Contemporary murals: Chemical characterisation of artists’ spray-paints

doi:10.21203/rs.3.rs-1519449/v1

This study outlines the chemical characterisation of various commercial spray-paints in different colours as used in contemporary public murals, street art, and graffiti. The analyses were focused on the identification of the binding media, pigments, and additives. The aim of the study was twofold; to establish a protocol for the diagnosis of aerosol paints and to provide useful information, which could help conservators remove overpaintings from both commissioned murals and any other form of cultural heritage. A multi-technique approach was developed using Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) for the identification of the main binder, pigments, and extenders, while Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDS) and Raman spectroscopy were used as complementary tools for the determination of organic and inorganic pigments, and fillers. Three kinds of binders based on alkyd or acrylic resins modified with nitrocellulose and/or styrene, along with a variety of organic pigments and inorganic components were detected.

contemporary murals

art conservation

chemical characterisation

spray-paints

binders

pigments

Outdoor murals have a long history since the beginning of the 20^th century when Mexican muralism appeared, followed by the Chicano mural movement in the late 1960s [1]. Since then, creating paintings on walls (or any other surface) has gone through various stages, permissioned or not [2]. Recently, the great impact of street art re-sparked the interest in this form of expression, giving the opportunity to artists to produce sanctioned large-scale murals [3].

Despite their broad public acceptance (social value), as it can be confirmed by the continuously growing touristic interest [4]-[5], the musealisation (artistic value) that followed [6], and even the trend in detaching and commercialising street artworks (financial value) [4], [7] conservation of commissioned contemporary murals is not so widespread yet. This is due to their ephemeral character (materials and meaning), uncontrolled environmental conditions, monumental size, high cost, lack of specialized training, ownership issues, and so forth [1]-[2], [5]. In the case of Greece, public murals and/or murals in public spaces have not yet been legally recognized as a protected form of art [8] to receive proper conservation treatments. At a global level, there has been a significant number of cases that acquired considerable cultural heritage value and protection, e.g. the murals of Keith Haring [9] and many more.

Spray-paints along with dispersion paints, commonly known as latex, constitute the most popular media used to produce street and urban art murals [8], [10]. Although several papers have dealt with artist’s waterborne paints providing information about their chemical characterization [6], [11]-[14], degradation mechanisms [15], and conservation interventions [9], little scientific research has been carried out concerning the spray-paints’ composition and ageing behaviour. However, the bibliography for spray-paint removal from cultural heritage monuments -and very recently from street art [16]-[18] is quite extensive.

Considering the vast variety of aerosol paints produced by different brands worldwide, the composition of which remains concealed due to intellectual property rights [19], there is undoubtedly a need for systematic conservation-driven research on spray-paints’ analysis. The understanding of these complex formulations constitutes the first and most important step for the successful implementation of conservation interventions [19] as it provides information regarding the technique, the degradation processes, and even the past treatments [10],[13]-[14]. In this study, the investigation on the chemical characterisation of various spray-paints used in contemporary murals aiming to establish a protocol for the documentation, analysis, and diagnosis of these materials, is presented.

It constitutes the first step in understanding what systems/methods could be used to remove overpaintings from contemporary murals, a very common phenomenon nowadays [20]. In addition, the determination of the chemical composition of these materials aims to provide a useful tool for selecting the appropriate cleaning method in the general field of cultural heritage. For this reason, initially, a questionnaire was shared to numerous Greek street artists and owners of spray-paint shops as well, to determine the most commercial spray-paint brands and colours. The spray-paints selected were investigated by means of Scanning Electron Microscopy/Energy Dispersive X-ray spectroscopy (SEM/EDS), Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and Raman spectroscopy.

2.1. Materials

The spray-paints selected in this study were obtained by four manufacturers (Flame Blue, Loop Colours, Montana Black, and Montana Gold) in five colours (white, black, red, green, and blue) as presented in Table 1. The exact description (code names) of each sample is not provided due to intellectual property purposes.

