## 3 Results and Discussion
### SG_37B Microsample  
This microsample is constituted by two layers:  
* Plaster made with a light-colored binder (calcium carbonate, CaCO\({}_{3}\)), consisting of aggregates of variable grain size and color;
* Continuous and homogeneous dark-gray layer. The XRF spectra show the peaks of calcium (Ca), iron (Fe), and lead (Pb), suggesting that the gray layer is made up of a mixture of several parts of ochres (iron oxides) and a black pigment (such as carbon black), probably the so-called _caput mortuum_ pigment, also known as _morellone_ (a mixture of red ochre and organic black). The presence of lead (Pb) indicates the use of white lead pigment (Figure 2). The FT-IR spectrum shows absorption bands at 1783, 1428, 870, and 716 cm\({}^{-1}\), attributed to the characteristic vibrations of the carbonate ion (CO\({}_{3}\)\({}^{2-}\)) [16], used as a binder, and the absorption bands at 2973, 2926, 1731, 1433, 1370, 1227, 1120, 1020, 945, 865, 795, 632, and 604 cm\({}^{-1}\) ascribable to a synthetic resin known as Mowilith DM 912 [17]. \begin{table}
\begin{tabular}{c c c} \hline
**ID Sample** & **Macro-Photos of the Cross-Section Observed under the Optical Microscope** & **Description of the Observed Layers** -from the Top to the Bottom- \\ \hline SG\_37B** & & 2 layers: - Dark gray (mixture of organic black and ochre) - Plaster \\ \hline SG\_38 & & 3 layers: - Green pigment - Grey primer (mixture of black, ochre, and white pigments) - Plaster \\ \hline SG\_39A & & 3 layers: - Green pigment - Grey primer (mixture of black, ochre, and white pigments) - Plaster \\ \hline SG\_39B & & 3 layers: - Blue pigment with white grains - Red pigment - Plaster \\ \hline SG\_41A & & 3 layers: - Blue pigment with white grains - Red pigment - Plaster \\ \hline SG\_42 & & 4 layers: - Green pigment - Blue pigment applied a secco - Red-violet pigment and gypsum - White primer - Plaster \\ \hline SG\_42 & & 4 layers: - Green pigment - Blue pigment applied a secco - Red-violet pigment and gypsum - White primer - Plaster \\ \hline \end{tabular}
\end{table} 
Table 2: ID sample, macro-photos of the cross-section observed under the optical microscope, and description of the observed layers. The EDS analysis, acquired along the dashed line reported in the SEM micrograph (Figure 8), showed the trends of the distribution of the characteristic elements Ca, zinc (Zn), Fe, and copper (Cu) along the cross-section of the sample from the plaster toward the surface paint layers. A high Ca content, attributable to the calcite of the plaster, was observed, followed by an increase in the Zn and Fe content, indicating the presence of zinc oxides in the white layer and of iron oxides in the red purplish layer, respectively, along with a subsequent increase in the Cu content in correspondence with the blue layer, attributable to the presence of a copper-based pigment such as azurite. Since the azurite is a water-soluble substance and reacts with the lime present in the fresh plaster, it was not applied a fresco, but a secco. On the other hand, the presence of green grains was evident under the optical microscope (Figure 9), which could be an indication of malachite, and an indication of the well-known reaction of conversion of azurite in malachite [21]. However, the results did not allow excluding the presence of other green-blue Cu-based pigments [22]. The barium (Ba) signals in the XRF spectra could indicate the presence of impurities in the copper-based pigment. Figure 7: **(top left,right)** SEM micrograph and the corresponding EDS profile of the SG_41 microsample cross-section. The EDS investigation was carried out on the dashed line to evaluate the distribution of Ca, Fe, and Mn from back to front of the sample. **(bottom left)** FT-IR and **(bottom right)** XRF acquired on the front and back of the SG_41 microsample before preparation of the cross-section.