3.2. Cross-Sections Analysis: Bluish Background (CS7), an Exemplificative Study
The preliminary analyses were fundamental to plan the sampling areas (see SM). Indeed, mapping the more recently retouched areas of over painting was necessary to avoid them during the sampling. Seven cross-sections were sampled from the painting and the
predella, and six from the frame. Each one was taken in order to answer specific questions about the materials employed in every layer of the stratigraphy at those precise points (
Figure 3). For the sake of brevity, only the most important and relevant results are presented (see SM).
The analysis of CS7, sampled in the bluish background behind the Virgin Mary and the clients, is the most exemplificative in the context of the present study.
Figure 4a shows the exact area where CS7 was sampled and, once embedded in a polyester resin block, it was analyzed using 50× magnification (
Figure 4b) and optical microscope in reflected light (
Figure 4c). At high magnifications (50×), all the layers composing the painting are visible. Starting from the bottom, we can find the ground layer, made of gypsum, which is necessary to make the support even and homogeneous, in this case, the wood panel. Then, it is possible to note the paint layer made up of a mixture of blue and white grains. The peculiar feature is that this sequence is repeated: a new ground and paint layer were added on the original ones in more recent times. Apparently, they may be composed of the same materials, but this must be investigated by means of elemental and molecular analyses.
First of all, the cross section was examined by SEM. The analysis of data in
Figure 5 confirms the presence of four layers: the 1st ground (I) (around 150 µm), characterized by light elements (darker areas); the 1st paint layer (II) (≈70 µm) composed by grains of quite light elements in a heavier matrix (brighter areas); the 2nd ground layer (III) (≈50 µm) and the 2nd paint layer (IV) (≈15 µm), both containing mainly light elements with some brighter spots which indicate the presence of heavy elements. EDS analysis provided important information on the elemental composition of the layers. EDS spectra inserted in
Figure 5 confirm the presence of Ca and S in the 1st ground layer, which agree with the likely use of gypsum (CaSO
4). The peak of Cu in the grains and the one of Pb in the matrix of the 1st paint layer suggest the presence of a blue copper pigment such as azurite [Cu
3(CO)
2(OH)
2] mixed with a white lead pigment, probably lead white [(PbCO
3)
2·Pb(OH)
2]. Ca and S are detected also in the 2nd ground layer, indicating again the presence of gypsum. Finally, the 2nd paint layer is mainly characterized by the presence of Ti, Si, Al, Na, S, Cu, and some Cr. The presence of Ti suggests the use of titanium white, TiO
2, a modern pigment introduced in 1920 [
30].
Additional EDS spectra were recorded, focusing on the bluish grains present in the 2nd paint layer, revealing the presence of S, Ba, Cr, and Co (see
Figure 6). The detection of S and Ba can agree with the presence of barite, BaSO
4. This natural occurring mineral has been synthetically produced from the beginning of the nineteen century to be extensively employed both as filler in the formulations of colors [
31] and used in the preparation of TiO
2 [
30]. Cr and Co could be attributed to the presence of a bluish chromium-based pigment: cobalt chromite, CoCr
2O
4 (PB36, cobalt chromite blue-green spinel) often substitutes the historically genuine cerulean blue (PB35, cobalt stannate), the latter being introduced in 1860 as a pigment [
32]. Unfortunately, no Raman or SERS spectral evidence allowed to confirm this supposition.
In order to obtain information at a molecular level, micro-Raman spectroscopy was applied.
Figure 7 shows the Raman spectra recorded analyzing defined points on the magnified cross section, indicated in
Figure 7a. The two ground layers were confirmed to contain gypsum, whose spectrum presents the typical band at 1009 (strong) cm
−1 (
Figure 7b) [
33]. Comparison with literature spectra [
33] proved that the 1st paint layer contains azurite, as indicated by the bands at 1578 (weak), 1423 (medium), 1096 (m), 832 (w), 770 (m), 401 (s), 247 and 83 (m) cm
−1 (
Figure 7c), and lead white, whose bands are detected at 1055 (s) and 401 (m) cm
−1 (
Figure 7d).
The results of the Raman analyses performed on the 2nd paint layer are exposed below. The presence of titanium white (PW6) in the form of anatase, was confirmed by the detection of its typical bands at 640 (m), 510 (m), 397 (m) and 143 (s) cm
−1 (
Figure 8) [
32].
The Raman characterization of the bluish pigment, due to the scarcity of grains and the tiny thickness of the layer, is particularly challenging. The Raman spectrum recorded on this layer allowed us to identify ultramarine blue: sodium polysulphide-aluminosilicate (PB29, Na
6-10Al
6Si
6O
24S
2-4, which is responsible for the Raman band at 548 cm
−1 (s) (the bands in gray belong to titanium white) (
Figure 9) [
34]. This attribution is supported by the detection of Si, Na, Al, S in the EDS spectrum reported in
Figure 5IV. Natural ultramarine is a mineral called
lazurite, a complex sulfur-containing sodium aluminosilicate based on a body-centered cubic lattice. Synthetic ultramarine was synthesized in 1828 by Jean Baptiste Guimet in Paris and then rapidly adopted by artists [
35]. Note that natural and synthetic ultramarine blue provide comparable spectral signatures and cannot be distinguished by Raman analysis.
During the analysis of the blue layer, we noticed that some spots gave strong fluorescence while others did not. Indeed, the spectrum in
Figure 9 refers to the non-fluorescent grains. Comparing the results obtained with the composition of the most common contemporary color tubes, it was clear that the presence of synthetic ultramarine alone was unlikely since this pigment is often mixed with synthetic organic dyes [
36]. Normal Raman spectroscopy did not provide spectra useful to solve this diagnostic issue because of the fluorescence, typically generated by some organic dyes and pigments (
Figure 10a). For this reason, we opted to employ the SERS technique using AgNSs as signal enhancers and fluorescence quenchers, in order to amplify the Raman spectrum of the fluorescent grains. After addition of the Ag nanostars [
14], the spectrum shown in
Figure 10b was collected. The fluorescent background is now dramatically lowered and a defined spectrum emerges being characterized by detectable bands at 1566 (m), 1514 (s), 1438 (w), 1400 (w), 1379 (m), 1349 (s), 1303 (m), 1142 (w), 1104 (s), 1002 (m), 720 (m), 679 (s), 649 (m), and 585 (w) cm
−1. Comparison with literature data indicate that these features corresponds to those of copper alpha-phthalocyanine (PB 15:2) [
37,
38,
39], a synthetic pigment. A detailed comparison of the experimental and the reference bands of PB 15:2 is reported in
Table S1 in Supplementary Materials. Interestingly, the presence of PB 15:2 agrees with the presence of the Cu signals in the EDS spectrum in
Figure 5II. Organic phthalocyanines and their metal complexes were synthesized at the beginning of the twentieth century and are widely used as blue and green pigments until the present time [
40]. Note that the band at 211 cm
−1 is produced by the interaction of Ag with ions present in the colloidal solution of nanoparticles [
41].