Table 1. Spray-paints investigated in this study.

Sample ID	Manufacturer	Colour
S_1	Flame Blue	White
S_2	Flame Blue	Black
S_3	Flame Blue	Red
S_4	Flame Blue	Green
S_5	Flame Blue	Blue
S_6	Montana Black	White
S_7	Montana Black	Black
S_8	Montana Black	Red
S_9	Montana Black	Green
S_10	Montana Black	Blue
S_11	Loop Colors	White
S_12	Loop Colors	Black
S_13	Loop Colors	Red
S_14	Loop Colors	Green
S_15	Loop Colors	Blue
S_16	Montana Gold	White
S_17	Montana Gold	Black
S_18	Montana Gold	Red
S_19	Montana Gold	Green
S_20	Montana Gold	Blue

2.2. Methods

2.3.1. Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDS)

SEM/EDS was used in this study for the elemental analysis of the spray-paint films (inorganic pigments and fillers). It was performed with Scanning Electron Microscope (SEM/EDS) Model JEOL JSM-6510LV equipped with an Elemental Analyzer Oxford Instruments PENTA FEI/PRECISION X-act/ and INCA software

2.2.2. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR)

ATR-FTIR was employed for providing information on the binders, organic pigments, and fillers [19]. IR spectra were collected on a Thermo Scientific Nicolet 6700 FTIR spectrometer (transmission mode), equipped with gold plated optics and an N₂ purging system. Spectra were acquired using a single reflection ATR (Attenuated Total Reflection) Quest (Specac) accessory equipped with ZnSe crystal (Spectral range: 7800–500 cm^-1, Penetration depth: 2μm, Angle of incidence: 45^o), in the 4000-400 cm^-1 region, at a resolution of 4cm^-1, and by summing 128 scans. ATR-FTIR spectra were processed on Spectragryph-optical spectroscopy software, v. 1.2.14, 2020. Baseline correction and smoothing were applied in all spectra.

2.2.3. Raman spectroscopy

Complementary information regarding the identification of pigments and additives was given using Raman spectroscopy since FTIR spectroscopy is poorly informative due to the overlapping of absorption bands of various components [19]. For the Raman microscopy measurements, a Renishaw inVia microscope was employed (Renishaw plc, New Mills, Wottonunder-Edge, Gloucestershire, GL12 8JR), with a near-infrared diode excitation laser at 785 nm (300 mW). The laser light was focused through the microscope lens (×50) onto the sample on a motorised XYZ stage. Individual point spectra (between 3 and 5) were obtained from each sample. Raman scattering was collected within a spectral range between 200-1800 cm⁻¹. The exposure time for each signal acquisition consisted of 2 accumulations of 10-60 seconds depending on the CCD saturation levels. The cosmic ray removal option of the software (Wire version 4.1) was applied, and an external standard of Si was measured as part of the calibration routines prior to each set of measurements. The analysis of the spectra was carried out with Spectragryph-optical spectroscopy software, v. 1.2.14, 2020. The Raman spectra were subjected to baseline correction and smoothing using custom algorithms.

2.3. Sample preparation

For SEM/EDS and ATR-FTIR analyses, the samples were prepared by spray-painting on Teflon sheets (~ 2 x 2 cm², 1 mm thickness), maintaining a constant pressure, from ~ 20 cm distance and after 2 minutes of shaking. For Raman spectroscopy, the samples were produced via spray-painting on glass slides (2.5 x 7.5 cm², 1 mmthickness). The samples were left to dry for two days at room temperature.

3.1. Binders

Initially, the binders were investigated by ATR-FTIR spectroscopy mainly in black spray-paints, to avoid possible overlapping of the binders’ peaks with the peaks of organic pigments. From this study, three different binder types were detected: (1) acrylic resins modified with nitrocellulose, (2) acrylic resins combined with nitrocellulose and modified with styrene, and (3) alkyd resins in combination with nitrocellulose and modified with styrene.

