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Article

Historical Pigments and Paint Layers: Raman Spectral Library with 852 nm Excitation Laser

by
Silvia Innocenti
1,2,
Diego Quintero Balbas
1,
Monica Galeotti
3,
Andrea Cagnini
3,
Simone Porcinai
3 and
Jana Striova
1,*
1
National Research Council—National Institute of Optics (CNR-INO), Largo E. Fermi 6, 50125 Florence, FI, Italy
2
Department of Sciences of Antiquity, “La Sapienza” University of Rome, Piazzale Aldo Moro 5, 00186 Rome, RM, Italy
3
Opificio Delle Pietre Dure, Viale F. Strozzi, 1, 50129 Florence, FI, Italy
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(6), 557; https://doi.org/10.3390/min14060557
Submission received: 24 April 2024 / Revised: 24 May 2024 / Accepted: 27 May 2024 / Published: 28 May 2024
(This article belongs to the Special Issue Spectral Behavior of Mineral Pigments, Volume II)
Browse Figures
Versions Notes

Abstract

:
Raman spectroscopy (RS), for its robust analytical capabilities under constant development, is a powerful method for the identification of various materials, in particular pigments in cultural heritage. Characterization of the artist’s palette is of fundamental importance for the correct formulation of restoration intervention as well as for preventive conservation of artworks. Here we examine the number and variability of research studies exploiting Bravo handheld Raman spectrophotometer relying on the excitation of Raman signal with temperature-shifted diode lasers emitting at 852 and 785 nm. To this end, we explore the spectral features of common historical pigments examined as powders and in the paint layer. We show that some materials may exhibit slightly different spectra as concerns especially the relative intensity of Raman lines with 852 nm laser excitation wavelength as compared to the standard 785 nm. The aim is to provide the research community with a reference spectral database that facilitates the identification of unknown pigments using the 852 nm excitation source.

Graphical Abstract

1. Introduction

The identification of pigments in artworks may provide relevant information to date/detect forgeries, establish the artwork’s origin, or formulate suitable restoration and preventive conservation procedures. Raman spectroscopy is nowadays a well-established method in the field of heritage science for its capacity to identify non-destructively and non-invasively a large variety of art materials [1]. The development of increasingly efficient low-size and lightweight Raman micro-probes enables in situ measurements and investigation of the artwork without any movement, sampling, or damage [2]. A robust spectral database of reference substances might ease the interpretation and exploitation of Raman spectral information, recently also through automated processes. Numerous Raman spectral databases have been developed (Table 1) with the first online databases of Raman spectra libraries dating back to the 1990s. Among the most frequently exploited libraries, we can list those provided by University College London (UCL) [3], Infrared and Raman Users Group (IRUG) [4], INFRA-ART by the Romanian database of Raman spectroscopy [5], RRUFF™ by University of Arizona [6], Infrared and Raman Discussion Group (IRDG) [7], SOPRANO [8], and Cultural Heritage Science Open Source (CHSOS) [9]. The last two databases are particularly focused on pigments of historical and artistic interest. In addition to the digitally available spectral datasets, publications containing Raman spectra compendia on art-related materials are available in the literature [10]. Despite the recent open-source policies that should provide findable, accessible, and reusable (FAIR) data, most of the spectral databases focused on cultural heritage materials display the Raman spectra only as images, therefore, limiting the usability of the information. A further limitation on the applicability of the databases is the fact that it is easier to obtain spectra informative enough when studying pure powder pigments; but when complex mixtures (i.e., paint layers containing a binder, pigment, and fillers) are analyzed, interference mainly arises from the organic substances employed as binders and/or varnishes. For this reason, it is fundamental to cross-check the applicability of the databases also in composite samples, for example, the SOPRANO and CHSOS databases.
As observed in Table 1, the libraries are commonly obtained with different excitation lines, because often the pigments’ response varies according to laser irradiation due to the Raman scattering cross-section and physical and optical properties. Generally, the selection of the excitation sources in the UV or NIR spectral regions may minimize the fluorescence interference. However, the Raman signal obtained with NIR lines is weaker than that obtained with lasers in the visible range because the Raman scattering strength is proportional to the fourth power of the excitation frequency [26]. In addition, excellent Raman enhancement can be obtained through the Resonance Raman effect, produced when the laser wavelength matches the absorption bands of the pigment. Moreover, the absorption of the laser radiation can lead to a reduction in the signal and induce interferences [1]. Photoluminescence from either the sample or the substrate may produce signals on the order of or sometimes greater than the Raman signal itself. These phenomena can have an impact on the final Raman spectra, depending on the employed laser wavelength, highlighting or hiding chemical information from the sample. As a consequence, the Raman spectrum of one pigment can appear different depending on whether the laser excitation is in resonance with an electronic transition or whether other photoluminescence bands appear. The literature reporting on the spectral pigments databases includes laser excitation sources, such as Ar+ (488 and 514 nm) or Kr+ (531 and 647 nm), He:Ne (633 nm), Nd:YAG (532 and 1064 nm), and diode laser (630 and 785 nm) [27].
The recently released handheld spectrophotometer by Bruker Optics, commercially named Bravo, employs two different excitation lasers (852 and 785 nm) to cover a spectral range initially comprised between 300 and 3200 cm−1 [28]. After 2020, the instrument spectral range was extended to cover a wider range comprised between 170 and 3200 cm−1, which was a significant improvement, particularly for the investigation of inorganic compounds. The sequentially shifted excitation technology (SSETM) and the PCA-based algorithm are exploited to provide the processed background-free and smoothed spectra. This processing procedure is particularly helpful in solving the common background collection produced by the short optical paths typical of portable instrumentation [29], as well as from the strong luminescence produced by cultural heritage organic materials. The application of Bravo Raman has increased considerably in the heritage science field as is evidenced by the recently published scientific literature (Figure 1). The trend of the employment of the instrumentation is exponential, with a decrease in 2020 corresponding to the COVID-19 pandemic. Bravo spectrophotometer proved powerful in addressing research problems pertinent to the identification of historical and modern pigments and lakes [30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53], minerals and gems [22,29,54], manuscripts [55,56], mosaics [2,57], and metal and metal corrosion products [58,59,60]. The present work aims to create a database using the 852 nm laser excitation wavelength for the most commonly employed art pigments, which can be regarded as the precursor of a larger open-source database of artistic colorants that is expected to be available online in the upcoming years. The goals of this study are to verify the possible differences in the Raman spectra acquired at 852 nm as compared to 785 nm when analyzing a set of different historical pigments frequently encountered during the analysis of polychrome surfaces, and based on the spectral characteristics of the materials, generate a summary to support researchers and conservators in the interpretation of the Raman spectra.
Additionally, with the aim to verify the applicability of the spectral references of pure pigments using the 852 nm laser excitation wavelength of the Bravo instrument, a mock-up panel painting, prepared in 1995 at the Opificio delle Pietre Dure (Florence, Italy) following the historical recipe for egg-tempera, was analyzed. Also, a comparison with spectra obtained with a 785 nm laser excitation wavelength allowed us to identify changes in the position and relative intensity of the characteristic peaks of some pigments in the spectra obtained with the 852 nm laser excitation wavelength.

