## 5 Discussion  
The enrichment factors (EFs) of chemical elements were calculated as the ratios of crust/substrate metals' concentration in order to discriminate the origin of the elements respectively attributable to the deposition process, or to the substrate. The results are reported in Table 5.  
Generally, an enrichment factor lower than or equal to 1 suggests that the source of the element comes from the substrate, while an enrichment factor greater than 1 may be imputable to an external \"provenance\", meaning that elements are linked to a deposition process. This secondary provenance is more accentuated as the enrichment factor increases. Considering Table 5, it is possible to observe these differences; specifically, cells in green show EFs values greater than 1, while red ones refer to values lower than 1. The high amount of Ba in the samples (Table 5) from the cloister of _San Marcellino e Festo_ (SM-P, SM-A, SM-S series) is probably a result of restoration interventions [53] carried out in the past, as barium hydroxide was widely used for the conservative treatment of marbles in the past [54]. This thesis is supported by the investigations carried out using OM. Thin section observations made it possible to highlight the presence of a layer of _scialbo_ between the marble substrate and the black crust (i.e., Figure 3, sample SD3). This could be attributed to a restoration intervention, since this practice is rather common in restoration of marble. The enrichment in barium is further provided by EF values (Table 5). In particular, different EFs can be ascribable to various factors such as the type of stone material, the particle deposit morphology (vertical surfaces), the different exposure to emission sources (mobile or fixed) and the wash-out phenomenon. In this way, specimens SD2 and SD3 showed a high enrichment factor for some elements such as Pb, Zn and Cu, because they were sampled from the church portal, which has been affected by mobile polluting sources--specifically, vehicles--in the past [55]. Figure 6, with respect to the complex of _San Domenico Maggiore_ (Figure 6a-c), shows good correlations among the different heavy metals, such as Zn vs. Pb, Cu vs. Zn and Sb vs. Cu. In detail, the high correlation between Zn vs. Pb (R2 = 0.8022) is in agreement with the use of leaded gasoline as fuel for auto-motion [4], used up until a few decades ago. Moreover, the correlation between Cu vs. Zn (R2 = 0.8406) suggests an enrichment of the samples with tire wear particles and other artefacts of friction and machine wear [56; 57; 58; 59]. Additionally, the latter could indicate soil contamination from lubricating oil and exhaust emissions from vehicles [60; 61]. Finally, the good correlation between Sb vs. Cu (R2 = 0.884) indicates a contribution from the brake wear [62; 63]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Complex of _San Domenico Maggiore_** & \multicolumn{3}{c}{**Cloiser of _San Marcellino e Festo_**} \\ \hline
**Chemical Elements** & **Sample SD2** & **Sample SD3** & **Sample SM-S1** & **Sample SM-A1** & **Sample SM-P1** \\ \hline
**As** & 0.9 & 0.7 & 3.3 & 4.6 & 5.2 \\
**Ba** & 0.8 & 1 & 20.1 & 41.6 & 38.4 \\
**Cd** & 2.2 & 1.6 & 0.8 & 0.8 & 5.5 \\
**Co** & 2 & 0.9 & 12.4 & 3.9 & 11.4 \\
**Cr** & 0.8 & 0.7 & 2.3 & 4 & 1.2 \\
**Cu** & 12.3 & 3.3 & 21.1 & 5.9 & 9.5 \\
**Ni** & 1.3 & 1 & 10.8 & 3.4 & 6.4 \\
**Pb** & 42.9 & 38.2 & 4.9 & 6.3 & 5.6 \\
**Sb** & 3.8 & 1.9 & 30.7 & 8.7 & 10.6 \\
**Ti** & 0.7 & 0.6 & 33.6 & 8.9 & 22.5 \\
**V** & 1 & 0.9 & 17.4 & 11.1 & 9.6 \\
**Zn** & 49.4 & 18.4 & 16.6 & 8.8 & 16.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Enrichment factors (EFs) of the chemical elements detected in the black crusts: EFs \(>\) 1 (values in green cells); EFs = 1 (values in white cells); EFs \(<\) 1 (values in red cells). On the contrary, the samples from the cloister of _San Marcellino e Festo_ (SM-S1, SM-A1 and SM-P1) show a low correlation between the heavy metals (Figure 6d-f), suggesting differences in the source.