In particular, peaks most possibly assigned to the p(nBMA-MMA) copolymer medium were identified in Flame Blue and Loop Colours spray-paints. In the ATR-FTIR spectrum of the S_2 sample (Fig. 1a; Table 2), a strong doublet at 2955 and 2933 cm^-1 with a shoulder at 2873 cm^-1, corresponding to C-H stretching vibrations, can be easily observed. The strong peak at 1724 cm^-1 is attributed to ester C=O stretching vibrations. The C-H bending bands can be seen at 1485, 1448, 1433, and 1385 cm^-1, whereas the C-O and C-C stretching bands appear at 1268, 1239, ~ 1180, 1144, 1061, 1018, 965, and 947 cm^-1. The C-H rocking bands were found at 809 and 748 cm^-1 [19], [21]. Nitrocellulose in small concentration seems to be present due to the peaks at 1652 and 842 cm^-1. Similar peaks were also observed in the ATR-FTIR spectrum of the S_12 sample (Fig 1b; Table 2). However, the characteristic absorptions of nitrocellulose in S_12 at 1650 and 844 cm^-1 are more intense than those in S_2, indicating a higher concentration of nitrocellulose in the former.

Montana Gold spray-paints show the main absorptions of a methacrylic binder ascribed to p(nBMA-MMA), in combination with nitrocellulose (almost 1:1 ratio) and modified with styrene. In S_17 spectrum (Fig. 1c; Table 2) the p(nBMA-MMA) part was identified by the following bands: 2956 and 2871 cm^-1 (C-H stretching); 1725 cm^-1 (C=O stretching); 1466, 1381 and 1365 cm^-1 (C-H bending); 1069, 1194, 1121, 1024, and 949 cm^-1 (C-O & C-C stretching). The modification with styrene can be inferred based on the absorptions at 1601, 1581, 1491, 742, and 701 cm^-1 [10], [19], [22], [26] while the presence of nitrocellulose can be clearly observed at 1645, 1276, and 1069 cm^-1 due to C-O, C-C (in glucose units of the polysaccharide structure), and 836 cm^-1 due to C-N stretching modes [10], [22]-[25].

Table 2. ATR-FTIR peaks of the major and minor binders of all spray paints of each company. The corresponding peaks of Table 2 are annotated as AC (for acrylic), AL (for alkyd), NC (for nitrocellulose), and Sty (for styrene).

Bond Type Assigned	Flame Blue, Loop Colors	Montana Gold	Montana Black
	(cm^-1)	(cm^-1)	(cm^-1)
C-H stretching	2956, 2931, 2873 (sh) AC	2956, 2929, 2871 (sh) AC	2953 (sh), 2925, 2854 AL
C=O stretching	1725 AC	1725 AC	1722 AL
N-O stretching	1650 NC	1645 NC 1276 NC	1649 NC
C-H bending	1484, 1448, 1384, 1367 AC	1466, 1381, 1365 AC	1466, 1383 AL
C-O & C-C stretching	1269, 1240, ~ 1180, 1145, 1060, 966, 945 AC	1069 AC & NC, 1194, 1121, 1024, 949 AC	1260, 1176, 1114, 1068 AL & NC, 1039, 1026, 972 AL
C-N stretching	844 NC	836 NC	846 NC
C-H rocking	ca .807, 749 AC	-	773 AL
Aromatic skeletal ring breathing (Sty)	-	1601, 1581, 1491	1601, 1581, 1489, 1451
Aromatic C–H out-of-plane bending vibrations (Sty)	-	742, 701	741, 709