2. Materials and Methods

2.1. Samples

A total of 32 pure pigments have been analyzed as dry powder. The analyzed pigments are divided into color categories. Table 2 lists all pigments with additional information. The samples represent a selection of the major pigments utilized in artworks.

2.2. Mock-Up Panel Painting

Restorers at Opificio delle Pietre Dure (Florence, Italy) realized the mock-up panel painting in 1995. It was prepared to employ wooden support with a ground layer of rabbit glue and gypsum (CaSO4·2H2O) (ratio 1:16 v/v) until saturation of the glue, applied with a brush in two steps (the second layer was applied once the first one was completely dry). On top of the ground layer, the imprimitura made of rabbit glue and H2O in a ratio of 1:32 was applied. Once completed, the prepared surface was divided into sections by drawing slots (1 cm × 3 cm). Egg tempera was prepared by mixing two parts of yolk, one of egg white and one of vinegar. The mock-up panel painting (Figure 2) includes blue pigments such as azurite, indigo, Lapis lazuli, smalt, cobalt blue, Prussian blue, and artificial ultramarine and was investigated using conditions specified in Table 3.

2.3. Sequentially Shifted Excitation (SSE) Raman Spectroscopy

The Bravo spectrometer uses a patented technology called SSE™ (Sequentially Shifted Excitation, patent number US8570507B1) to mitigate fluorescence. The device is equipped with two excitation lasers (DuoLaserTM, 785 nm, and 852 nm) that can be temperature-shifted three times over a small wavelength range (about 0.4 nm). Both laser beams impinge on the sample sequentially in every measurement, covering a broad spectral range from 170–2000 cm−1 and 2000–3200 cm−1, exploiting the 852 nm and 785 nm lasers, respectively. The laser power was set automatically, reaching a maximum of 100 mW for an 852 nm laser, in an area spot of about 100 µm × 500 µm [45]. In this work, we considered the spectral range 170–2200 cm−1, corresponding to the 852 nm laser excitation, and only the first channel raw data (called CH2) are reported.
The pigment powders were placed on a sheet of aluminum foil, and the Bravo instrument was placed in a vertical position over two aluminum square supports (Figure 3). In this configuration, the laser beam impinges vertically on the pigment powders without any interference.
The measurements were first performed with parameters automatically set by the instrument. These are determined by the internal algorithm that evaluates the signal-to-noise ratio. Starting from the so obtained values of integration time, the number of scans was further increased to improve the quality of the obtained spectrum (i.e., 0.5–50 s detector integration time and 5–30 accumulations—Table 3) depending on the response of the investigated materials.

2.4. Micro-Raman Renishaw (785 nm)

A benchtop Raman confocal microscope (Renishaw inVia) equipped with a Leica DM2700 optical microscope was employed to acquire micro-Raman spectra using a 785 nm excitation diode laser. We performed the measurement in the spectral range 170–1800 cm−1, using a grating 1200 lines/mm and a thermoelectrically cooled CCD pixel (functional resolution 400–1060 cm−1). The laser powder was kept below 7 mW, using a 10 s exposure time and 5 accumulations.

2.5. Stereomicroscopy

The powder pigments were examined under a Leica M205C stereomicroscope with a camera Leica DFC 295 at 0.78×. White balance over a white surface was performed before capturing the images that were processed with LAS v4.6 software.

3. Results

Raman spectra acquired with an 852 nm laser excitation wavelength of each pigment are reported here. The results are conventionally organized into sections according to the pigments’ color as follows: 3.1 blue, 3.2 green, 3.3 yellow, 3.4 red, and 3.5 white and black pigments. A summary of the peaks’ assignment with relevant references is reported in Table A1 of Appendix A. The Raman spectra obtained also with 785 nm laser excitation wavelength are reported for comparison (in green lines) only when notable differences in Raman peak position/intensity or spectral shape have been encountered.

