## 3 Results
As shown, C-H stretching bands (2954, 2920, and 2852 cm\({}^{-1}\)), a shoulder of C=O at 1725 cm\({}^{-1}\) and absorptions in the amide I and II range at 1676, 1635, and 1550 cm\({}^{-1}\) are consistent with a protein-based binding medium [14]. Noteworthy are the slightly perceivable signals at 3320 and 3113 cm\({}^{-1}\) and the one at 1676 cm\({}^{-1}\), which could be assigned firmly to guanine but even to proteins [15]. More detailed information of the outermost layer may be retrieved from the \(\upmu\)-ATR FT-IR stratigraphic analysis of the painting cross-section shown in Figure 2 (Figure 5B spectrum a). It is possible to confirm that the topcoat (i.e., layer 1) is likely constituted of oil, as suggested by the intense C=O stretching band at 1730 cm\({}^{-1}\) and by the C-H stretching (2926-2934 and 2857 cm\({}^{-1}\)) and bending (1455-1460 cm\({}^{-1}\)) vibrations. Moreover, the very weak out-of-plane CH bending vibration at 720 cm\({}^{-1}\) is diagnostic for the presence of a lipidic component. The weak band at 1377-1382 cm\({}^{-1}\) may also suggest the use of a mixture with a natural resin [16]. However, a direct identification is not  
Figure 4: Normalized UV-Induced VIS Fluorescence (UVIVF) spectra showing the different fluorescence emission of the organic layers applied on top of the silver leaf. straightforward, as from FTIR spectra of preheated and aged oil-resin varnishes individual signatures of each component are no longer recognizable [17; 18]. The broad shoulders at about 1650-1660 and 1550 cm\({}^{-1}\) may be attributed to amide I and amide II, respectively, thus confirming the presence of a proteinaceous substance [19]. In this regard, the \(\upmu\)-ATR FT-IR spectrum collected over the surface of a sampled fragment showing a severely crackled and tarnished silver leaf (Figure 5B, spectrum b) resulting consistently with that of an aged egg yolk, as suggested by the appearance of very weak signal at 3070, the shoulder of C-H stretches at 2954 cm\({}^{-1}\), the C=O stretching vibration at 1735 cm\({}^{-1}\), and signals at 1650, 1576-1550, and 1460 cm\({}^{-1}\) of amide I, amide II, and amide III, respectively [15; 20]. The thin film over the silvered surface was also characterized by DRIFT, which is well-known to generate high-quality IR spectra in reflection-absorption configuration (Figure 5A, spectrum c). Besides the presence of overlapping signals originating from the underlying gypsum layer (1680 and 1632 cm\({}^{-1}\)), a mixture of proteinaceous material (signals at 1650, 1576, and 1550 cm\({}^{-1}\)) and calcium oxalates (1636, 1320, and 780 cm\({}^{-1}\)) was identified [21; 22; 23]. The band presence at 1320-1325 cm\({}^{-1}\) is typical of whewellite (Ca(C\({}_{2}\)O\({}_{4}\)) H\({}_{2}\)O) [24]. In the spectrum c of Figure 5A, additional signals requiring further attention were the C-H stretches near 2900 and 1460 cm\({}^{-1}\) and the sharp features of signals at 1735 cm\({}^{-1}\) (stretching vibrations of C-O-C groups) and 1170 cm\({}^{-1}\) (stretching vibrations of C-O-C groups). Most of these features were detected also by \(\upmu\)-ATR FT-IR  
Figure 5: Qualitative comparison of IR spectra of different sampled paint fragments using Diffuse Reflectance Infrared Fourier Transform (DRIFT; **A**) and Microattenuated Total Reflection Fourier Transformed-Infrared (\(\upmu\)-ATR FT-IR; **B**) techniques. DRIFT (A) spectra refer exclusively to different non-embedded samples (**a**: fragment with varnish on top, **b**: fragment with less varnish on top **c**: degraded silvered surface, **d**: ground). \(\upmu\)-ATR FT-IR (B) spectra were acquired on embedded (i.e., cross-sections) and non-embedded samples (**a**: layer 1 of Figure 2, embedded sample, **b**: top of the silvered surface, non-embedded sample, **c**: top of the non-silvered surface, non-embedded sample, **d**: ground, embedded sample). Red labels in the plot chart B indicate absorption bands of the polyester embedding resin.