analysis of SOPs. The study demonstrated the chronological misplacement of the paintings and concluded that the paintings were faulty attributed to the artist. Russel et al. in 2011 [38] study the potentials (discussing problematics) of pyrolysis–gas chromatography–mass spectrometry (Py-GC–MS) for analysis of SOPs equally pigments and SOPs in paints and paintings. Defeyt et al. in 2012 [39] used XRD, attenuated total reflectance micro-Fourier transform infrared spectroscopy (μ-FTIR-ATR) and micro-Raman spectroscopy on copper phthalocyanine (CuPc) polymorphs measured as pigments and paints with different binders and in 2013 [40] combined micro-Raman spectroscopy and linear discriminant analysis (LDA) for the differentiation of CuPc polymorphs. Vagnini et al. in 2017 [41] employed a handheld Raman spectrometer coupled to fluorescence reduction engineering (also used in the current research) and mobile, X-ray fluorescence (XRF) and reflection infrared systems, for the overall characterization of art with Raman spectroscopy used successfully for the analysis of SOPs (among others materials). Steger et in 2019 [42] combined portable Raman spectroscopy and lengthened reflectance infrared Fourier transform spectroscopy (DRIFTS) (handheld FTIR) for the molecular and not-invasive identification of SOPs found on glass reverse paintings. Ghelardi et al. in 2015 [three] reported the results of artificial ageing (photochemical) of SOPs by using colorimetric measurements, Py-GC–MS and FTIR-ATR. Next to the colour change, the application of the last ii techniques, suggested alterations in the anile samples and carbonyl groups germination, respectively. Germinario et al. in 2016 [43] when studying the components included in spray paints, stated that generally micro-Raman spectroscopy gave more information regarding the inorganic pigments and SOPs (and extenders/fillers) compared to FTIR and Py-GC–MS. The last two techniques performed meliorate on the binders' characterization, providing some information on the organic/inorganic pigments, extenders and fillers. Combined FTIR (ATR and transmission) and nuclear magnetic resonance (NMR) were employed by Ciccola et al. in 2017 [44] to investigate the role of SOPs in the degradation of the Acrylem AC 33 binder nether UVB influence. 2 years later, the same thematic was investigated by Ciccola et al. [17], past adding micro-Raman spectroscopy to the previous protocol. Sundberg et al. in 2021 [45] performed an in-depth comparative study on the performance of Py-GC–MS, ultra-loftier pressure LC-PDA, UPLC-PDA-HRMS, and micro-Raman spectroscopy for the SOPs identification for fine art samples. In the example of characterization of materials found in street art murals, multiple techniques were employed, including Raman spectroscopy [9, 33].

In the electric current study nosotros present the extensive in situ assay of the street fine art murals together with the micro-Raman results in society, non only to identify the artists' palette but too to understand possible degradation mechanisms that affect the outdoor murals.

Materials and methods

In situ Raman assay was performed on the street murals 'The Big Mother" past Gola Hundun, "Big Sacral Bird" by Kenor, 'Oriental Carpet' by H101, and 'The Economy Subdues You' by Zosen. All the mobile Raman spectrometers were brought simultaneously on the field, and the analyses were carried out directly on the colourful surfaces of the street murals without jeopardising the works of art. An aerial work platform was used to mensurate the large-sized murals 'Large Sacral Bird' and 'The Economy Subdues You lot' (Fig. 1a and b) (up to 15 k) while a ladder with no additional equipment was used for investigating the 'Oriental Carpet' and 'The Large Mother'. During a unmarried day of on-site analysis, 47 different color regions accept been analysed, covering all the dissimilar chromatic hues, plainly stable and unstable, painted both by spray and by roller, of the four wall paintings. Every region was analysed with at to the lowest degree two unlike spectrometers, in virtually cases with three spectrometers, repeating the measurements in dissimilar (usually 3) spots for each expanse. A huge corporeality of spectral data was obtained in a very brusque time, minimizing the affect on the environs and on the inhabitants of the buildings. The analysed points are reported in Fig. S2 (supplementary material), also equally with the hue changes of the different chromatic areas.