3.1. Blue Pigments

The 852 nm Raman spectra of eight blue pigments—cobalt blue, Prussian blue, Antwerp blue, ultramarine, azurite, indigo, smalt, and Lapis lazuli—are displayed respectively in Figure 4a–h. For Cobalt blue, smalt, and Lapis lazuli, some differences between 852 and 785 nm-generated Raman spectra were detected; therefore, comparative spectra are reported in Figure 4a,e,f.
Cobalt blue (CoO·Al2O3, Figure 4a) is detected by the bands positioned at 204 cm−1 (Co-O tetrahedral sites vibrational), 410 cm−1 (symmetric bending of AlO4), 514 cm−1 (asymmetric stretching of AlO4), 615 cm−1 (antisymmetric stretching of AlO4), and 754 cm−1 (symmetric stretching vibration of the AlO4) within the spinel lattice [64]. As compared to the spectrum with the 785 nm laser excitation line (Figure 4a, green line), the number of bands and their positions are the same; however, their intensities at the low wavenumbers are inverted, being the ratio 204/410 cm−1 lower for the 852 nm laser.
The Raman spectrum of Prussian blue (Figure 4b) is detected by the two low-frequency bands located at 275 cm−1 and 538 cm−1 due to Fe-CN-Fe deformation vibration and Fe-C stretching vibration, respectively [65]. The major bands of Prussian blue, positioned at 2090 and 2159 cm−1 (C≡N stretching vibration), are absent in the 852 nm generated spectrum owing to the detection range limits; however, they can be detected by the other 785 nm laser excitation wavelength of instrumentation (not reported in Figure 4b).
Antwerp blue (Figure 4c) is a combination of Prussian blue and Cobalt blue [66]. The Raman spectra obtained with 852 nm laser excitation wavelength highlight only the presence of Prussian blue for the two low-frequency bands located at 275 and 533 cm−1, and no information about the presence of cobalt blue was detected [65].
Ultramarine (Figure 4d), a synthetic pigment of an approximate formula Na6–10Al6 Si6 O24 S2–4, is identified by its principal band at 548 cm−1 (and its shoulder at 585 cm−1) due to the symmetric stretching mode of S3− in the sulfur-containing sodium-silicate pigment. Its first overtone at 1096 cm−1 is also detected [67].
The overall intensity and quality of the Raman spectrum of azurite at 852 nm is low (Figure 4e). In agreement with the spectra obtained at 785 nm, it is characterized by a band placed at 404 cm−1 (lattice mode), by the bands related to the symmetric and asymmetric stretching of carbonate at 1095 cm−1 and 1425 cm−1, respectively, and by the symmetric bending mode detectable at 835 cm−1. Low-intensity bands present at lower frequencies are assigned to lattice modes (220 and 284 cm−1). Furthermore, the out-of-plane bending mode of the OH group present in the azurite molecule is located at 939 cm−1 [68]. In the considered spectral range, however, it is not possible to detect the bands at 3427 and 3453 cm−1 assigned to the hydroxyl-stretching modes of the OH unit of azurite [69].
The characteristic Raman bands of indigo (Figure 4f) are placed at 674 cm−1, 756 cm−1, 1225 cm−1 (rocking vibration of N-H), 1470 cm−1 (rocking of C-H), and 1360, 1571, and 1622 cm−1 (stretching vibration of conjugate system C=C, C=O, and N–H group) [70], the last two being the most intense.
The Raman spectrum of smalt (Figure 4g) is dominated by the intense band assigned to the Si-O-Si vibration mode of silicate at 689 cm−1 [71], and the lower frequencies are assigned to the breathing vibration mode of the T–O–T substructure and correlate with the size of the TO4 tetrahedra ring, where T = Si or Al [72]. Similarly to Cobalt blue, as compared to the spectrum with a 785 nm laser excitation line (Figure 4g, green line), the band intensities at the low wavenumbers are inverted, being the 195/481 cm−1 ratio lower for the 852 nm laser.
Lapis lazuli, as ultramarine, is detected by the band at 548 cm−1 and a shoulder at 583 cm−1 (Figure 4h) attributed to lazurite S3− and S2− symmetric stretching mode. On the contrary to the spectrum of the artificial pigment (Figure 4c) and to that obtained with 785 nm (Figure 4h), this spectrum obtained through 852 nm laser excitation wavelength is very complex and characterized by several sharp and broad bands (namely at 312 vs, 350 m, 405 w, 513 m, 712 w, 835 s, 985 m), not correlated directly to lazurite. This spectral behavior is probably due to luminescence phenomena. In general, the activators of luminescence phenomena—including rare earth elements, Fe2+, Fe3+, Co2+, and Ni2+—are chemical impurities in the mineral structure. The 785 nm laser excitation wavelength Raman spectrum gave rise to two sharp bands, one related to lazurite (548 cm−1) and the luminescent band at 1306 cm−1 attributed to the presence of diopside [73]. Gonzáles-Cabrera et al. [74] exploit these luminescence phenomena to discriminate natural from synthetic ultramarine blue. Other researchers [75,76] link these wavelength-dependent luminescence spectral patterns to the geographical provenance of the samples. It is most likely that the observed luminescent pattern at 852 nm laser excitation wavelength is associated with diopside; however, more in-depth research is necessary to explore its precise origin.
To showcase a different pigment spectral behavior under different laser excitation wavelengths, the example of Lapis lazuli is reported in Table 4. Lapis lazuli is detected by the band at 548 cm−1 referred to mineral of lazurite in the spectra acquired with all the laser excitation wavelengths. The spectra generated with 532 nm and 638 nm laser excitation wavelengths, in addition to 260 a cm−1 and 548 cm−1 (bending and stretching of S3− ion, respectively), also show overtone bands (548 × 2 = 1096 cm−1, 548 × 3 = 1644 cm−1, etc.) or combinations [260 + 548 = 808 cm−1; 260 + (2 × 548) = 1358 cm−1; etc.]. The shoulders at ca 258 and 583 cm−1 correspond to asymmetric modes of lazurite. This Raman fingerprint is typical of resonance Raman spectra of lazurite [77].
Other laser excitation wavelengths, i.e., 785 nm, 852 nm, and 1064 nm, do not generate resonance Raman spectra and exhibit a lower number of bands. The Raman spectrum acquired with 1064 nm laser excitation wavelength is very noisy, and only 364 cm−1 and 548 cm−1 peaks are detected.

3.2. Green Pigments

Five green pigments—green earth, cobalt green, chromium oxide, malachite, and viridian—were analyzed. The 852 nm laser excitation spectra are reported in Figure 5a–e. Spectra with 785 nm are reported for green earth (Figure 5a) and viridian (Figure 5d).
Raman spectrum of green earth (Figure 5a) is characterized by several broad bands that cover the spectral range of 200–550 cm−1. The Raman bands centered at 200 and 277 cm−1 are due to internal vibrations of the MoO6 octahedra, where Mo is the inter-layer metal atom, and those located at 393 and 533 cm−1 are the bands mainly due to the vibrational modes of the SiO4 tetrahedra [20]. These bands indicate the presence of celadonite, as the main mineral component of green earth. In these spectra, because of the natural origin of the pigment, Raman bands of gypsum are also present (1006 and 1135 cm−1) [78]. By changing the excitation to 785 nm (Figure 5a, green line), an apparent shift towards higher wavenumbers of the 533 cm−1 band is observed (the shift is from 533 to 547 cm−1), probably due to intensity changes in an unresolved doublet. Similar behavior was previously reported by Ospitali et al. when changing the excitation from 514.5 to 780.0 nm [79].
Cobalt green (Figure 5b)—Co2TiO4—is characterized by an intense band at 713 cm−1 (symmetric stretching of CoO6 octahedra in CoTiO3) [20], and other bands are at 520, 460, and 340 cm−1. The major bands of chromium oxide (Figure 5c) are positioned at 297, 350, 528, 615 cm−1 (bending mode CrO3), and 554 cm−1 (symmetric stretching of Cr2O) [80].
Viridian (Figure 5d) is a mixture of anhydrous and hydrated chromium oxides, as the recorded Raman bands at 487 and 584 cm−1 are attributed to the hydrated oxide and that at 262 cm−1 to the anhydrous one [81]. On the other hand, the spectrum obtained with 785 nm laser excitation wavelength yields a large fluorescence background and accompanying noise; therefore, its identification is not possible.
Malachite (Figure 5e) is detected by the band at 430 cm−1, together with the peaks at 219, 270, and 350 cm−1 that are related to lattice modes and bands due to symmetric stretching (1095 cm−1) and symmetric bending mode (816 and 834 cm−1) of CO32− [69]. In this spectrum, unlike the one from azurite, the band at 1425 cm−1 (asymmetric stretching vibration) is not detected, and hence, the spectral range reported is cut off at 1200 cm−1.

