Results of samples characterization are summarized in Table 2. The detailed characterization, divided by each technique, is reported in Table S1 in SI.
Table 2
Results of the multi-analytical characterization performed on the samples.
Sample name
|
Elemental composition
|
Crystalline phases
|
Molecular
composition
|
PL properties
|
Interpretation
|
ARTWORKS
|
A2
|
Cd, S (Zn)
|
Poorly crystalline CdS/Cd1 − xZnxS
|
CdS/Cd1 − xZnxS (Raman)
Drying Oil (FTIR, GC-MS: much likely linseed oil)
Carboxylates (FTIR)
Cd sulfates (FTIR, Raman)
Oxalates (FTIR)
|
Homogeneous µs emission along the paint stratigraphy, broad and weak, peaked at 700 nm (DTS CdS/Cd1 − xZnxS)
|
Poorly crystalline Cd1 − xZnxS (x < < 1), oil binder, degradation products/compounds due to oil/pigment interaction/additives: sulfates, carboxylates, oxalates
|
A7
|
Cd, S (Zn)
|
Poorly crystalline CdS/Cd1 − xZnxS (bulk), 3CdSO4·8H2O (surface)
|
CdS (Raman, µXANES)
Drying oil (FTIR, GCMS: much likely linseed oil)
carboxylates (FTIR)
Cd sulfates (FTIR, Raman, µXANES)
|
Spatially heterogeneous emission, with the paint surface displaying a broad and intense µs emission peaked at 695 nm (DTS CdS/Cd1 − xZnxS), very weak emission from the bulk
|
Poorly crystalline Cd1 − xZnxS (x < < 1), oil binder, degradation products/compounds due to oil/pigment interaction/additives: sulfates, carboxylates
|
A9
|
Cd, S (Al, Si)
|
Poorly crystalline CdS (bulk), 3CdSO4·8H2O (surface)
|
CdS (Raman, µXANES)
Drying oil (GCMS: much likely safflower oil)
Cd sulfates (Raman, µXANES)
|
Spatially heterogeneous emission, with the paint surface displaying a broad and intense µs emission peaked at 690 nm (DTS CdS/Cd1 − xZnxS), very weak emission from the bulk
|
Poorly crystalline CdS, oil binder, degradation products: sulfates
|
OIL PAINT TUBES
|
A3
|
Cd, S (Mg, Al)
|
Crystalline hexagonal CdS
|
Drying oil (FTIR, GCMS: much likely linseed oil)
Carboxylates (FTIR)
Mg carbonate (FTIR)
Mg sulfate (FTIR)
Traces of beeswax and alkyd-based resin (GCMS)
|
Homogeneous emission in the paint layer, with a sharp and strong ns peak at 517 nm (NBE CdS) and a broad µs peak above 800 nm (DTS CdS)
|
Crystalline CdS, oil binder, magnesium carbonate additive
|
A5
|
Cd, S (Zn)
|
Poorly crystalline CdS/Cd1 − xZnxS
|
CdS (Raman, µXANES)
Drying oil (FTIR, GCMS: much likely safflower or sunflower)
Carboxylates (FTIR)
Cd sulfates (FTIR, Raman, µXANES)
Traces of Pinaceae resin (GCMS)
|
Homogeneous emission with a weak ns sharp peak at 485 nm (NBE Cd1 − xZnxS) and a broad µs emission peaked above 750 nm (DTS Cd1 − xZnxS)
|
Poorly crystalline Cd1 − xZnxS (x < < 1), oil binder, degradation products/compounds due to oil/pigment interaction/additives: sulfates, carboxylates
|
A6
|
Cd, S (Mg, Al)
|
Crystalline hexagonal CdS
|
Drying oil (FTIR, GCMS: much likely linseed oil)
Mg carbonate (FTIR)
Mg sulfate (FTIR)
Oxalates (FTIR)
|
Homogeneous emission, sharp and strong ns peak at 515 nm (NBE CdS), broad µs emission peak above 800 nm (DTS CdS)
|
Crystalline CdS, oil binder, magnesium carbonate additive, compounds due to oil/pigment interaction: oxalates
|
ARTIST PALETTES
ARTIST PALETTES
|
A1
|
Cd, S (Zn)
|
Poorly crystalline CdS/Cd1 − xZnxS
|
Drying oil (FTIR, GCMS: much likely linseed oil)
Carboxylates (FTIR)
Cd sulfates (FTIR, Raman)
Oxalates (FTIR)
Traces of Pinaceae resin (GCMS)
|
Spatially heterogeneous emission, with the paint surface displaying a broad and intense µs emission peaked at 655 nm (DTS CdS/Cd1 − xZnxS), weak emission from the bulk
|
Poorly crystalline Cd1 − xZnxS (x < < 1), oil binder, degradation products/compounds due to oil/pigment interaction/additives: sulfates, carboxylates, oxalates
|
A4
|
Cd, S (Zn)
|
Poorly crystalline CdS/Cd1 − xZnxS
|
CdS (Raman, µXANES)
Drying oil (FTIR, GCMS: much likely linseed oil)
Carboxylates (FTIR)
Cd sulfates (FTIR, Raman, µXANES)
Traces of Pinaceae resin (GCMS)
|
Homogeneous µs emission at 700 nm (DTS CdS/Cd1 − xZnxS)
|
Poorly crystalline Cd1 − xZnxS (x < < 1), oil binder, degradation products/compounds due to oil/pigment interaction/additives: sulfates, carboxylates