Fig. ane

Mobile Raman spectroscopy was applied on the street fine art murals a 'Large Sacral Bird' and b 'The Economy Subdues You'. The scaffold on a truck was used in order to reach the higher parts of the street murals; c The mobile Raman systems involved in the current study. Clockwise appearance: the dispersive i‐Raman ^® EX portable organisation, the Bravo handheld Raman spectrometer, the handheld Raman analyzer RaPort. The i‐Raman ^® EX is connected to a laptop and a portable battery; d The fibre-optics probehead of i‐Raman ^® EX positioned against the region of involvement; e The fixed optical head of Bravo positioned against the mural 'The Big Female parent' (foreground) while the researchers are simultaneously investigated the graffiti 'Sacral Bird' with the RaPort handheld spectrometer (background)

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Some millimeter-sized samples were collected during the measurements past means of a scalpel from the positions indicated in Fig. S2 (supplementary fabric). Iv of them were embedded in epoxy resin and polished. The as-obtained samples and the cantankerous-sections were and so analyzed by a micro-Raman spectrometer.

In situ and direct assay

All the mobile Raman spectrometers used for the on-site analysis of the outdoor murals tin can be visualized in Fig. 1c.

The Bravo handheld Raman spectrometer (Bruker, Ettlingen, Deutschland)

The Bravo handheld Raman system is a dual laser (785 and 853 nm) meaty spectrometer able to acquire spectra in an extended spectral region from 300 to 3200 cm⁻¹ _. The system is coupled to a CCD detector, its spectral resolution is 10–12 cm⁻¹ and the light amplification by stimulated emission of radiation power is fixed (less than 100 mW). This battery-operated spectrometer that weights 1.5 kg uses an automated integrated calibration. Bravo is using the sequentially shifted excitation (patented) technology to reconstruct a spectrum that is free from fluorescence effects.

The Bravo Raman system is actually collecting slightly shifted Raman spectra from slightly unlike laser excitations produced by changing the lasers' electric current. In practice, information technology uses two spectral regions; from 300 to 2200 cm⁻¹ for the 785 nm light amplification by stimulated emission of radiation and from 1200 to 3200 cm⁻¹ for the 853 nm laser, to collect 3 shifted Raman spectra per laser per spectral region and reconstructs a 7th final spectrum gratis of fluorescence. In theory, if the differences in laser excitations are small (slightly different), the broad fluorescence features can remain unchanged while the actual Raman signal is shifted. The user tin separately view all the spectra recorded together with the final one. This is very useful in cases where the in-built algorithm of the instrument produces artefacts other than the actual Raman bands.

All the measurements were conducted in the automatic style bringing the plastic protective caput (used to secure the lens/light amplification by stimulated emission of radiation optics) of the fixed probehead of the spectrometer in contact with the surface nether study. The working distance is defined at effectually 4 to 5 mm while the spot size is smaller than 1 mm (both as reported past the manufacturer).

The dispersive i‐Raman® EX portable arrangement (BWTek, Newark, USA)

The i‐Raman® EX is a fibre optics dispersive Raman spectrometer coupled to a 1064 nm light amplification by stimulated emission of radiation and equipped with a TE‐cooled InGaAs detector. The organization records spectra in the spectral range from 100 to 2500 cm⁻¹, with a spectral resolution of 10 cm⁻¹. The adjustable light amplification by stimulated emission of radiation power can reach up to 499 mW, while the main unit weighs 3.4 kg. The spectrometer is coupled to an external laptop, to permit for setting the experimental weather and collecting the spectra. In general, the experimental weather condition used were prepare at 10 accumulations of half dozen southward with a laser ability reaching 50%. In some cases, the total measuring fourth dimension and laser power were modified (e.k. 20 accumulations of 3 s at 50% or 20% laser power; 10 accumulations of 6 s at 20% light amplification by stimulated emission of radiation power) to avoid detector saturation. The objective lens has a 5.ix mm working distance and a low-cal-blocker with a protective foam was slid over the probehead. The spot size is less than 0.v mm (measured with the lite blocker). No post-calibration of the information was performed but the arrangement was checked on the field and prior to the analysis using sulphur (UCB) and cyclohexane (Kaiser).

The RaPort handheld Raman analyzer (EnSpectr, San José CA, U.s.)

The EnSpectr RaPort Raman spectrometer is equipped with a frequency-doubled Nd:YAG laser with a wavelength of 532 nm. The maximum output power is thirty mW. To avert thermal harm to the organic colorants, the laser power was lowered via software between 20 and 50% of the maximum. The musical instrument is handheld, with a weight of 2.v kg. During the measurement session, the power was supplied past the internal bombardment, allowing wireless performance upward to 6 h. The measurement range is fixed but very wide, from 125 to 4070 cm⁻¹, with a maximum spectral resolution of seven cm^−one. The minimum spot size is 0.5 mm. No scale is required before measurements. Measurement time ranged from 10 to 30 s per spectrum. The instrument is USB-connected to a laptop, where the spectra are visualized and saved in ASCII format for identification and processing.