3.3. Yellow Pigments

The 852 nm Raman spectra of six yellow pigments investigated—massicot, lead–tin yellow type I, Naples yellow, chromium oxide, cadmium yellow, yellow ochre, and raw sienna—are reported in Figure 6a–g. No significant spectral changes are recorded compared with the 785 nm laser excitation wavelength.
In general, the low wavenumber region provides a confident recognition of massicot (PbO); in particular, the major band at 142 cm−1 (symmetric stretching of Pb-O) [45] is not detected by the Bravo instrument due to limited spectral region (>170 cm−1). However, the diagnostic bands of massicot at 290 and 385 cm−1 are well discerned (Figure 6a).
Lead–tin yellow type I (Pb2SnO4) spectrum (Figure 6b) at 852 nm is well characterized by the bands at 197, 275, 290, 455, and 524 cm−1. The main band of lead–tin yellow at 130 cm−1 (due to symmetric stretching vibration of Pb-O) [82] is out of the detectable spectral range of the instrument. No significant differences as compared to 785 nm were observed.
Naples yellow (Pb2Sb2O7) (Figure 6c) has a typical spectral pattern in the 200–400 cm−1 spectral region (290, 318, and 348 cm−1) of the vibrational mode of both Sb-O and Pb-O bonds; instead, the band at 510 cm−1 is due to Sb-O stretching of the SbO6 octahedra [83]. Other characteristic bands of Naples yellow are placed at 440 and 613 cm−1.
Chrome yellow (PbCrO4, crocoite, Figure 6d) is detected by the Raman instrument at 852 nm, unveiling the most informative 774–942 cm−1 spectral range related to symmetric stretching of the Cr-O bond [45] detected at 841 cm−1, together with the bands around 360 cm−1.
Cadmium yellow (CdS, ZnO) (Figure 6e) is characterized by the bands at 298 and 597 cm−1 assigned to the longitudinal optic phonon (LO) + 2E2 and the overtone 2LO + 2E2 of the CdS crystal lattice [84]. The band at 212 cm−1 is related to the longitudinal acoustic (LA) phonon modes of the ZnS crystal lattice [85]. The unlabeled bands at 987 cm−1 and 457 cm−1 correspond to the symmetric stretching mode of barite SO4 tetrahedra, probably added as a filler.
Yellow ochre and raw sienna (Figure 6f,g)—which are mainly constituted by goethite chromophore—are detected by bands placed at 300 cm−1 (Fe-OH symmetric bending), 387 cm−1 (Fe-O-Fe/-OH symmetric stretching), and 549 cm−1 (Fe-OH asymmetric stretching) [86]; in addition, the gypsum bands are present (1008 and 1134 cm−1).

3.4. Red Pigments

The 852 nm Raman spectra of six red pigments—caput mortuum, umber, red bole, cinnabar, Carmin naccarat, and Alizarin crimson—are reported in Figure 7a–f. As for the yellow pigments, also the Raman spectra of red pigments at 852 nm are very similar to those generated with a 785 nm laser.
Caput mortuum, umber, and red bole are detectable through their major intensity bands related to iron oxides modes placed at 222 and 494 cm−1 (symmetric stretching vibration of Fe-O), 295 and 612 cm−1 (symmetric bending of Fe-O), and 407 cm−1 (symmetric stretching of Fe-O-Fe/-OH) [45]. While caput mortuum and umber (Figure 7a,b) have the same spectral trend in terms of relative intensity between the bands, being the band at 295 cm−1 the most intense, the major band of red bole (Figure 7c) is that at 407 cm−1.
The Raman spectrum of cinnabar (Figure 7d) is characterized by a very intense band at 251 cm−1 with a shoulder at 284 cm−1, and a band at 343 cm−1 is also present. These bands are assigned to HgS stretching modes and are orientation-dependent [87].
Carmin naccarat (Figure 7e), an aluminum lake of carminic acid, also known as cochineal lake, is detectable by the bands placed at 471 cm−1 (skeletal vibrations); 1110 and 1530 cm−1 (δ(OH) + δ(CH)); 1250, 1358, 1426 cm−1 (Gly); 1312 cm−1 (δ(OH) + δ(CH) + δring); 1491 cm−1 (δCH(m)) [88].
Raman spectrum of Alizarin crimson (Figure 7f) is characterized by bands placed at 1480, 1328, and 1292 cm−1 and other characteristic bands at 238, 483, 655, 840, 903, 1020, 1047, 1164, 1190, 1223, 1358, 1522, 1574, and 1639 cm−1.

3.5. White and Black Pigments

The spectra of the five white pigments investigated—lead white, gypsum, calcite, barite, and zinc oxide—are reported in Figure 7a–d together with that of ivory black (Figure 7f). Lead white (PbCO3)2·Pb(OH)2) (Figure 8a) is detected by the Raman band at 1051 cm−1 (symmetric stretching of CO32− ion) [89] and by the bands at 1365 and 1484 cm−1 due to asymmetric stretching mode of CO32− [68]. Other lower-intensity bands are placed at 329, 417, and 681 cm−1.
Raman spectrum of selenite (Figure 8b), a variety of gypsum, is characterized by intense bands at 1008 cm−1 (symmetric stretching of SO42− group) and 1133 cm−1 (asymmetric stretching mode of SO42−). Bands at 316 cm−1 (translational mode of H2O, Ca) [78], 413 and 495 cm−1 (δ SO42−), and 620 and 674 cm−1 (ν4, SO4) are also observed [78].
Raman spectrum of calcite (CaCO3) (Figure 8c) is characterized by the band placed at 285 cm−1 due to external vibration of the CO32− groups that involve translator oscillations of these groups (relative translation between cation and anionic group) and by the band at 1089 cm−1 due to symmetric stretching of CO32−. Even the bands at 714 cm−1 (ν4, CO32−) and 1437 cm−1 (ν3, CO32−) are present in the Raman spectra of calcite [68].
Barite (BaSO4) (Figure 8d) is identified by intense Raman bands at 989 cm−1 and 460 cm−1 symmetric stretching of SO42−; 619 and 644 cm−1 (asymmetric bending of SO42−); 1079, 1105, 1141, and 1166 cm−1 (asymmetric stretching of SO42−) [90].
Raman band at 440 cm−1 is characteristic of zinc oxide (ZnO) (Figure 8e), together with 336 and 581 cm−1. Bands at 1082 and 1156 cm−1 are assigned to the second-order Raman spectrum arising from zone boundary phonons [91].
In the Raman spectrum of Ivory black ([Ca3(PO4)2]+CaCO3+C) (Figure 8f), a carbon black pigment is detected by a couple of broad bands at 1336 and 1600 cm−1; the former (C-C D1 band) is assigned to edge effect, plane defects, and heteroatoms as oxygen or double bonds. Other impurities are considered responsible for this broad band. The D1 band position is found to cover the whole range of 1240 to 1400 cm−1, and the C-C D2 band at 1600 cm−1 is assigned to C-C in-plane symmetrical stretching [19]. Notably, the 961 m (ν1(a1) PO43−) is not observed. The broad bands characterizing carbon-black pigments are occasionally subtracted by PCA pSSE elaboration. which has proven slightly inefficient [45]. As a result, it is always recommended to view the raw spectra. In this spectrum, a band at 1475 cm−1 marked with an asterisk is due to an internal optical contribution of the employed instrumentation.