ATR-FTIR spectra of Montana Black spray-paint enabled the identification of an alkyd binder in combination with nitrocellulose (1:1) and, possibly, modified with styrene. In the S_7 spectrum (Fig. 1d; Table 2), peaks at ca. 2953 (sh.), 2925, and 2854 cm^-1 are related to the antisymmetric and symmetric stretching vibrations of C-H groups, whilst a sharp carbonyl peak at around 1722 cm^-1 can be easily observed. In the fingerprint region, bands at 1260, 1176, 1114, 1068, 1039, 1026, and 972 cm^-1 can be ascribed to C-O and C-C stretching. Bands at 1466 and 1383 cm^-1could be attributed to C-H bending, whereas the band at 773 cm^-1to C-H rock vibration [10], [25]-[31]. The modification with styrene can be inferred from the presence of the aromatic C-H out-of-plane bending vibrations with absorptions at 741 and 709 cm^-1. Additional peaks attributed to styrene are observed at ~ 3070 cm^-1 (aromatic C-H stretching), as well as at 1601, 1581, 1489, and 1451 cm^-1, although alkyd binder shows the same peaks. The presence of nitrocellulose could be recognized from the peaks at 1649, 1068, and 846 cm^-1.

Based on the above assignments, the Flame Blue, Loop Colors, and Montana Gold spray paints are based on acrylic resins, while the Montana Black products are based on alkyd binders, all containing various amounts of nitrocellulose. Montana Gold and Montana Black are also modified with styrene.

3.2. Pigments and additives

3.2.1. SEM/EDS

EDS analysis revealed the presence of titanium dioxide (rutile, PW6) in the majority of spray-paints. In addition, barium, aluminium, silicon, magnesium, calcium, and iron appear as major constituents of all investigated spray-paint samples. Barium sulfate (PW21), kaolinite (hydrated aluminium silicate, PW19), and talc (hydrated magnesium silicate, PW26) were found in most cases in this study; they were used as extenders to improve the optical and rheological properties [10]. Also, gypsum (calcium sulfate, PW25) and chalk (calcium carbonate, PW18) were detected as extenders in some cases. The presence of chlorine and copper in green spray-paints spectra suggests the use of chlorinated phthalo green pigments. All the above have been confirmed by ATR-FTIR and Raman analyses.

3.2.2. ATR-FTIR

Both organic pigments and inorganic additives/fillers were detected by ATR-FTIR spectroscopy in almost all spray-paints in a very satisfactory way. Particularly, ATR-FTIR spectra of S_3 (Fig. 2a; Table 3) revealed the presence of a polycyclic red pigment, probably PR254, inferred by its characteristic bands at: 1608, 1404, 1327, 1307, 1089, 1014, 825, 810, 751, 713, and 623 cm^-1[32]. The monoazo Naphthol AS red pigment PR112 was identified in S_8 (Fig 2b; Table 3) and S_18, based on the absorbance bands at: 1672, 1624, 1553, 1537, 1204, 1159, 1115, 1039, 1014, 931, 907, 893, 867, 760, and 687 cm^-1 [19], [22]. The chlorinated phthalocyanine pigment PG7 along with the monoazo acetoacetyl yellow pigment PY74 could be successfully identified in S_9 and S_19 (Fig. 2c; Table 3). Characteristic bands of phthalocyanine green PG7 can be found at 1554, 1209, 1151, 1121, 950, 778, and 769 cm^-1[10], [22], whereas the peaks at 1519, 1338, 1254, 1225, 1180, 1024, and 802 cm^-1 are assigned to PY74 [19], [22]. Different polymorphs of phthalocyanine blue were clearly visible in all blue spray-paints. In S_15 (Fig. 2d; Table 3) PB15:3 may have been used, recognised by its characteristic peaks at: 1610, 1508, 1464, 1421, 1335, 1286, 1174, 1122, 1089, 900, 878, 780, 753, and 729 cm^-1 [12], [33].