Laboratory measurements

Micro-Raman spectroscopy

Micro-Raman analysis was performed on all the samples taken during the on-site campaign, without farther grooming, and on cross-sections embedded in resin obtained from three samples. Non-polarized Raman spectra were recorded at 632.viii nm (He–Ne laser) in a nearly backscattering geometry with a Horiba LabRam microspectrometer equipped with an integrated Olympus BX40 microscope. The spectral resolution was about ii cm⁻¹. The power on the sample was kept nether x mW using neutral density filters. A 100X objective was used to collect the Raman point, with a space resolution of one μm. A Raman profile containing xx points with 2 μm of pace was obtained on a selected cantankerous-section.

Software

All data acquisition was performed with software provided past the manufacturers. Mail manipulation of the in situ Raman spectra was performed with OPUS™ software (Bruker) and Thermo Grams/AI viii.0^® suite software (Thermo Fischer Scientific). Horiba LabSpec five was used for the acquisition and processing of the micro-Raman maps.

Results and discussion

In situ Raman spectroscopy: bug and comparison of the instruments

For the purpose of the assay of the street art belonging to the Cooperative Pop Houses of Mancasale and Coviolo in Reggio Emilia (Italia), three mobile Raman spectrometers, coupled to different laser excitations were brought to the field. Although the straight Raman analysis is the optimal approach for characterizing cultural heritage artefacts without jeopardising them, its application is not always straightforward.

The measurements conducted on the murals colourful surfaces were all ended in one day during the day time. Raman spectroscopy is challenged by calorie-free interferences, and thus, the identification of the unknown tin be hampered. In Fig. 2a, it is illustrated that the RaPort Raman system suffered calorie-free interferences, when measuring a coloured surface area (red faded to lite red) from the 'Large Sacral Bird'. The excess fluorescence present in the spectrum, made information technology difficult to place the synthetic organic pigment present on the painting. The i‐Raman ^® EX and Bravo Raman spectrometers were able to characterize the main pigment used equally the diketopyrolo-pyrole (DPP) paint PR254 (Fig. two b, c and Table ane.

Fig. two

Representative Raman spectra of the cherry-red faded to light carmine area of the outdoor mural 'Large Sacral Bird' collected with the a the handheld Raman analyzer RaPort, b dispersive i‐Raman ^® EX portable system (PR254) and c the Bravo handheld Raman spectrometer (PR254)

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Table ane Overview of the street art murals' colours together with their materials identification, for all the mobile Raman systems discussed in the study. The colours fading is indicated (when possible) in the colour cavalcade. The band positions are referring to the Bravo handheld Raman organization and are assigned according to [29,30,31, 33, 52, 53]. Particular assignments with other Raman systems/lasers are underlined in the band positions cavalcade. The band positions in brackets denote unresolved bands. + , positive identification; x, no identification

Total size table

When measuring in situ, ambience light interferences can be avoided past ensuring night conditions between the Raman probehead of the spectrometer and the surface under study. In general, this can be achieved either by measuring at night or in dark weather condition, e.chiliad. measuring in calorie-free-blocking tents, switching off the lights when measuring in museums, etc. In this instance, dark atmospheric condition were created past bringing the probeheads in complete contact with the surfaces. For the i‐Raman ^® EX Raman spectrometers, the long fibre optics probehead can easily accomplish any distant surface and by sliding a light-blocker cup with a protective cream over the probehead (Fig. 1d) the darkened measuring conditions tin can be met. Indeed, in none of the spectra collected with this mobile system from the 3 street art murals, light interferences were observed. In the example of RaPort and Bravo Raman systems, these are coupled to fixed optical heads (Fig. 1e). Although in the Bravo data, no major interferences were observed, in several spectra from RaPort the daylight was reflected in the spectra. Spectra of Bravo seem unaffected to these interferences as these are effectively eliminated by the sequentially shifted excitation algorithm that the spectrometer uses to care for and produce the concluding spectrum.