|
A12
|
Cd, S (Zn, Cl)
|
Poorly crystalline CdS/Cd1 − xZnxS (bulk), 3CdSO4·8H2O (surface)
|
CdS (Raman, µXANES)
Drying oil (FTIR, GCMS: much likely linseed oil)
Carboxylates (FTIR)
Cd sulfates (FTIR, Raman, µXANES)
Oxalates (FTIR)
Traces of Pinaceae resin (GCMS)
|
Spatially heterogeneous emission, with the paint surface displaying a broad and intense µs emission peaked at 650 nm (DTS CdS/Cd1 − xZnxS), weak emission from the bulk
|
Poorly crystalline Cd1 − xZnxS (x < < 1), oil binder, degradation products/compounds due to oil/pigment interaction/additives: sulfates, carboxylates, oxalates
|
Elemental composition and crystalline phases
Elemental analyses revealed that the samples from artworks, palettes and Lucien Lefebvre-Foinet paint tube (sample A5) are similar in elemental composition, containing as main elements Cd, S and low quantity of Zn (Fig. S3 and S4 in SI). In samples A3 and A6, only Cd and S were detected, together with small peaks for Mg, Al and Si. These elements may be indicative of the presence of additives such as kaolin and magnesium carbonate. Al and Si traces were also found in sample A9, while sample A12 present small traces of Cl.
X-ray diffraction analyses showed that the all artworks, palettes and paint tube A5 are composed by poorly crystalline CdS or Cd1 − xZnxS in mixed crystalline form (hexagonal and cubic). Due to the broad diffraction peaks (Fig. 2), it was not possible to establish the amount of Zn present evaluating the shift of the diffraction peaks with respect to the refence CdS pattern [34]. Instead, paint tubes A3 and A6 show the presence of crystalline hexagonal CdS (Fig. 2b). Moreover, most of the samples from artworks or palettes analysed through SR µXRD (A7, A9, A12), show the presence of cadmium sulfates hydrate (3CdSO4·8H2O) at the paint surface or within the paint layer (Fig. S5 in SI), ascribed to paint degradation.
Molecular composition
FTIR spectra from the pictorial/paint fragments and cross sections (Fig. S6 in SI) show the typical peaks associated with fatty acids at ca. 2920, 2850 [v(CH)], and 1710–1730 [ν(CO)] cm− 1, suggesting a lipid-containing binding medium [35]. In most of the samples, bands at 1560 − 1530 cm− 1 were also detected, ascribed to the presence of metal carboxylates (Cd and Zn ones) [36, 37], formed as a consequence of the reaction of free fatty acids with metal cations or intentionally added to the paint tube formulation as additives [38, 39]. Magnesium carbonate (ca 1450, 1415, 875, 820 cm− 1) was also detected in Mir paint tubes (samples A3 and A6). This compound was used in the formulation of 20th century oil paints tubes as an additive [38–41].
Cadmium sulfate, in hydrated form (3CdSO4·8H2O) was detected in all the samples from artworks and palettes. The sulfate was particularly evident in all the samples from artworks (broad band in 1000–1100 cm− 1 range and below 650 cm− 1) and from palettes A1 and A12. Interestingly, sulfates were also found in samples from the Lucien Lefebvre-Foinet paint tube (sample A5). Cd and/or Zn oxalates were also found in samples A1 and A2, through the absorption bands at ca 1620, 1320 and 820 cm− 1. These compounds can form due to the reaction between Zn2+ and Cd2+ ions of the pigment and the oxalate ions resulting from the oxidation of the oil binder [42, 43].
The presence of cadmium sulfate hydrate was also confirmed using Raman microscopy (Fig. S7 in SI), which detected the bands at 476, 983 and 1006 cm− 1, characteristic of the SO4 bending mode, and the oscillation of the free and bound SO4 ion respectively [44, 45]. The Raman spectra from the samples investigated also indicated bands related to cadmium sulfide and its multi-phonon resonant scattering (bands at 218, 238, 305, 346 and 633 cm− 1) [46, 47]. Bands related to a drying oil present as binding medium were detected at 869, 1063 and 1080 cm− 1 and, in sample A5, which was examined in the range 180 to 3000 cm− 1, additional bands for oil at 1304, 1441, 1657, 1740, 2853, and 2909 cm− 1 could also be detected [48].
