## 3 Results
A second version of this figure (Figure S1) reporting the vibrational modes assignments of the IR bands in the spectra is shown in supplementary materials. (Figure 5B, spectrum b) and matched very well with the signatures of waxes (e.g., beeswax, Carnauba, shellac wax, etc.) [16]. Concerning with layers underneath the silver leaf, the use of a protein-based glue was clearly confirmed, as shown by the typical spectral profile (Figure 5B, spectrum c). Calcium sulphate dihydrate was detected as well, which is consistent with the presence of gypsum in the ground (O-H stretching bands at 3540 and 3401 cm\({}^{-1}\), bending vibration of water at 1625 cm\({}^{-1}\) and vibration of sulphate at 1117 cm\({}^{-1}\)), particularly evident in the bulk of this layer (Figure 5B, spectrum d). In spectrum d, some weak bands not ascribable to the ground and red-labeled were attributable to the polyester embedding resin (1731, 1458, 1256, and 1275 cm\({}^{-1}\)). Moreover, DRIFT spectra of the ground (Figure 5A, spectrum d) figured out a strong and broad \(v_{1}+v_{3}\) (SO\({}_{4}\)\({}^{-2}\)) overtone and combination bands in the 2500-1900 cm\({}^{-1}\) range, with maxima at 2230 and 2130 cm\({}^{-1}\), and a strong inverted reststrahlen band at 1150-1160 cm\({}^{-1}\). These features are unequivocally ascribed to calcium sulphate [25]. The bands at 1636 and 1550 cm\({}^{-1}\) were also of amide I and II due to the proteinaceous material present in the ground. SEM-Back-Scattered Electrons(BSE) images of the Ag layer (Figure 6) in the cross-section showed a very thin (\(\approx\)100 nm) metal leaf following the shape of the ground underneath. At some points, the Ag layer followed precisely the shape of even single gypsum grains (i.e., the area in the cross-section indicated by a white arrow in Figure 6A) demonstrating an amazing accuracy in the execution technique and suggesting burnishing afterward. As observed in the UV microscope images, a classic bole did not appear here to have been applied since the presence of a layer of organic matter between the metal and the ground was not observed (Figure 6 A1, B). The Ag layer appeared rather be attached directly to the preparation layer most probably trough the protein-based glue detected by FTIR spectra (Figure 5B spectrum c). White arrows in Figure 7 indicate scratches in the ground filled with Ag that might suggest the application of more than one single metal leaf overlapped [26]. The elemental composition of the Ag layer was detected performing a SEM-EDS analysis on the surface of a sample not used for resin casting and cross-section preparation. The SEM-EDS spectrum and the area of analysis are shown in Figure 7. In particular, an area devoid of the uppermost varnish layers along with the metal leaf in a relatively good state of conservation (not blacked) was selected for the analysis. Results led to 80% of Ag and 2% of gold (Au) along with other elements such as calcium (Ca), silicon (Si), and aluminum (Al) mainly due to the ground [27]. The presence of chlorine (Cl) and sulphur (S) was ascribable to the degradation products of the silver leaf and Ag sulphide was detected by micro-Raman spectroscopy (see after). The confirmation of the provenance of Au from the Ag layer was achieved mapping the elemental composition of an extended area. In this respect, Figure 8 shows the SEM compositional map of the area A2 (Figure 7) confirming a distribution of gold totally overlapped to that of Ag. Conversely, no presence of gold was detected on the varnish layers ruling  
Figure 6: SEM-BSE images of the silver leaf. Figure A and B show the silver leaf in two different areas of the cross-section. Figure A1 shows a magnified view of the area inside the white-dashed square in figure A. The black color of the silver layer suggests a critical conservation state of the metal leaves due to corrosion and oxidation processes.