Regarding the fluorescence observed, in general, this is unavoidable and information technology is a phenomenon inherited in Raman spectroscopy. I tin utilise college light amplification by stimulated emission of radiation wavelengths to try to avoid electronic transitions, hence minimise the fluorescence emission. However, by increasing the wavelength, the decreasing of the Raman scattering is expected. RaPort is equipped with a 532 nm laser, a wavelength that often is ideal for resonance Raman spectroscopy [46]. Several components of the coloured surfaces of the murals were identified by using the 532 nm laser, cheers to the resonance of some pigments. Unfortunately, in some cases, fluorescence was overwhelming the Raman signals. In general, as better evidenced in the following, for the assay of landscape paintings the reward of having a amend efficiency past using a curt-wavelength (e.g. 532 nm) laser, does not compensate for the big disadvantage of inducing also much fluorescence. The 1064 nm laser of the i‐Raman ^® EX worked adequately in near of the cases with the materials characterization, although fluorescence still occurred in some cases. Moreover, in order to reach an acceptable signal-to-noise ratio in a short measurement time, the i‐Raman ^® EX was operated at 50% of its maximum power. The same state of affairs was valid when using the RaPort spectrometer: a compromise between short acquisition time (essential when using a handheld musical instrument without tripods or other supports) and a practiced signal-to-racket ratio, would require the use of elevated light amplification by stimulated emission of radiation power. However, excessive power can crusade alteration to the measured material, which could be observed every bit the Raman signal of the colorant was decreasing when increasing light amplification by stimulated emission of radiation power, evidencing their photo-degradation [47,48,49]. So the careful choice of the laser power when analysing different colours (different light assimilation) was highly important.

To eliminate fluorescence or signals other than the Raman bands, Bravo uses the sequentially shifted excitation technology. For each laser (785 and 853 nm), the instrument acquires iii spectra in slightly unlike excitations in social club to reconstruct two fluorescence-costless spectra, that are finally merged to a single final spectrum. Indeed, Bravo produced the all-time results from the mobile Raman systems used in this study and was able to measure out all of the loftier- and most of the low-intensity bands for the components present on the surfaces. Drawbacks of the organisation are the not-flexible probehead and the stock-still full light amplification by stimulated emission of radiation power used. Moreover, although this handheld Raman spectrometer can be easily operated past a non-skilful, the interpretation of its data is sometimes challenging. One should always cross-bank check the final spectrum with the raw data, to check for possible spectral artefacts: in some measurements, the algorithm produced an erroneous and difficult to interpret concluding spectrum.

From all the aforementioned, the necessity of collecting data with several Raman instruments using unlike lasers and technologies is underlined. Specifically, from the in situ measurements conducted on the outdoor street art murals belonging to the Cooperative Popular Houses of Mancasale and Coviolo in Reggio Emilia (Italy), Bravo instrument outperformed the other two. The spectra were easily collected as the measurements were conducted on an automatic mode and the materials identification was more complete compared to i‐Raman ^® EX and RaPort. The i‐Raman ^® EX amend characterized the components of the murals than the RaPort, with the latter positioned at the lesser place out of the iii.

Although working on unstable scaffolding with directly sunlight were challenging conditions, the different mobile Raman systems were able to characterize the materials establish on the surfaces of: the 'Big Sacral Bird' by Kenor, 'The Big Mother' by Gola Hundun and 'The Economic system Subdues You' by Zosen. In Table 1, the identification of materials collected from different areas on the murals, with the three mobile systems used, is described together with the identified Raman band positions for each component.

In situ identification of the chromophores

In all the analysed areas, a suitable dye or pigment was identified, representing the vast majority of the chromophores present in the paintings. The results are summarized in Table one, with the indication of the instruments able to identify them. In the following, some details of the results obtained on the different paintings are shown.

'Big Sacral Bird' by Kenor

The monoazopigment, acetoacetic arylide PY74 was institute on the discoloured surfaces of the 'Big Sacral Bird'. In Fig. 3a, the spectrum of the PY74 of an orange faded to pink surface area is presented. In this spectrum the bands at 1349 and 1297 cm^−one (indicated in Fig. 3a with an *) are unresolved. Although the low spectral resolution of Bravo might pose several identification problems on bands found in close spectral proximity, as here the bands accept a spectral difference of more than than 10–12 cm^−ane, in this instance, the organisation did not endure this trouble. Thus, other explanations involving a real change in the spectrum should be involved: a small ring shift or broadening can exist attributed to degradation phenomena that could take occurred naturally (weathering) or due to the laser ability. The same unresolved design was noticed also in some spectra recorded with the i‐Raman ^® EX system, although this was non frequently observed, every bit in most cases these bands were resolved. In the spectrum in Fig. 3b, the bands of PY74 seem more resolved compared to the spectrum of Fig. 3a.