4. Mock-Up Panel Painting

To test the applicability of the 852 nm laser excitation wavelength of the Bravo instrument in possible real cases, a wooden panel tempera painting with blue pigments (Figure 2) was analyzed. Raman spectra (raw CH2) acquired directly on the mock-up panel painting are compared with reference powder pigments in Figure 9, showing a good spectral correspondence.
The azurite paint layer (Figure 9a) is characterized by the main bands at 404 and 320 cm−1, the spectrum obtained from the mock-up being more intense. Indigo (Figure 9b) in the mock-up is detected by its major band at 1571 cm−1, and the spectrum is dominated by the bands of gypsum (marked with red dots in the spectrum) from the ground layer.
The spectrum of Lapis lazuli (Figure 9c) is characterized by the band of lazurite at 548 cm−1. Other photoluminescence signals are observed as discussed earlier (Figure 4h). The results obtained from the smalt paint layer (Figure 9d) contain all the characteristic bands of the pigments in addition to those arising from the preparation layer.
Among all the spectra registered from the paint model, the spectrum from cobalt blue (Figure 10a) is the most challenging to interpret because the bands from the pigment are located in proximity to the characteristic peaks arising from gypsum in the ground layer. Despite this inconvenience, it is possible to identify the peaks of cobalt blue.
Ultramarine pigment (Figure 10b) is well detected in the mock-up by the band at 548 cm−1; interestingly, the spectrum is free from the photoluminescence phenomena as observed earlier for pigment powder. For the Prussian blue layer, the entire spectrum (obtained by merging the spectra from 852 nm and 785 nm excitation laser) is reported in Figure 10c. In fact, the major bands of Prussian blue are placed at 2095 and 2150 cm−1, as observed for both the mock-up and the pigment powder.

5. Conclusions

The creation and the availability of a spectral database of reference materials are crucial to enable conservation scientists to rapidly identify and study unknown pigments commonly employed in artworks. This work provides the Raman spectra of 32 pigments of historic and artistic importance acquired at 852 nm laser excitation wavelength. This wavelength proved suitable for the detection of all the pigments investigated even though some of them present a strong fluorescence background. Indeed, despite the weak signal in the Raman spectra of azurite, malachite, viridian, and cadmium yellow, their identification possibly outperforms the 785 nm laser excitation wavelength in some cases.
Some differences in the spectral shape and relative intensity of the Raman peaks between spectra obtained with 852 nm and 785 nm laser excitation wavelength were detected for some blue and green pigments. The results suggest that the spectra of pigments such as cobalt blue and smalt exhibit inversions in the relative intensity of the bands at low wavenumbers. An interesting case is represented by Lapis lazuli that upon 852 nm laser wavelength excitation produces a characteristic photoluminescence pattern that differs from that of synthetic ultramarine. The comparison between the two laser excitation wavelengths also highlighted the main advantage of using longer wavelengths for the detection of viridian. The 785 nm spectra of viridian are completely masked by fluorescence; while with 852 nm laser excitation wavelength, the main bands are detectable.
Among the aspects to be considered is the covered spectral range by the 852 nm excitation laser (170–2000 cm−1) that excludes the main bands of some pigments, e.g., Prussian blue, massicot, and lead–tin yellow. Anyway, the presence of the other characteristic bands of each pigment leads to their identification. Also considering the 785 nm excitation laser employed by Bravo, the spectral range is extended to 3200 cm−1 enabling to identify the Prussian blue uniquely.
Our future prospects concern the expansion of this online database with other natural and synthetic pigments as well as organic dyes.

Author Contributions

Conceptualization, J.S.; methodology, S.I. and J.S.; software, S.I.; validation, J.S., A.C., S.P. and M.G.; formal analysis, S.I.; investigation, S.I.; data curation, S.I. and D.Q.B.; writing—original draft preparation, S.I.; writing—review and editing, J.S., D.Q.B., A.C., S.P. and M.G.; supervision, J.S.; project administration, J.S.; funding acquisition, J.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by PNRR H2IOSC (Humanities and Cultural Heritage Italian Open Science Cloud) Project (IR0000029), CUP_B63C2200073005, founded by Next Generation EU. The contents reflect only the authors’ view, and the European Commission is not responsible for any use that may be made of the information it contains. The SHINE project funding (Strengthening the Italian Nodes of E-RIHS, Avviso 424/2018 dell’Azione II.1 PON R&I 2014–2020, DD n. 461 del 14-03-2019, PIR01_00016, CUP B27E19000030007) is acknowledged.

Data Availability Statement

The data presented in this study are available on request from the corresponding author and will be made available online at https://hsg.ino.cnr.it/ (accessed on 26 May 2024).

