# Deterioration of White Tempera Mock-Ups Paints in a SO\({}_{2}\)-Rich Atmosphere
## 4 Discussion
In addition, Cl was also identified in the calcium-based samples using SEM-EDS. These elements (K, Na, Mg and Cl) could be derived from the water used in the SO\({}_{2}\) test environment, leading to the precipitation of sulfated or hydrated salts, such as K\({}_{2}\)SO\({}_{4}\), K\({}_{0.67}\)Na\({}_{1.33}\)SO\({}_{4}\) and Mg\({}_{5}\)O\({}_{3}\cdot\)6H\({}_{2}\)O (detected by XRD). Considering the formation of diverse phases of sulfites and sulfates after the aging test, it is important to highlight that the climatic chamber, in addition to having exposure to SO\({}_{2}\) gas (diluted at 3% in 3000 ppm of nitrogen and dosed at a concentration of 200 ppm), was also set to 25 \({}^{\circ}\)C and 45% RH for two months. The relative humidity value chosen was not too high, but it is important to clarify that RH is a highly influential factor in the chemical-mineralogical alteration process of carbonate materials. Water in the vapor phase can condense on and inside the carbonated paint and, in the presence of SO\({}_{2}\), contribute to the dissolution of calcite, causing the release of more Ca\({}^{2+}\) ions that can precipitate as sulfites or sulfates [40]. Therefore, it is very likely that if this research had been conducted with higher RH values (around 90%), more phases of sulfites and sulfates would have appeared. This topic could be further investigated in the future by subjecting paintings made with these pigments to different levels of temperature and relative humidity. Concerning the reflectance measurements, the results confirmed changes on the aged paints. Indeed, the increase on the intensity of the reflectance detected in the aged paints could be explained considering the formation of new sulfur-based phases [18; 19]. In this study, however, a greater susceptibility of the egg yolk-based paints to the SO\({}_{2}\) deposition process was not confirmed, unlike what was found in previous studies [18; 19] where blue (small, azurite, lapis lazuli) and green (malachite) tempera paints were subjected to the same SO\({}_{2}\) aging test. Firstly, neither by XRD nor by SEM-EDS, was a greater quantity of S-rich deposits detected on the egg-yolk-based paints, unlike in [19] where a greater quantity of sulfated salts (as product of SO\({}_{2}\) deposition) was found in egg-based paints. Furthermore, the deterioration of the binder was unequivocally confirmed in the previously cited studies [18; 19], namely, a higher degree of cracking in rabbit-glue-based paints and an opacification of the egg-yolk-based paints surfaces [18]. The egg yolk binder opacification was explained as a result of the photo-oxidation of their lipids, which, following several studies [6], led to a formation of peroxyl radicals and the denaturation of proteins (polymerization of proteins and/or crosslinking of peptide chains). In our study, only differences between egg-yolk- and rabbit-glue-based paints were detected in terms of color and roughness. That is, in egg-yolk-based paints, color suffered a whitening, and roughness varied more intensely (showing increases) than in rabbit-glue-based paints. Considering that fewer salt deposits were detected in egg-yolk-based paints compared to those made with rabbit glue, these changes in color and roughness were due to a differential degradation of the binders (organic phase) during the aging test. However, no evidence of this deterioration was detected by FTIR or SEM. FTIR only identified changes in the bands assigned to the S-O group (which were associated with the newly formed sulfates and sulfites), and no changes were detected in the functional groups of the binder. On the other hand, SEM did not detect a more intense physical deterioration in egg-yolk-based paints. Probably, the analytical techniques here applied were not the most suitable for detecting minor chemical or physical modifications in the binders. However, it is indisputable from the point of view of roughness and color, that a greater physical deterioration in egg-yolk-based paints took place.