Fig. iii

Representative Raman spectra collected from the street mural 'Large Sacral Bird', with the Bravo handheld Raman spectrometer, corresponding to a an orangish faded to pink surface area (PY74 and rutile (R)), b a lemon yellow faded to pale yellow surface area (PY 74 and rutile (R)) c gilded yellow faded to light brownish area (PY83, bismuth vanadate (BV) and rutile (R)) and d a red area (PR112)

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Additional examples of unresolved bands of constructed organic pigments can be also found in several cases with several Raman systems, with some examples demonstrated in Table i, and in Figs. 3d, and 4c, d. Again, this might point out to a deposition visualized in the Raman spectra, but the source of the degradation is not straightforward (weathering, photograph degradation or post-laser degradation). Ciccola et al. in 2019 [17], when studying the role of SOPs in the degradation of Acrylem AC 33 under UVB, observed sure alterations on the Raman signals before and later on exposure. For some SOPs band broadening was observed (while for others not). After averaging spectra nerveless from Raman mappings and by band plumbing equipment, the visualization of changes was possible for the Pb 15:1, PG 7 and PR 264 pigments. The Raman bands before and after the deposition experiments were contrasted on the footing of normalization with the unaltered bands. Pause et al. in 2021 [fifty] evaluated the BRAVO Raman arrangement for the identification of SOPs in varnished paints. Micro-Raman spectroscopy was besides used for comparing reasons. The authors, amidst other aspects regarding Raman spectroscopy in full general and the Bravo handheld instrument in detail (fluorescence, spectral resolution, laser-induced damage) are discussing the shift or merge or broadening of the Raman bands, and they attributed that phenomenon to the elevated laser power of the handheld instrument. Although, shifting of the Raman bands may be attributed to other reasons [30, 39]. Moreover, broadening and merging can occur when bands are overlapping, especially in cases of molecular vibrations found on close wavenumbers.

Fig. four

Representative Raman spectra: collected from the mural 'The Big Female parent' with the Bravo handheld Raman organisation, respective to a blue area (PB15), b a dark violet area (PV23, calcite (C) and rutile (R)); collected from the landscape 'The Economy Subdues You' with c the RaPort handheld Raman spectrometer, corresponding to an orange faded to light yellow expanse (PY74 and rutile (R)) and d with the dispersive i‐Raman^®EX portable system, corresponding to a red faded to majestic surface area (PO34 and rutile (R))

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For the murals discussed in the current study, if the spectral resolution is not responsible for the unresolved bands, broadening, shifting, and merging can be the last product of either natural exposure to the outdoor ecology conditions or light amplification by stimulated emission of radiation degradation. For the Bravo Raman organisation, the light amplification by stimulated emission of radiation power is stock-still (less than 100 mW) with literature reporting that light amplification by stimulated emission of radiation powers of 45 mW [l] and 50 mW [51] are reaching the sample surface. For the other mobile Raman systems likewise elevated lasers powers were used. Shifting of the Raman bands is not only so straightforward equally it can ascend from natural or laser-induced degradation only also from focusing, stability, calibration, sample grade, shape or size of the granules or crystals, etc. then no major conclusions are discussed regarding this attribute in the electric current study.

Co-ordinate to Scherrer et al. [29], PY74 demonstrates strong vibrations at 1593 and at 1352 cm^−one and a very stiff band at 1328 cm⁻¹. In a gold xanthous faded to light brown area (Fig. 3c) the combination of the disazopigment, diarylide PY83 and bismuth vanadate (BiVO_iv) were responsible for the colouring [52, 53]. The phthalocyanine pigment PB15 together with bismuth vanadate were responsible for the shade of one of the measured green colours. The symmetric (Ag) and antisymmetric (Bg) bending modes of bismuth vanadate (BiVO₄) [53] seemed partially unresolved in the electric current study. Although, the aforementioned reasons may play a part for the unresolved and broad features, this might also be indicative of the presence of monoclinic and tetragonal BiVO₄ [52]. Also, monoclinic and tetragonal BiVO₄ differ on their symmetric Five–O stretching fashion (Ag symmetry) with tetragonal sharing a band at 850 cm⁻ⁱ and monoclinc below 830 cm⁻¹ [53,54,55].

Apropos the crimson pigments, PR254 (Fig. 2b and c) and the naphthol AS monoazopigment PR112 (Fig. 3d) were positively identified on different areas of the mural, while a disazopigment (pyrazolone PO34) was found for the orange-red faded to brown and calorie-free royal areas. In a deep blueish area, the polycyclic dioxazine paint PV23 was positively identified together with the possible incorporation of PB15.