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Appendix A

Table A1. Summary of the main Raman bands at 852 nm laser excitation (the most intense band is underlined) and their tentative assignment as from the literature.
Table A1. Summary of the main Raman bands at 852 nm laser excitation (the most intense band is underlined) and their tentative assignment as from the literature.
NameBand Wavenumber (cm−1) and Relative Intensity at 852 nm of Laser Excitation Raman AssignmentsFigure
Cobalt blue [64] 204Complete transition of AO4Figure 4a
410 δs AO4
514, 615νas AO4
754νs AO4
Prussian blue [65]275, 538 Fe-CN-Fe deformation vibration and νs Fe-C Figure 4b
Antwerp blue275, 533* See Prussian blueFigure 4c
Ultramarine blue [67]258δS3−Figure 4d
375-
548νsS3-
10963νsS3−
Azurite [68]320, 404Lattice mode
Figure 4e
939δO-H out-of-plane
1095νsCO3
Indigo [70]674-Figure 4f
756-
1225ρN-H
1360, 1571, 1622νsC=C, C=O, and N–H
1470ρC-H
1571νs C=C, C=O, and N–H
Smalt [71]195, 481, 620The breathing vibration mode of the T–O–T substructure Figure 4g
689Si-O-Si vibration mode of SiO4
Lapis lazuli [74]548νsS3-Figure 4h
1306Luminescence band of minerals accompanying genuine lazurite
Green earth [20]200, 277Internal vibrations of the MoO6Figure 5a
393, 545vibrational
Modes of the SiO4
Cobalt green [20]340, 460, 520-Figure 5b
713νs CoO6
Chromium oxide [80]297, 350, 528, 615Eg Figure 5c
554A1g
Viridian [81]262CrO2Figure 5d
487, 584α-CrOOH
Malachite [69]219, 270, 350, 430Lattice modeFigure 5e
816, 834δs CO3
1095νs CO3
Massicot [45]290, 385-Figure 6a
Lead–tin yellow [82]197νs Pb-OFigure 6b
275, 290, 455, 524-
Naples yellow [83] 290, 318, 348, 440Vibrational mode of Sb-O and PbOFigure 6c
510νs Sb-O
613-
Chrome yellow [45]339, 360, 379, 402-Figure 6d
841νs Cr-O
Cadmium yellow [84,85]212LA of ZnS crystal lattice Figure 6e
298, 597LO+2E2 + overtones 2LO+2E2 of CdS crystal lattice
Yellow ochre [86]240, 480-Figure 6f
300δsFe-OH
387νs Fe-O-Fe/-OH
549νas Fe-OH
Raw sienna [86]240, 480-Figure 6g
300δsFe-OH
390νs Fe-O-Fe/-OH
550νas Fe-OH
Caput mortuum and umber [45]222, 494νs Fe-OFigure 7a,b
295,612δs Fe-O
407νs Fe-O-Fe/-OH
Red bole [86]225νs Fe-O Figure 7c
296, 614Symmetric bending Fe-O
407νas Fe-O-Fe/-OH
Cinnabar [87]251, 284, 343 νs HgS Figure 7d
Carmin naccarat [88]471Skeletal vibrationsFigure 7e
1110δ(OH) + δ(CH)
1252Gly
1313δ(OH) + δ(CH) + δring
1358Gly
1426Gly
1491δCH(m
1530δ(OH) + δ(CH)
-
Alizarin crimson238, 483, 655, 840, 903, 1020, 1047, 1164, 1190, 1223, 1292, 1328, 1358, 1480, 1522, 1574,1639-Figure 7f
Lead white [68,89]329, 417-Figure 8a
681, 1051CCO32−
1365, 1484 νasCO32−
Selenite, gypsum [78]316T(H2O, Ca) Figure 8b
413, 495 δSSO42−
620, 674δasSO42−
1008νsSO42−
1133, 1476νasSO42−
Calcite [68]285Translatory oscillations of CO3Figure 8c
714δasCO32−
1087νaCO32−
1437νasCO32−
Barite [90]460δsSO42−Figure 8d
619, 644δasSO42−
989νsSO42−
1079, 1105, 1141, 1166νasSO42−
Zinc oxide [91]440, 336, 581-Figure 8e
1156Boundary photons
Ivory black [19]1336C-C A1gFigure 8f
1603C-C in-plane stretching
δs: symmetric bending; δas: asymmetric bending; νs: symmetric stretching; νas: asymmetric stretching; ρ: rocking; LO: longitudinal optic phonon; LA: longitudinal acoustic phonon; Gly: normal modes localized on glycosidic moiety; A1g: A: symmetric species concerning the principal axis of symmetry, 1 (subscript): symmetric concerning a C2 axis that is perpendicular to the principal axis, g (subscript): symmetric concerning a center of symmetry; E2g: E: doubly degenerate, two-dimensional irreducible representation, g (subscript): symmetric concerning a center of symmetry; T: transitional model.

