## 3 Results and Discussion
### After Aging: Evaluation of Paint Samples
#### 3.3.2 Inorganic Fraction  
The morphological observations carried out with Optical Microscopy are confirmed by the SEM-EDX analysis. Indeed, the crystals formed after exposure to SO2 in a humid environment can be observed with greater resolution in the SEM. In particular, three different crystal types can be distinguished (Figure 6). The first crystal (namely CRY_1) appears to have a cubic structure, the second (CRY_2) appears to have a monoclinic structure, and the third (CRY_3) appears less defined and more rounded. These three types of crystals were observed in all samples, except for the Oil_V sample, where only the first type was distinguishable. Figure 6: (**a**) Photo of a paint sample taken after 45 days from the end of the accelerated aging. (**b**) Micrograph of a painting portion acquired after 45 days from the end of the accelerated aging. (**c–e**) SEM-BSD images of different types of euhedral crystals appeared after accelerated aging. (**c**) CRY_1, (**d**) CRY_2, and (**e**) CRY_3. Considering the EDX spectra obtained for each crystal type (Supplementary Figure S9), the qualitative elemental composition of the three crystals appears to be the same but in different percentages. The crystals CRY_1 and CRY 2 contain the same main elements: oxygen (O), nitrogen (N), manganese (Mn), phosphorus (P), and sulfur (S). The weight percentages of the latter four elements, however, differ greatly between the two crystals (Table 2). It would therefore appear that the CRY_1 crystal contains mainly manganese, sulfur, and nitrogen, whereas CRY_2 crystals contain mostly phosphorus, sulfur, and nitrogen. The carbon detected in these two crystals is probably linked to the organic matrix on which the crystals were formed, and the aluminum and silicon could be linked to the presence of kaolinite identified in the samples before the accelerated aging. Regarding CRY_3, in addition to oxygen, the main elements found are manganese (22.73% \(\pm\) 0.7), phosphorus (16.52% \(\pm\) 0.3), and sulfur (2.01% \(\pm\) 0.1). ATR-FTIR spectroscopy was employed to obtain additional chemical information on the crystal composition. The spectra after aging were acquired mainly on the grey area, due to the high presence of crystals. In Figure 7, it can be observed that in all samples, the pigment peaks disappear or decrease, such as the peaks at 989, 590, and 563 cm\({}^{-1}\). This phenomenon indicates that changes have occurred in the inorganic fraction. Moreover, new peaks are visible. In Oil_V and Oil_V_COMM samples, a new peak around 1000 cm\({}^{-1}\) could be related to the asymmetric _stretching_ of the PO\({}_{4}\)\({}^{3-}\) anion [61], and the band at 600 cm\({}^{-1}\) could be related to the PO\({}_{3}\) asymmetric _bending_[33]. In addition, in the Oil_V sample, a new band at 691 cm\({}^{-1}\), probably related to the _twisting_ vibration of water molecule [32], and a band at 616 cm\({}^{-1}\), attributable to the vibration of the sulfate group (SO\({}_{4}\)\({}^{2-}\)), are visible [62; 63; 64]. In Oil_V_COMM, new bands are visible at 503 and 449 cm\({}^{-1}\), possibly due to the _stretching_ of the Mn-O bond of manganese sulfate crystals and the asymmetrical _bending_ of the P-O-P bond, respectively [33; 65]. Regarding the alkyl resin sample, a shift of the peak related to the symmetric _stretching_ of the PO\({}_{3}\) group (at 979 cm\({}^{-1}\)) and the formation of two new bands at 602 and 470 cm\({}^{-1}\) can be observed, probably due to the asymmetric _bending_ and _rocking_ of the PO\({}_{3}\) group [32; 33]. There is also a new band at 533 cm\({}^{-1}\) that could be due to the _stretching_ of the Mn-O bond [65]. Concerning the acrylic and styrene-acrylic-based samples, a peak around 970 cm\({}^{-1}\) can be related to the _stretching_ of the sulfate group [44; 65]. It is also visible that a new peak at 665 cm\({}^{-1}\) is probably related to the _bending_ of the MnOH or PO\({}_{3}\) group [33; 66].