'The Big Female parent' past Gola Hundun

PB15 was institute in different blue areas measured on the 'The Large Mother' mural (Fig. 4a). The Bravo Raman arrangement was unable to identify the second pigment present in the mixture. In the data from the blue areas collected from the RaPort spectrometer, a band at ca. 1390 cm⁻¹ is possibly attributed to the presence of PV23. PV23 is the main colour of the nighttime violet area measured (Fig. 4b).

'The Economic system Subdues Yous' by Zosen

PY74 was positively characterized equally an orange faded to light yellow (Fig. 4c) and to white areas, respectively. In Fig. 4d the paint PO34 was plant in a red faded to royal area. Pigment PB15 is responsible for the turquoise hue found on the outdoor mural's surface.

Other components

In most of the areas, measured C-H stretching vibrations were found above 2900 cm⁻¹, while measuring with the Bravo instrument. Its extended spectral region (until 3200 cm⁻¹) and the lasers used are ideal for Raman signals constitute in this spectral area. A band at ca. 1723 cm^−one present in many spectra (sometimes together with bands at ca. 1042 and 1002 cm⁻ⁱ) might exist due to the resin used [56]. Alkyd and modified alkyd solvent-based spray paints have been mainly employed by the artists, i.e. MTN 94 spray cans, as suggested by annal documentations [9, 43].

Calcite (CaCO_iii) is positively identified in the spectrum of Fig. 4b while barites (BaSO_four) can exist a possible interpretation of the band at 985 cm⁻ⁱ (although downshifted and in absence of other bands) from an orange faded to the pinkish area (Fig. 3a).

Rutile (TiO₂) is undoubtedly the protagonist in most of the spectra collected from the 'Large Sacral Bird' and the 'The Big Mother' (simply a few did not contain spectral features of rutile) and in all the spectra collected from 'The Economic system Subdues You'. As mobile Raman spectroscopy is a surface-sensitive technique, by using in situ analysis information technology is not possible to decide whether the TiO₂ originates from the paint mixtures or whether information technology migrated to the surface from an underlying layer. The migration of rutile is a phenomenon that has been demonstrated before on outdoor street murals [9, 57]. Possible answers to the presence of rutile were given with the aid of micro-Raman spectroscopy performed on cross-sections.

Assay of degradation and fading

The four investigated artworks, exposed to farthermost sunlight and weather condition conditions, are characterized by visible chromatic alterations and problems of swell with flaking and losses with the fissure pattern that follows the bricks underneath.

'The Big Mother' by Gola Hundun, the 'Big Sacral Bird' by Kenor, the 'Oriental Carpeting' past H101 and 'The Economic system Subdues Yous' by Zosen are exposed to sunlight and weather. In 'The Economy Subdues Yous', the most exposed of the murals (absence of trees and south orientation), the general fading is more evident than in other artworks. In two artworks (past Kenor and H101), at that place are also visible problems of cracking with flaking and losses with the scissure design that follows the bricks underneath.

In locations where the prime number coating is absent, the damage is slightly more severe, with pronounced cracks, larger flakes, and in the lower part, too subfluorescence between the footing layer and the plaster.

During the condition written report compilation and the photographic documentation, recurring alterations on some colors emerged. For case, an orange color used in iii artworks has turned into greyish purple. Some orange, purple and yellow colours faded partially, while a night pink used in almost every artwork disappeared completely.

Micro-Raman spectroscopy

During the measurement campaign performed with mobile instruments, a few millimetre-size painting samples were taken on positions subject area to the major color changes to be studied with micro-Raman, in order to understand the fading mechanism.

Micro-Raman analyses have been carried out on all the samples, without farther preparation, showing the same pigments detected with the mobile spectrometers. The rutile underlayer was detected in most all samples, allowing to amend understand the stratigraphy of the painting. From 3 samples (1 for each painting, except "The Big Mother") showing the virtually axiomatic fading problems, cross-sections were obtained. In all cases, the point-to-signal micro-Raman analysis showed a similar situation: a rutile underlayer, a painting layer containing one or more dyes and minor rutile, and a very thin outer layer with rutile and a small amount of dye. To better understand this phenomenon, a linear Raman micro-map was carried out on the sample "H101" taken from "oriental Rug", selected equally representative for its clear stratigraphy and its large change in hue.

'Oriental Carpet' by H101

The sample "H101" (from the name of the artist) comes from a light pinkish-purple area of the 'Oriental Carpet' painting. From the photographic record, it is evident that the original hue was an intense orangish-red. The cross-department (Fig. v) reveals the presence of a sparse, white footing layer (less than 10 μm) over the plaster, and so a thicker vivid orange layer, with variable thickness (boilerplate value of ca. 40 μm) and a very thin greyish outer layer (few μm).

