## 4 Results
### Micromorphological and Elemental Analysis of the Black Crusts by SEM-EDS
The red dotted line demarcates the layer of black crust from the substrate. Elemental analyses were carried out by EDS on the black crust to evaluate the chemical composition in terms of major elements, as well as their distribution within the sample (i.e., black crust). Finally, SM-P1 (Figure 4g,h) displays a poorly degraded stone substrate, with a well adherent crust only at some points of the analyzed surface. The crust shows a heterogeneous and irregular morphology, with a thickness ranging from 790 to 50 \(\upmu\)m (Figure 4g), where lamellar gypsum and calcite crystals could be identified, probably coming from the stone substrate. Additionally, it combustion particles with different shapes (spherical, sub-spherical and irregular), sizes (thickness between 2.5 and 125 \(\upmu\)m) and surface morphologies (smooth, porous and wrinkled) can be recognized, distributed variously along the entire surface investigated (Figure 4h). Black crusts are mainly composed of SO3 and CaO, clearly attributable to the gypsum. Moreover, SiO2, Al2O3, Fe2O3 were detected, along with smaller amounts of K2O, P2O5, Na2O, MgO and TiO2, whose presence is ascribable to the abovementioned particles being embedded in the crust (Table 3). Since the crusts in the SD2 and SD3 samples show a similar composition, as do SM-S1, SM-P1 and SM-A, Table 3 only reports the crust compositions of the SD2 and SM-P1 samples, as representatives. The distributions of SO3 and CaO, detected in the analyzed window for SD2 and SM-P1 were obtained by means of false color maps regarding S and Ca (Figure 5a-c), as well as their combination (Figure 5d). Figure 5d further confirms that the BC surface is mainly composed of gypsum, with calcium and sulfur being the main components, with a more marked yellow coloring resulting from the combination of red and green colors (i.e., overlapping of red colors indicating 100% S and green indicating 100% Ca, according to their greater or lesser amount). Additionally, the US surface shows a high concentration of calcium oxide, demonstrated by the greener coloring. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{3}{c}{**Complex of _San Domenico Maggiore_**} & **Cloister of _San Marcellino e Festo_** \\ \hline \multirow{2}{*}{**Major Elements**} & **Sample SD2** & **Sample SM-P1** \\  & **Average Analysis No. 8** & **Average Analysis No. 8** \\ \hline \multirow{2}{*}{**Na2O**} & \textless{}0.1 & 1.99 \\  & \textless{}0.1 & 1.19 \\ Al2O3 & 6.70 & 8.82 \\ SiO2 & 7.82 & 13.60 \\ P2O5 & 0.99 & \textless{}0.1 \\ SO3 & **40.90** & **38.60** \\ K2O & 3.49 & 2.50 \\ CaO & **35.50** & **28.40** \\ TiO2 & \textless{}0.1 & 0.57 \\ Fe2O3 & 5.59 & 4.36 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Average concentrations of major elements expressed in oxides (wt%) in the black crust of sample SD2 as representative of the complex of _San Domenico Maggiore_ and SM-P1 as representative of the cloister of _San Marcellino e Festo_. Measurements were obtained by SEM-EDS analysis performed in raster mode. Figure 5: False color maps by SEM-EDS related to SD2 sample as representative. (**a**) EDS analyzed window of the SD2 sample, consisting of the crust (BC) and stone substrate (US); (**b**) distribution of S (red); (**c**) distribution of Ca (green); (**d**) overlapping and distribution map of S and Ca.