## 4 Results
### Micromorphological and Elemental Analysis of the Black Crusts by SEM-EDS  
Scanning Electron Microscopy observations highlighted many similarities between SD samples. Both show a stone substrate (US unaltered) overlayed by well adherent and homogeneous crusts with different thicknesses (Figure 4a,b). The SD2 crust has a size that varies from 175 to 10 \(\upmu\)m (Figure 4a), while SD3 ranges from 150 to 5 \(\upmu\)m (Figure 4b). Black crusts consist of gypsum, which has an acicular and lamellar crystal habit, in which combustion particles of various sizes and morphologies were identified (Figure 4c,d). Particles have diameters ranging from 80 to 3 \(\upmu\)m, and they are spherical, sub-spherical, and irregular in shape, displaying smooth, porous, or rough surface (Figure 4c). These are homogeneously distributed over the whole examined surface. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & & \multicolumn{6}{c}{**Complex of _San Domenico Maggiore_ (SD series)} \\ \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Sample** \\ **ID** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Grain Size** \\ **\(\upmu\)m** \\ \end{tabular} } & \multirow{2}{*}{**Texture**} & \multirow{2}{*}{**Fabric**} & \multirow{2}{*}{\begin{tabular}{c} **Mineralogical Phases** \\ **in the Substrate** \\ \end{tabular} } & \multicolumn{6}{c}{**Mineralogical phases in the black crust**} \\ \cline{5-10}  & & & & & **Cal** & **Gp** & **Qz** & **Whw** & **Wed** \\ \hline SD2 & 450–100 & Ho & Mosaic & Cal & +++ & ++ & – & + & – \\ \hline SD3 & 450–100 & Ho & Mosaic & Cal & +++ & ++ & + & ++ & – \\ \hline \multicolumn{10}{c}{**Cloister of San _Marcellino e Festo_ (SM-P, SM-A, SM-S series)} \\ \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Sample** \\ **ID** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Grain Size** \\ **\(\upmu\)m** \\ \end{tabular} } & \multirow{2}{*}{**Texture**} & \multirow{2}{*}{**Fabric**} & \multirow{2}{*}{
\begin{tabular}{c} **Mineralogical phases** \\ **in the substrate** \\ \end{tabular} } & \multicolumn{6}{c}{**Mineralogical phases in the black crust**} \\ \cline{5-10}  & & & & **Cal** & **Gp** & **Qz** & **Whw** & **Wed** \\ \hline SM-P1 & 550–100 & Ho & Mosaic & Cal & ++++ & ++ & – & ++ & + \\ \hline SM-S1 & 690–100 & He & Mosaic & Cal & ++++ & ++ & – & ++ & + \\ \hline SM-A1 & 470–100 & He & Mosaic & Cal & ++++ & ++ & + & + & + \\ \hline \multicolumn{10}{c}{Minerals abbreviations according to [52]: Cal. calcite; Qz. quartz; Gp. Gypsum; Whw. Whewellite; Wed. Weddellite; ++++. very abundant; ++++. abundant; ++. moderate; +. poor; -. not present.} \\ \end{tabular}
\end{table} 
Table 2: Main textural features (OM) and mineralogical phases (XRPD) occurring in the analyzed samples, considering both unaltered substrate and black crusts. Micromorphological investigations on SM-S1 (Figure 4e) showed a crust with variable thickness (from 520 to 75 \(\upmu\)m), irregular and heterogeneous morphology. Inside, acicular-lamellar gypsum crystals and numerous combustion particles were recognized; the latter are characterized by a morphology ranging from sub-spherical microporous to smooth spherical. The crust appears to be well adherent to the underlying substrate, which is degraded and characterized by many micro-fractures. The crust in SM-A1 (Figure 4f) is well adherent to the substrate, with higher thickness than the other samples (up to 350 \(\upmu\)m), a jagged outer edge, and a heterogeneous morphology. Acicular and lamellar gypsum crystals and few combustion particles were also recognized. These particles, ranging in size from 70 to 5 \(\upmu\)m both have partly sub-spherical morphology and porous surface and partly irregular morphology and rough surface. Figure 4: SEM microphotographs of some examined samples (i.e., SD2, SD3, SM-S1, SM-A1 and SM-P1) with details of combustion particles and gypsum crystals (**c**,**d**,**h**). The holes caused by LA-ICP-MS spot analyses (carried out on black crusts = BC and unaltered substrates = US) are also visible (**a**,**b**,**e**,**f**,**g**).