Fig. 5

Microscope image of the cross-section obtained on the altered light pink-purple area of 'Oriental Carpet'. From bottom to summit: plaster, white basis layer, thick orange paint layer, thin pink altered layer

Total size image

Although a limited number of in situ measurements were performed with all the mobile Raman instruments, we are able to confirm the presence of PO34 and rutile in the particular area discussed here. In general, rutile was identified in most of the measured areas.

Micro-Raman measurements, performed in different points of the 3 layers, demonstrated the presence of rutile in the ground layer and of PO34 in the orange layer, together with a minor amount of rutile. The outer layer, which is clearly related to the faded color, contains more often than not rutile, with modest concentrations of PO34. To better follow the relative concentrations of the two compounds, a micro-Raman profile was obtained along the cross-department, from plaster to the outer layer (Fig. 6a and b). The Raman profile clearly shows how the corporeality of rutile (marked with 'R'), maximum in the white preparation layer, increases from the orangish layer to the outer grey ane. On the reverse, the signal of PO34 (marked with 'P') decreases significantly from the orange layer to the outer 1.

Fig. 6

a Linear profile of Raman spectra obtained through the paint layer, of the sample collected from a lite pink-purple surface area, from the plaster (depth − 20 μm), through the white basis layer (from − 15 to − 5 μm nearly) and the paint layer (− 5 to 15 μm nearly) to the outer altered layer (from xv to 20 μm). R indicates the primary Raman bands of rutile. All the sharp bands at higher wavenumbers are attributed to the PO34; b False-color bi-dimensional representation of a linear profile of Raman spectra obtained through the pigment layer of the sample (nerveless from a light pink-majestic area) with a microscope picture show of the analysed surface area. In the Raman profile, the lightness is proportional to the Raman signal. R = rutile, C = calcite (of the plaster), P = PO34

Full size image

The fading is related to the relative enrichment of rutile in the outer layer. Two possible mechanisms could explain this phenomenon. The commencement one is the photograph-degradation of PO34 due to the exposition of the sunlight, depleting the outer layer and producing a local higher concentration of rutile. This is the simplest possible caption, but this would mean that the lightfastness of PO34 (and many other pigments showing fading in these paintings) is very poor, less than expected. 1 should take into account the fact that nanocrystalline TiO₂ rutile is an efficient photograph-catalyst, able to induce degradation of organic molecules exposed to sunlight [58,59,60,61]. The absence of whatever PO34 degradation products, which have not been found in the outer layer, can exist supported by the efficient photocatalytic activity of TiO_two. PO34 molecules have been locally degraded by the photo-catalyst into volatile species (H₂O, CO₂) which are no longer on the artwork surface.

The second possible explanation is the migration of rutile from the inner layers to the outer surface. The migration seems to starting time from the inner layer, because the concentration of rutile in the orange layer is very low. The migration could happen through micro-cracks in the paint layers or through diffusive phenomena that need further investigation. Even if this explanation seems more complex, information technology appears to better stand for to the observations. The migration of rutile particles through a pigment layer was already observed by one of the authors during a master thesis work on the degradation of the Klein Blue, presented at different conferences (eastward.m. Sodo et al. Non-destructive and Microanalytical Techniques in Art and Cultural Heritage Enquiry Conf., Lisboa 2007) Further studies on artificially anile synthetic paint layers could help to investigate the machinery at the ground of the migration of rutile and to better quantify the lightfastness of the involved organic dyes. It should be noted that during the final years, the hypothesis of the migration of rutile particles was already invoked in like cases [9, 57].

Conclusions

In situ Raman spectroscopy and micro-Raman spectroscopy were employed to identify the artists' palette and investigate deposition phenomena affecting these colourful works of art. In particular, the 3 mobile Raman spectrometers employed, immune us to place the vast majority of the pigments used and underline the extensive presence of rutile on the murals' surfaces. Micro-Raman spectroscopy performed on selected cantankerous-sections not merely confirmed this merely was also able to locate the rutile in the stratigraphy. Nosotros notice those recent developments in the apply of the Raman spectroscopy, in particular of the SORS (Spatially Offset Raman Spectroscopy) technique, could allow to obtain stratigraphic data on paintings without the demand for sampling [34].