Titanium dioxide (PW6) was easily detected through the broad absorption band at 750-400 cm^-1 (S_15; Fig. 2d) [10], [19] confirmed by its elemental analysis through SEM/EDS. Talc (PW26) was detected in most spray-paints, recognised by its characteristic peaks at ca. 668 and 1014 cm^-1 (S_3; Fig. 2a) [26], [34]. Barium sulfate (PW21) was evidenced too based on the bands at 1176, 983, 633, and 610 cm^-1 (S_8; Fig. 2b) [12], [34]-[35], although its characteristic bands are often overlapped. Kaolinite (PW19) was observed (S_19; Fig. 2c) at 1121, 1024, 1005, and 917 cm^-1 [12], [26], [35]-[36]. Calcium carbonate (PW18) and calcium sulfate (PW25) are most likely present in S_15 (Fig. 2d) due to peaks at 1421 and 878 cm^-1 (PW18) [12], [34], [36], and 1122, 665, and ca. 600 cm-¹ (PW25) [19], [22].

3.2.3. Raman spectroscopy

Raman spectra acquired on S_3 (Fig. 3a; Table 3) and S_18 (Fig. 3b; Table 3) confirmed the presence of PR254 and PR112 pigments, respectively. Also, Raman spectroscopy proved quite useful for the identification of the synthetic organic pigments (SOPs) used in S_13 (Fig. 3c; Table 3), that could not be detected exclusively through ATR-FTIR analysis. More specifically, PR48:3 (1593, 1553, 1488, 1456, 1391, 1378, 1361, 1264, 1181, 1104, 1043, 1019, 970, 954, 875, 786, 768, 748, 717, 578, 531, 509, 497, 463, and 346 cm^-1) [37], alongside with PY74 (1667, 1593, 1553, 1510, 1488, 1456, 1439, 1403, 1353, 1327, 1298, 1264, 1244, 1160, 1126, 1116, 1089, 1067, 1043, 1019, 919, 848, 828, 802, 646, 626, 600, 560, 463, 441, and 404 cm^-1) [37]-[40], may be present. All the data collected are presented in Table 3.

Table 3. Organic pigments identified by ATR-FTIR and Raman spectroscopy.

Pigment (Colour Index)	Chemical class	ATR-FTIR bands (cm^-1)	Raman bands (cm^-1)
PR254	Polycyclic pigment, Diketopyrrolo-Pyrrole (DPP)	1608, 1404, 1327, 1307, 1089, 1014, 825, 810, 751, 713, 623	1663, 1593, 1576, 1552, 1498, 1444, 1402, 1344, 1319, 1306, 1255, 1200, 1090, 1053, 1014, 955, 927, 727, 687, 644, 622, 524, 453, 421, 352, 290, 277 [37], [39]
PR112	Monoazopigment, Naphthol AS	1672, 1624, 1553, 1537, 1204, 1159, 1115, 1039, 1014, 931, 907, 893, 867, 760, 687	1579, 1553, 1483, 1461, 1448, 1391, 1375, 1358, 1332, 1316, 1282, 1258, 1243, 1230, 1205, 1162, 1109, 1062, 1041, 1015, 967, 894, 814, 746, 728, 681, 618, 573, 528, 469, 431, 387, 348, 299, 275, 247 [11], [37], [40]
PY74	Monoazopigment, acetoacetic arylide	1519, 1338, 1254, 1225, 1180, 1024, 802	1667, 1592, 1552, 1510, 1488, 1454, 1439, 1403, 1353, 1326, 1298, 1264, 1244, 1160, 1126, 1116, 1089, 1068, 1044, 1019, 916, 847, 828, 802, 646, 628, 601, 560, 462, 440, 405
PG7	Polycyclic pigment, Phthalocyanine	1554, 1209, 1151, 1121, 950, 778, 769	1537, 1507, 1484, 1445, 1424, 1390, 1338, 1283, 1213, 1187, 1084, 979, 958, 818, 777, 741, 686, 644, 349, 335
PB15:3	Polycyclic pigment, Phthalocyanine	1610, 1508, 1464, 1421, 1335, 1286, 1174, 1122, 1089, 900, 878, 780, 753, 729	PB15:1 1529, 1453, 1342, 1306, 1214, 1186, 1159, 1143, 1108, 1008, 954, 848, 837, 781, 749, 720, 682, 595, 486, 259
			PB15:3 1530, 1451, 1429, 1342, 1307, 1216, 1196, 1186, 1144, 1109, 1008, 954, 849, 835, 781, 749, 718,703, 682, 595, 485, 260, 236
			PB15:6 1532, 1486, 1451, 1430, 1344, 1306, 1216, 1186, 1144, 1110, 1037, 1007, 955, 837, 784, 751, 718, 683, 595, 485, 260, 238