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Minerals 14 00557 g001
Figure 1. Number of papers exploiting the Bravo spectrometer in heritage science from its marketing year to date.
Figure 1. Number of papers exploiting the Bravo spectrometer in heritage science from its marketing year to date.
Minerals 14 00557 g001
Minerals 14 00557 g002
Figure 2. Mock-up panel painting of blue pigments. The pigment is indicated under each square. Azz.: azurite; INDACO: indigo; LAPIS.: Lapis lazuli; Smalt; COB. S.: cobalt blue; PRUS.: Prussian blue; OLT. ART.: ultramarine.
Figure 2. Mock-up panel painting of blue pigments. The pigment is indicated under each square. Azz.: azurite; INDACO: indigo; LAPIS.: Lapis lazuli; Smalt; COB. S.: cobalt blue; PRUS.: Prussian blue; OLT. ART.: ultramarine.
Minerals 14 00557 g002
Minerals 14 00557 g003
Figure 3. The overall set-up for Raman measurements of pigments placed on a sheet of aluminum (left) and shown in detail (right).
Figure 3. The overall set-up for Raman measurements of pigments placed on a sheet of aluminum (left) and shown in detail (right).
Minerals 14 00557 g003
Minerals 14 00557 g004aMinerals 14 00557 g004b
Figure 4. Raman spectra of blue pigments: (a) cobalt blue, (b) Prussian blue, (c) Antwerp blue, (d) ultramarine, (e) azurite, (f) indigo, (g) smalt, and (h) Lapis lazuli. All the Raman spectra acquired with 852 nm laser excitation wavelength are reported in black lines, while those acquired with 785 nm in green lines (a,g,h).
Figure 4. Raman spectra of blue pigments: (a) cobalt blue, (b) Prussian blue, (c) Antwerp blue, (d) ultramarine, (e) azurite, (f) indigo, (g) smalt, and (h) Lapis lazuli. All the Raman spectra acquired with 852 nm laser excitation wavelength are reported in black lines, while those acquired with 785 nm in green lines (a,g,h).
Minerals 14 00557 g004aMinerals 14 00557 g004b
Minerals 14 00557 g005
Figure 5. Raman spectra of green pigments: (a) green earth, (b) cobalt green, (c) chromium oxide, (d) viridian, and (e) malachite. Raman spectra acquired with 852 nm laser excitation wavelength are reported in black lines, while those with 785 nm are in green lines (a,d).
Figure 5. Raman spectra of green pigments: (a) green earth, (b) cobalt green, (c) chromium oxide, (d) viridian, and (e) malachite. Raman spectra acquired with 852 nm laser excitation wavelength are reported in black lines, while those with 785 nm are in green lines (a,d).
Minerals 14 00557 g005
Minerals 14 00557 g006aMinerals 14 00557 g006b
Figure 6. Raman spectra of yellow pigments: (a) massicot, (b) lead–tin yellow type I, (c) Naples yellow, (d) chromium yellow, (e) yellow ochre, (f) raw sienna, and (g) raw sienna.
Figure 6. Raman spectra of yellow pigments: (a) massicot, (b) lead–tin yellow type I, (c) Naples yellow, (d) chromium yellow, (e) yellow ochre, (f) raw sienna, and (g) raw sienna.
Minerals 14 00557 g006aMinerals 14 00557 g006b
Minerals 14 00557 g007aMinerals 14 00557 g007b
Figure 7. Raman spectra of red pigments: (a) caput mortuum, (b) umber, (c) red bole, (d) cinnabar, (e) Carmin naccarat, and (f) Alizarin crimson.
Figure 7. Raman spectra of red pigments: (a) caput mortuum, (b) umber, (c) red bole, (d) cinnabar, (e) Carmin naccarat, and (f) Alizarin crimson.
Minerals 14 00557 g007aMinerals 14 00557 g007b
Minerals 14 00557 g008
Figure 8. Raman spectra of white and black pigments: (a) lead white, (b) gypsum, (c) calcite, (d) barite, (e) zinc oxide, and (f) ivory black. The asterisk indicates a band due to the glass of the tip.
Figure 8. Raman spectra of white and black pigments: (a) lead white, (b) gypsum, (c) calcite, (d) barite, (e) zinc oxide, and (f) ivory black. The asterisk indicates a band due to the glass of the tip.
Minerals 14 00557 g008
Minerals 14 00557 g009
Figure 9. Raman spectra of blue pigments in mock-up panel painting: (a) azurite, (b) indigo, (c) lapis lazuli, and (d) smalt. Pink lines (named mock-up) indicate the Raman spectra acquired on mock-up panel painting, and cyan lines (named powder) refer to Raman spectra of pigments in powder. All the spectra are acquired with an 852 nm laser excitation wavelength.
Figure 9. Raman spectra of blue pigments in mock-up panel painting: (a) azurite, (b) indigo, (c) lapis lazuli, and (d) smalt. Pink lines (named mock-up) indicate the Raman spectra acquired on mock-up panel painting, and cyan lines (named powder) refer to Raman spectra of pigments in powder. All the spectra are acquired with an 852 nm laser excitation wavelength.
Minerals 14 00557 g009
Minerals 14 00557 g010
Figure 10. Raman spectra of blue pigments in mock-up panel painting: (a) cobalt blue, (b) ultramarine blue, and (c) Prussian blue (spectral range 170–2500 cm−1). Pink lines (named mock-up) indicate the Raman spectra acquired on mock-up panel painting, and cyan lines (named powder) refer to the Raman spectra of pigments in powder. All the spectra are acquired with an 832 nm laser excitation wavelength, and Prussian blue is also acquired with a 785 nm laser excitation wavelength.
Figure 10. Raman spectra of blue pigments in mock-up panel painting: (a) cobalt blue, (b) ultramarine blue, and (c) Prussian blue (spectral range 170–2500 cm−1). Pink lines (named mock-up) indicate the Raman spectra acquired on mock-up panel painting, and cyan lines (named powder) refer to the Raman spectra of pigments in powder. All the spectra are acquired with an 832 nm laser excitation wavelength, and Prussian blue is also acquired with a 785 nm laser excitation wavelength.
Minerals 14 00557 g010
Table 1. Overview of the main published (1997–2019) reference Raman spectral libraries of artistic materials, their accessibility, examined material, and excitation wavelength.
Table 1. Overview of the main published (1997–2019) reference Raman spectral libraries of artistic materials, their accessibility, examined material, and excitation wavelength.
LibrariesData FormatAnalyzed MaterialsExcitation Wavelength (nm)
ULC [3]OnlineNatural and synthetic pigments514, 633
IRUG [11]OnlineMinerals, pigments, organic dyes, synthetic resins532, 633, 785
Romanian Database of Raman Spectroscopy [12]OnlineMinerals532
RRUFF [6]OnlineMinerals and pigments532, 785
IRDGOnlinePigments1062
SOPRANO [13]OnlineSynthetic organic pigments (SOPs)785
CHSOS—Pigments Checker [9]OnlinePigments 532, 638, 785, 830, 1064
CAMEO Materials database (Museum of Fine Arts—Boston)OnlinePigments and other art materials 514, 532, 632, 785
Vandenabeele et al. [14]Published paperAzo pigments785
Burgio and Clark [10]Published paperPigments, minerals, pigments media, and varnishes514, 633, 647, 780, 1064
Bouchard and Smith [15]Published paperMinerals on corroded metals and colored glass514, 632
Scherrer et al. [16]Published paperSynthetic organic pigments514, 633, 785
Martina et al. [17]Published paperSilver corrosion products532
Pozzi et al. [18]Published paperWatercolor’s dyes488, 785
Coccato et al. [19,20]Published paperCarbon-based black pigments and green minerals532, 785
Marucci et al. [21]Published paperMedieval pigments488, 532, 633, 785, 830
Culka and Jehlicka [22]Published paperGemstones785, 852
ColoRaman database [23]No longer available onlineOil, tempera, and fresco paint pigments531, 633, 780