'The Large Mother' by Gola Hundun, the 'Big Sacral Bird' by Kenor, the 'Oriental Carpet' by H101 and 'The Economy Subdues You' by Zosen are exposed to sunlight and weather condition. In 'The Economy Subdues You', the most exposed of the murals (absence of trees and south orientation), the full general fading is more evident than in other artworks. In two artworks (past Kenor and H101), at that place are besides visible problems of nifty with flaking and losses with the crack pattern that follows the bricks underneath.

In locations where the prime coating is absent, the damage is slightly more severe, with pronounced cracks, larger flakes, and in the lower function, besides subfluorescence betwixt the ground layer and the plaster.

During the status report compilation and the photographic documentation, recurring alterations on some colors emerged. For instance, an orange color used in three artworks has turned into greyish imperial. Some orange, majestic and xanthous colours faded partially, while a dark pinkish used in nearly every artwork disappeared completely.

Taking into account the higher damages, in terms of changes of hue and nifty, experienced by the paintings with higher solar exposition and the results obtained on the stratigraphic section, information technology is possible to hypothesize that the alterations present are mostly due to the disappearance of the organic pigments present in the spray products (first microns) of the paint layer, leaving the TiO₂-based extender and therefore returning a gray-violet tone in some cases (due to the underlying orangish color still present) or in many other cases a white background. This provides interesting information on spray products and is of smashing use for their conservation. It can also exist used by artists, to avoid unstable tints or use other types of colors such as quartz dispersion or acrylic-siloxane paints. Moreover, the micro-Raman stratigraphic analysis suggests that a second mechanism, the migration of rutile from the ground layer to the surface, can too play an important role in the fading procedure. In order to sympathize the relevance of this phenomenon, further investigations are required.

Data Availability Statement

This manuscript has associated data in a data repository. [Authors' annotate: Data associated with the present work and other investigations related to the CAPuS project tin can be found at https://www.capusrepository.unito.it/. The datasets generated and analysed during the electric current study are bachelor from the corresponding author on reasonable request.]

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Acknowledgements

This work was supported by the project "Conservation of Art in Public Spaces (CAPuS)", funded past the European Committee, Programme Erasmus Plus—Primal Activity two: Cooperation for innovation and the exchange of practiced practices—Knowledge Alliances (Call EAC/A03/2016), Project Due north° 588082-EPP-A-2017-1-Information technology-EPPKA2-KA. The European Commission's support for the product of this publication does not constitute an endorsement of the contents, which reflect the views only of the authors, and the Commission cannot be held responsible for whatever utilise which may be made of the information contained therein. Anastasia Rousaki greatly acknowledges the Research Foundation-Flemish region (FWO‐Vlaanderen) for her postdoctoral fellowship with projection number: 12X1919N. The authors would also like to give thanks Ylenia Cobelli for her aid during the on-site analysis of the outdoor mural paintings and data estimation, as these were parts of her degree thesis.

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Affiliations

1. Raman Spectroscopy Inquiry Group, Department of Chemistry, Ghent University, Krijgslaan 281 (Due south-12), 9000, Ghent, Belgium

   Anastasia Rousaki & Peter Vandenabeele

2. Archaeometry Enquiry Group, Department of Archæology, Ghent University, Sint-Pietersnieuwstraat 35, 9000, Ghent, Belgium

   Peter Vandenabeele

3. AN.T.A.RES srl, Via Aldo Moro, 24/a, 40068, San Lazzaro di Savena, BO, Italian republic

   Michela Berzioli

4. CESMAR7- Centro per lo Studio dei Materiali Per il Restauro APS, Viale dei Mille 32, 42121, Reggio Emilia, Italy

   Ilaria Saccani

5. ICCOM-CNR, Found of Chemical science of Organometallic Compounds, National Research Council, Via G. Moruzzi, 1, 56124, Pisa, Italian republic

   Laura Fornasini

6. Dipartimento di Scienze Matematiche, Fisiche eastward Informatiche, Università di Parma, Parco Surface area delle Scienze 7/A, 43124, Parma, Italy

   Danilo Bersani

Corresponding writer

Correspondence to Danilo Bersani.

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Rousaki, A., Vandenabeele, P., Berzioli, One thousand. et al. An in-and-out-the-lab Raman spectroscopy study on street fine art murals from Reggio Emilia in Italian republic. Eur. Phys. J. Plus 137, 252 (2022). https://doi.org/10.1140/epjp/s13360-022-02423-1

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• Received: 08 July 2021

• Accepted: 25 January 2022

• Published: 21 Feb 2022

• DOI : https://doi.org/10.1140/epjp/s13360-022-02423-1

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