Phthalocyanine green PG7 (1537, 1507, 1484, 1445, 1424, 1390, 1338, 1283, 1213, 1187, 1084, 979, 958, 818, 777, 741, 686, 644, 349, and 335 cm^-1)[37], [41]-[43], combined with PY74 (1589, 1459, 1290, 1264, 1158, 1064, 1044, 915, 802, and 720 cm^-1) were identified in S_9 (Fig. 4a; Table 3) and S_19. For S_4 and S_14 (Fig. 4b; Table 3), PB15:1 and PB15:3 (1531, 1451, 1427, 1342, 1307, 1217, 1197, 1185, 1160, 1144, 1109, 1008, 955, 848, 835, 782, 749, 719, 682, 645, 595, 485, 260, 236 cm^-1), respectively, in combination with PY74 (1667, 1592, 1404, 1327, 1264, 1089, 1067, 1043, 803, 702, and 497 cm^-1) seem to be present as observed in the Raman spectrum. Small concentration of PG7 may have been added too, as indicated by SEM/EDS analysis; nevertheless, its peaks are indistinguishable due to the overlapping bands of the phthalo blue pigment PB15:3.

Raman analysis enabled the discrimination of different copper phthalocyanine (CuPc) polymorphs (blue pigments). More specifically, based on the position of the bands, a-CuPc PB15:1 was found in S_5 (Fig. 5a; Table 3), β-CuPC PB15:3 in S_15 (Fig. 5b; Table 3), and ε-CuPc PB15:6 in S_10 and S_20 (Fig. 5c; Table 3). This discrimination of CuPc polymorphs was also confirmed by the intensity ratio of various bands, and especially the intensity ratio of the bands 749:683, according to the literature [33], [37], [43].

Large quantities of titanium dioxidein rutile form (PW6) was detected at 450 and 612 cm^-1 in the white spray paint sample S_6 (Fig. 6a) [11],[45]. Additionally, the band at 988 cm^-1indicates the presence of barium sulfate (PW21) [14], [46], while the bands at 1004 and 1087 cm^-1 could be attributed to calcium sulfate (PW25) [47], and calcium carbonate (PW18), respectively [48]. In addition to the above, the more indistinguishable inorganic additives, i.e. talc (PW26) with bands at 236, 363, and 683 cm^-1 [49] and kaolinite (PW19) with bands at 749 and 931 cm^-1 [50] were also detected in the white spray paint sample S_16 (Fig. 6b). Finally, the detection of the inorganic pigments usually used in the black spray paints was not possible under the present experimental conditions, due to significant fluorescence observed.

From the present study on the chemical characterisation of spray-paints used in public murals, the high complexity in the chemical composition of these formulations is highlighted. The complementary application of ATR-FTIR and Raman spectroscopies was proved to be fundamental for the identification of both binders and synthetic organic pigments. Inorganic components were identified mostly by SEM/EDS and ATR-FTIR techniques, supported by Raman spectroscopy. Three kinds of synthetic binders have been identified: alkyd resins combined with nitrocellulose (almost 1:1 ratio) and modified with styrene, acrylic resins combined with nitrocellulose (almost 1:1 ratio) and modified with styrene, and acrylic resins modified with nitrocellulose. Black spray-paints were proved more suitable for identifying the binding media of the paints with ATR-FTIR spectroscopy. On the other hand, white spray paints were proved more appropriate for the identification of inorganic pigments and additives with Raman spectroscopy. The identification of synthetic pigments, although it was quite challenging, led to the detection of a wide variety of organic pigments mainly with Raman spectroscopy (and secondarily with ATR-FTIR spectroscopy) that resulted even in the discrimination of different CuPc polymorphs in blue spray-paints samples.

