## 3 Results
### ATR-FTIR Analysis
The sample taken from Panel D (SG_3) also showed high concentrations of sulphate and calcium ions (Figure 8a), attributable to the same degradation phenomena, which was also confirmed by observing the spectrum obtained by infrared spectroscopy (Figure 8b, Table 3). In fact, the sampled area is characterized by the complete loss of the plaster and of the pictorial layer caused by the crystallization cycles of these salts (Figure 1). Figure 8: Sample SG3 taken from Panel D. (**a**) Ion concentrations of sodium, magnesium, chlorides, nitrates, calcium, sulphate in ppm (\(\mu\)g g\({}^{-1}\)); (**b**) ATR-FTIR spectrum: marker bands of carbonates (*) and sulphates (*) are highlighted. Figure 7: ATR-FTIR spectra of samples SG_4, SG_5 and SG_229 taken from Panel C. Marker bands of carbonates (*) and sulphates (*) are highlighted. Finally, samples taken at the top of the vault do not show high values of sulphate and calcium ions (moderate concentrations were only obtained in sample SG_37A) (Figure 9). Based on the results obtained, it is difficult to assume the precipitation of a specific salt. In fact, from the infrared spectra of samples taken from the detached surface (Figure 10, Table 3), carbonate bands were observed [19; 20] that can be attributed either to newly formed carbonate crystals or to the paint substrate. Figure 10: ATR-FTIR spectra of samples SG_37A and SG_41B taken from the top of the vaults. Marker bands of carbonates (*) and silicates (\({}^{\circ}\)) are highlighted. Figure 9: Ion concentration of samples SG_37A and SG_41B taken from the top of the vaults. (**a**) Ppm (\(\mathrm{\SIUnitSymbolMicro g}^{-1}\)) concentrations of sodium, ammonium, potassium, magnesium and calcium cations; (**b**) ppm concentrations (\(\mathrm{\SIUnitSymbolMicro g}^{-1}\)) of chloride, nitrate and sulphate anions. The graph shown in Figure 11a indicates that the calcium and sulphur trends are perfectly aligned for many of the analyzed samples in which efflorescence produced by poorly soluble gypsum salts was assumed. Gypsum has a low solubility and hygroscopicity compared to most salts, and its deliquescence occurs at a very high relative humidity (over 99.9%, according to [12]). Hence, it is stable in a variety of environments, even extremely humid ones. It should be noted that studies in literature [12] show that the decay rate of wall paintings due to salt contamination originating from gypsum crystallization causes significant deterioration, but also that it is 10 to 100 times slower than that caused by more soluble salts. In fact, the areas where newly formed gypsum was found did show efflorescence, but detachment of the paint surface was not always very evident (except for the area where SG_4 and SG_3 was sampled). This is supported by other studies which indicate that the deterioration caused by gypsum in stone and similar building materials is not produced by hydration pressures [12], but is the result of its crystallization within the porous matrix of the material, as with any other non-hydrating salt. Because of these characteristics, once gypsum is deposited in the pores of a building material or on a pictorial surface, such as a fresco, it will tend to accumulate over time. Its extremely low mobility also explains why desalination of materials contaminated with calcium sulphate is very difficult [12; 23; 24]. Figure 11: Trend of the concentrations (\(\mu\)g g\({}^{-1}\)) of certain ions observed in the samples analyzed. (**a**) Calcium and sulphate; (**b**) chloride and sodium; (**c**) potassium and nitrates. Furthermore, a comparison was made between the concentrations of sodium and chloride ions (Figure 11b), and a good correlation was observed for samples taken from Panel A (SG_6, SG_7, SG_8), Panel C (except for sample SG_29) and the samples taken from the top of the vault (SG_37A). This suggests that sodium chloride salts, such as halite, are present in these areas in addition to newly formed gypsum.