The distribution of sulfides/sulfates in the paint cross-section was achieved through SR µXRF chemical state maps of exemplary samples A4, A7, A9 and A12 (Fig. 3 and Fig. S8 in SI). In the bulk of all the samples sulfides were detected, which can be associated to the preserved pigment (Cd1 − xZnxS). In the samples A7, A9 and A12, a layer of sulfates was found on the surface, while in sample A4 sulfides have been mainly detected in the whole paint layer, with a localized agglomerate of sulfates that well corresponds to the elemental distribution of Zn.
The organic composition of the paints analysed using GC-MS analysis showed that all the samples from paintings and studio materials are bound in dying oils. All chromatograms obtained after transesterification of the paint fragments and subsequent GC-MS analysis showed the presence of the typical fatty acids characteristic of siccative oils: saturated mono-fatty acids, from C8 to C26 (palmitic and stearic being the most abundant); saturated di-fatty acids (azelaic, suberic and sebacic acids in particular), as the most abundant oxidation products; unsaturated fatty acids (mainly oleic acid); glycerol (and glycerol derivatives); oxo-, hydroxy- and metoxy- octadecanoic acids as by-oxidation products of unsaturated fatty acids.
Based on the ratio between palmitic to stearic acid (P/S) and the presence of specific markers [30, 33, 39], samples from artworks were found to contain cold-pressed linseed oil (A2), stand-linseed oil (A7) and safflower oil (A9). This diversity of siccative oils used reflects the complexity of the composition of 20th century manufactured art oil paintings [30, 33, 39, 49]. Similarly, in the samples taken from the artist's oil paint tubes, the presence of a binder consisting of linseed oil was detected in A3 and A6, while safflower or sunflower oil was detected in sample A5. In addition, several organic additives were found, for example, in A3 traces of beeswax acting as a stabilizer, in A5 traces of a Pinaceae resin (i.e., Colophony), probably added as a thickener, and in A6 probably the presence of stearates added as dispersants.
Linseed oil and a Pinaceae resin were found in the samples taken from the artist's palette; the resin may be present as a commercial additive or added by the artist for the purpose of thickening the paint on the palette [30]. Different drying rates were registered, according to the Azelaic to Palmitic (A/P) and Oleic to Stearic (O/S) molar ratios, which may be related to both the different thickness of the samples and their exposure to oxygen. In addition, as is well known, the presence of metal cations can strongly influence the drying of oil paints. It is significant to note that in correspondence with the identification of Zn by elemental analysis, there was also a higher abundance of oleic acid, which can be quantified by the O/S molar ratio, which was already reported in literature as typical of XX century Zn-containing white paint [50, 51].
Photoluminescence properties
Under UV light, many of the paintings present a bright pink/orange luminescence from the deteriorated CdY areas, an emission that is absent in the well preserved yellow paints (Fig. 4). In the recent past this peculiar and intense emission has been observed also in other paintings with degraded CdY paints [15, 52] and its origin has been related with reactive cadmium yellow paints, possibly made starting from nanocrystalline or poorly crystalline CdS pigments [16, 17].
To characterize the luminescent properties of the paints, all samples were analysed with time-gated hyperspectral micro-imaging and results are summarized in Table 2. Paint tube samples containing crystalline CdS (samples A3 and A6) have a sharp nanosecond emission (peaked at 515 and 517 nm, respectively) and a microsecond emission spectrally broad and peaked above 850 nm. These two emission bands are related to the near band edge (NBE) and deep trap states (DTS) emissions of pure CdS, respectively [46, 53]. Instead, poorly crystalline CdY paint tube (sample A5) shows a weak NBE emission peaked at 485 nm, and a broad microsecond emission peaked above 750 nm, ascribed to Cd1 − xZnxS pigment (with \(\text{x~16}\)) accordingly to the position of the NBE emission [46, 53].
Most of the degraded samples from artworks and palettes present an emission, occurring at the microsecond timescale, heterogeneously distributed along the paint stratigraphy (Fig. S9 in SI). Indeed, as illustrated in Fig. 5a for sample A7, the paint surface displays an emission much more intense than the one from the bulk, suggesting that it should be linked to degradation at the paint surface. The strong intensity of the emission at the sample surface, which optically diffuses in the surrounding paint layers, prevents a quantitative comparison of the spectral and lifetime behaviour from the innermost parts of the paint. Despite this, it is worth noting that the emission at the paint surface, peaked at around 650–700 nm, occurs at shorter wavelengths than the emission observed in the poorly crystalline tube sample A5 (Fig. 5b). Indeed, the features of this intense microsecond emission resembles the ones observed in other artworks with degraded CdY [16] and can be ascribed to DTS emission of cadmium-based paints with a high density of crystal defects. It is worth noting that, due to the absence of NBE emission, it was not possible to exactly establish the possible Zn content on the basis of the NBE shift [46]. Nonetheless, the low percentage of Zn detected through elemental analysis does not account for the shift observed in the trap state emission.