In the forthcoming future, the chemical characterisation of spray-paints will be further deepened with the Py-GC/MS method for the identification of plasticizers and other possible additives present in these formulations and extended with the investigation of the thermal and physical properties of these materials. The understanding of the physicochemical properties of spray paints is a crucial point both for the investigation of the degradation mechanisms and the selection of appropriate cleaning methods of these materials.

AC	Acrylic
AL	Alkyd
ATR-FTIR	Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy
C.I.	Color Index
Ca.	Circa
CCD	Charge-Coupled Device
cm	Centimeter
cm²	Square centimeters
cm^-1	Wavenumber
CuPc	Copper Phthalocyanine
Fig.	Figure
Mm	Millimeter
NC	Nitrocellulose
p(nBMA-MMA),	Poly(n-butyl methacrylate-methyl methacrylate)
PB15(x)	Pigment Blue Phthalocyanine Blue
PG7	Pigment Green Chlorinated Phthalocyanine Green
PR112	Pigment Red Naphthol Red AS-D
PR254	Pigment Red Pyrrole Red
PR48:3	Pigment Red BONA Lake Red
PW18	Pigment White Calcium Carbonate (chalk)
PW19	Pigment White Hydrated Aluminum Silicate (kaolinite)
PW21	Pigment White Barium Sulfate
PW25	Pigment White Calcium Sulfate (gypsum)
PW26	Pigment White Hydrated Magnesium Silicate (talc)
PW6	Pigment White Titanium dioxide
PY74	Pigment Yellow Arylide Yellow
Py-GC/MS	Pyrolysis Gas Chromatography Mass Spectrometry
S_	Sample
SEM/EDS	Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy
Sh.	Shoulder
Si	Silicon
SOPs	Synthetic organic pigments
Sty	Styrene
v.	Version
ZnSe	Zinc selenide

Availability of data and materials

The data during the current study are available from the corresponding author on reasonable request.

Competing interests

The authors declare no conflict of interest.

Funding

The research was funded by the Special Account for Research Grants, University of West Attica.

Authors' Contributions

V.M.: Conceptualisation, Methodology, Project administration, Investigation, Data Curation, Visualization, Writing Original Draft; A.M.D.: Supervision, Investigation, Writing-Review & Editing; F.K.: Resources, Investigation, Writing-Review & Editing; P.K.: Investigation, Writing-Review & Editing; C.C.: Resources, Writing-Review & Editing; V.G.G.: Resources, Writing-Review & Editing; S.B.: Supervision, Validation, Writing-Review & Editing; Y.F.: Supervision, Validation, Writing-Review & Editing. All authors have read and agreed to the published version of the manuscript.

Acknowledgments

Authors are grateful to A. Karampotsos (Special Technical Laboratory Staff, Department of Conservation of Antiquities and Works of Art, UNIWA) for the valuable collaboration and advice on SEM-EDS analyses. Varvara Marazioti would like to thank University of West Attica for funding the Ph.D. research.

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No competing interests reported.

Contemporary murals: Chemical characterisation of artists’ spray-paints

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1. Materials

2.2. Methods

2.3.1. Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDS)

2.2.2. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR)

2.2.3. Raman spectroscopy

2.3. Sample preparation

3. Results and Discussion

3.1. Binders

3.2. Pigments and additives

3.2.1. SEM/EDS

3.2.2. ATR-FTIR

3.2.3. Raman spectroscopy

4. Conclusions

abbreviations

Declarations

References

Additional Declarations

Status:

Version 1