## 3 Results and Discussion
### Panel A \"Il Bacio di Giuda e la Cattura di Cristo\"
The SEM-EDS elemental analysis (Table 2) confirms the hypothesis of colour shading in the surface pigment; indeed, copper and chlorine were detected on the A layer, in relation to the formation of atacamite, a copper chloride (Cu\({}_{2}\)Cl(OH)\({}_{3}\)) and a degradation product of azurite. The presence of iron in the red layer attests to the use of the _morellone_ (B). Figure 4: (**a**) Micrographs of SG_20 cross section obtained using OM, showing the three-layer stratigraphy; (**b**) SEM-EDS back-scattering electron image of the three layers. Figure 5: (**a**) Micrographs of SG_21A cross section obtained using OM, showing the four-layer stratigraphy; (**b**) SEM-EDS back-scattering electron image showing the four layers. A supplementary fragment of the sample SG_22 was investigated using XRD (Table 3). The analysis reveals and confirms the presence of atacamite, resulting from azurite alteration under specific conditions; the presence of quartz, calcite, plagioclase, mica, and chlorite are to be attributed to the composition of the mortar that polluted the analysis of the pigment. In the case of the SG_23B micro-fragment, the \(\upmu\)-Raman spectrum (Figure 7) collected on a blue area shows the presence of lazurite, as a blue pigment, mixed with carbon black (smoke black), calcite, and gypsum. Importantly, considering the specific historical-geographical context and execution technique of the whole pictorial cycle, the presence of gypsum can be considered as intentionally added by the painter as binding material for the pigments. However, the existence of gypsum, resulting from environmentally activated alteration processes that took place over time onto the topmost pictorial surface, cannot be excluded. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Sample ID** & **P\({}_{2}\)O\({}_{5}\)** & **MgO** & **Al\({}_{2}\)O\({}_{3}\)** & **SiO\({}_{2}\)** & **SO\({}_{3}\)** & **Cl** & **K\({}_{2}\)O** & **CaO** & **FeO** & **CuO** & **ZnO** \\ \hline SG\_19 & - & 3.58 & 1.89 & 12.33 & 3.83 & - & - & 17.75 & 3.02 & 55.54 & - \\ Greenish layer & - & 1.84 & 1.29 & 13.27 & 1.96 & - & 1.38 & 41.76 & 3.24 & 34.50 & - \\ \hline SG\_20 & - & 1.54 & - & 10.99 & 2.14 & - & 0.90 & 45.24 & - & 38.48 & - \\ Blue layer & - & - & 2.01 & 20.10 & 3.44 & - & - & 5.70 & 2.06 & 7.49 & 58.43 \\ \hline SG\_21A & 29.93 & - & - & - & 3.04 & - & - & 36.19 & - & - & 30.13 \\ A layer, black grains & - & - & - & - & - & - & - & 1.06 & - & 98.94 & - \\ \hline SG\_22 & - & 1.86 & 1.68 & 4.77 & - & 24.73 & 0.63 & 4.95 & - & 61.38 & - \\ Greenish layer & & & & & & & & & & \\ \hline \end{tabular}
\end{table} 
Table 2: EDS analysis (weight %) of samples SG_19, SG_20, SG_21A, and SG_22 cross sections. Figure 6: (**a**) Micrographs of SG_22 cross section obtained using OM, showing the three-layer stratigraphy; (**b**) SEM-EDS back-scattering electron image of the three layers. The FT-IR spectrum of this sample (Figure 8) clearly highlighted the presence of the characteristic bands of calcite and gypsum. The band between \(\sim\)890 cm\({}^{-1}\) and \(\sim\)1230 cm\({}^{-1}\) is associated with silicates. No organic binders are observed. This experimental evidence is in agreement with the use of the \"_a fresco_\" technique for the application of the pigment. The spectrum (Figure 9) measured on the brown-green pictorial layer of SG_24 sample shows the presence of carbon black, probably used to darken the used pigment, that does not show any characteristic peak. Figure 8: FT-IR spectrum of SG_23B sample. Figure 7: (**a**) Micro-photo of the analyzed blue area, (**b**) \(\upmu\)-Raman spectrum collected on the blue area of SG_23B sample. The SG_31 blue sample shows a Raman spectrum (Figure 10) with the characteristic peak of calcite and those of carbon black. No marks of a blue pigment are visible.