E-Visart [24]No longer available onlinePigments785, 1062
Sessa et al. [25]Published paperSynthetic organic pigments (SOPs) in watercolor, tempera, and oil paints532, 638, 785
Table 2. Analyzed pigments: name and stereoscopic image, chemical composition, color index and number, supplier, and article number. The scale bar shown in the first row is valid for all the images.
Table 2. Analyzed pigments: name and stereoscopic image, chemical composition, color index and number, supplier, and article number. The scale bar shown in the first row is valid for all the images.
Name and Stereoscopic ImageChemical Composition [61,62,63]Colour Index (C.I.) Name and Number Supplier and Article Number
Blue pigmentsCobalt blueMinerals 14 00557 i001Cobalt(II) oxide–aluminum oxide, cobalt(II) aluminate
CoOAl2O3 (CoAl2O4)		PB28
No. 77346		Kremer, 45710
Prussian blueMinerals 14 00557 i002Iron(III) hexacyanoferrate(II)
Fe4[Fe(CN)6]3		PB27
No. 77510		F.lli Maimeri
Antwerp blueMinerals 14 00557 i003Prussian blue + Cobalt blue-n.d.
Ultramarine blueMinerals 14 00557 i004Sodium silicate containing sulfur and aluminum
(Na,Ca)8[(SO4,S,Cl)2(AlSiO4)6]		PB29
No. 77007		Kremer, 10500
AzuriteMinerals 14 00557 i005Basic copper carbonate
2CuCO3·Cu(OH)2		PB30
No. 77420		Kremer, 10201
IndigoMinerals 14 00557 i0062-(3-oxo-2,3-dihydro-1H-indol-2-ylidene)-2,3-dihydro-1H-indol-3-one
C16H10N2O2		NB1
No. 75780		Kremer, 36000
SmaltMinerals 14 00557 i007Potassium glass containing cobalt
SiO2 + K2O + Al2O3 + CoO		PB32
No. 77365		Kremer, 10010
Lapis lazuliMinerals 14 00557 i008Sodium silicate containing sulfur and aluminum
(Na,Ca)8[(SO4,S,Cl)2(AlSiO4)6]		PB29
No. 77007		Kremer, 10530
Origin Afghanistan
Green pigmentsGreen earthMinerals 14 00557 i009Hydrated iron (II-III) potassium silicate
K(Mg,Fe2+)(Fe3+,Al)[Si4O10](OH)2, and/or (K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2		N/A
C.I. PG23
No. 77009		Hopkins
Cobalt greenMinerals 14 00557 i010Cobalt(II) zincate
Co2TiO4
PG50
No. 77377		Kremer, 44100
Chromium oxideMinerals 14 00557 i011Chromium(III) oxide
Cr2O3
PG17
No. 77288		Winsor & Newton
MalachiteMinerals 14 00557 i012Basic copper (II) carbonate
CuCO3·Cu(OH)2
PG39
No. 77492		Kremer, 10310
ViridianMinerals 14 00557 i013Hydrated chromium(III) oxide
Cr2O3·2H2O		PG18
No. 77289		Winsor & Newton
Yellow pigmentsMassicotMinerals 14 00557 i014Orthorhombic lead oxide
β-PbO		PY46
No. 77577		Kremer, 43010
Lead tin yellow
type I		Minerals 14 00557 i015Lead(II) stannate
Pb2SnO4		N/A
No. 77629		Kremer, 10110
Naples YellowMinerals 14 00557 i016Lead (II) antimonate
Pb2Sb2O7		PY41
No. 77588		Kremer, 13100
Chrome yellowMinerals 14 00557 i017Lead (II) chromate
PbCrO4		PY34
No. 77600		Kremer, 43700
Cadmium yellowMinerals 14 00557 i018Cadmium (II) sulfide, zinc oxide
(CdS, ZnO)		PY35
No. 77117		Roberson Ltd.
Yellow ochreMinerals 14 00557 i019Iron(III) oxide–hydroxide
FeO(OH)		PBr6
No. 77499		Kremer, 10220
Raw siennaMinerals 14 00557 i020Iron (III) oxide–hydroxide
FeO(OH)		PBr6
No. 77491		Winsor & Newton
Red pigmentsCaput mortuumMinerals 14 00557 i021Iron (III) oxide
Fe2O3		PR101
No. 77491		Kremer, 48750
UmberMinerals 14 00557 i022Iron (III) oxide
Fe2O3		PBr7
No. 77492		Kremer, 40700
Red boleMinerals 14 00557 i023Iron (III) oxide
Fe2O3		PR102
No. 77015		Kremer, 40503
CinnabarMinerals 14 00557 i024Mercury(II) sulfide
HgS		PR106
No. 77766		Kremer, 10600
Carmin naccaratMinerals 14 00557 i0252-Anthracene-carboxylic acid, 7-β-D-glucopyranosyl-9,10-dihydro-3,5,6,8-tetrahydroxy-1-methyl-9,10-dioxo-
C22H2OO13		NR4:1
No. 75470		Kremer, 42100
Alizarin crimsonMinerals 14 00557 i026Dihydroxy-9,10-anthracene-dione calcium salt (1:1)
C14H8CaO4		PR83
No. 58000:1		Winsor & Newton
White and black pigmentsLead whiteMinerals 14 00557 i027Lead (II) carbonate, basic
(PbCO3)2·Pb(OH)2		PW1
No. 77597		Kremer 46060
GypsumMinerals 14 00557 i028Calcium (II) sulfate dihydrate
CaSO4·2H2O		PW25
No. 77231		Kremer, 11800
CalciteMinerals 14 00557 i029Calcium (II) carbonate
CaCO3		PW18
No. 77220		Merck, 2066
BariteMinerals 14 00557 i030Barium (II) sulfate
BaSO4		PW22
No. 77120		CarloErba, 425497
Zinc oxideMinerals 14 00557 i031Zinc (II) oxide
ZnO		PW4
No. 77947		Merck, 8849
Ivory blackMinerals 14 00557 i032Calcium (II) phosphate, calcium (II) carbonate, carbon
[Ca3(PO4)2]+CaCO3+C		Bk9
No. 77267		Winsor & Newton
Table 3. Experimental conditions (spectral range, detector integration time, and number of scans) for the samples in powder and mock-up panel painting for the measurement with the pSSE instrument.
Table 3. Experimental conditions (spectral range, detector integration time, and number of scans) for the samples in powder and mock-up panel painting for the measurement with the pSSE instrument.
Samples Spectral Range
(cm−1)		Integration Time (s)Number of Scans
powdersCobalt blue170–1200220
Prussian blue170–12001025
Antwerp blue170–1200505
Ultramarine170–1800130
Azurite170–1450520
Indigo170–18000.510
Smalt170–1200510
Lapis lazuli170–1800120
Calcite,
Barite		170–2000
Green earth170–12001010
Colbalt green170–1200510
Chromium oxide170–120055
Viridian170–1200515
Malachite170–1200205
Massicot170–120055
Lead tin yellow,
Chromium yellow,
Cinnabar		170–1200110
Alizarin crimson170–1800
Naples yellow,
Gypsum		170–1200
170–2000		15
Cadmium oxide170–1200215
Yellow ochre,
Caput mortuum,
Umber,
Red bole		170–1200515
Carmin naccarat170–1800
Ivory black170–2000
Raw sienna170–1200105
Lead white170–20001.510
Zinc oxide170–14001.530
temperaAzurite 170–145052
Indigo
Lapis lazuli		170–1800
170–1200		1.515
Smalt 170–1200115
Cobalt blue 170–1200120
Ultramarine blue
Prussian blue 		170–1200
170–2500		1.510
Table 4. Peak frequencies and their relative intensities of Lapis lazuli under different laser excitation wavelengths.
Table 4. Peak frequencies and their relative intensities of Lapis lazuli under different laser excitation wavelengths.
Measured Peaks (cm−1) in This Work
Lapis Lazuli Kremer #10530		References Peaks (cm−1) [9]
Lapis Lazuli Kremer #10510
785 nm852 nm532 nm638 nm785 nm1064 nm
260 m260 m
285 sh285 sh
312 vs *
350 m *
405 w *
513 m *
364 m364 m
548 s548 s548 vs548 vs548 vs548 s
583 vw583 sh583 sh583 sh583 m
712 w *
808 w808 w
835 s *
985 m *
1096 m1096 m
1306 s * 1346 m
1358 vwSplit into two bands
1644 m1644 m
v: very; s: strong; m: medium; w: weak; * luminescence phenomena.
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Innocenti, S.; Quintero Balbas, D.; Galeotti, M.; Cagnini, A.; Porcinai, S.; Striova, J. Historical Pigments and Paint Layers: Raman Spectral Library with 852 nm Excitation Laser. Minerals 2024, 14, 557. https://doi.org/10.3390/min14060557

AMA Style

Innocenti S, Quintero Balbas D, Galeotti M, Cagnini A, Porcinai S, Striova J. Historical Pigments and Paint Layers: Raman Spectral Library with 852 nm Excitation Laser. Minerals. 2024; 14(6):557. https://doi.org/10.3390/min14060557

Chicago/Turabian Style

Innocenti, Silvia, Diego Quintero Balbas, Monica Galeotti, Andrea Cagnini, Simone Porcinai, and Jana Striova. 2024. "Historical Pigments and Paint Layers: Raman Spectral Library with 852 nm Excitation Laser" Minerals 14, no. 6: 557. https://doi.org/10.3390/min14060557

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