## 3 Results and Discussion
### After Aging: Evaluation of Paint Samples
#### 3.3.2 Inorganic Fraction
Additional bands at 600 and 530 cm\({}^{-1}\) can be assigned to the PO\({}_{3}\) asymmetric _bending_ and the Mn-O bond _stretching_, respectively [33; 65]. Furthermore, in the Sty_V sample, a new band is visible at 469 cm\({}^{-1}\), probably related to the _rocking_ of the PO\({}_{3}\) group [32]. As just reported, ATR-FTIR analysis revealed new signals mainly related to the PO\({}_{3}\) group, and in some samples (Oil_V, Acr_V, and Sty_V), absorptions related to the sulfate group are also visible. This identification could indicate a reaction between the pigment and the sulfuric acid that was presumably formed inside the weathering chamber. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **CRY\_1** & **CRY\_2** & **CRY\_3** \\ \hline
**Mn** & 17.65\% \(\pm\) 0.4 & 1.04\% \(\pm\) 0.1 & 22.73\% \(\pm\) 0.8 \\ \hline
**S** & 15.73\% \(\pm\) 0.2 & 12.63\% \(\pm\) 0.2 & 2.01\% \(\pm\) 0.1 \\ \hline
**P** & 2.01\% \(\pm\) 0.1 & 14.48\% \(\pm\) 0.2 & 16.52\% \(\pm\) 0.3 \\ \hline
**N** & 8.37\% \(\pm\) 0.3 & 11.65\% \(\pm\) 0.5 & - \\ \hline
**O** & 49.69 \(\pm\) 0.4 & 41.22 \(\pm\) 0.5 & 45.12 \(\pm\) 0.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Average weight percentages, calculated with ZAF matrix correction, of the main elements in the crystals detected with SEM-EDX. To better understand the nature of the crystals formed, it is necessary to consider the results of \(\upmu\)-Raman spectroscopy: acquisitions were carried out by trying to focus on the crystals; however, due to instrumental limits, it was hard to distinguish the three crystal types already differentiated with SEM-EDX. The main characterization was obtained for CRY_1. \(\upmu\)-Raman analysis (Figure 8) detects the co-presence of phosphate and sulfate groups. New peaks can be observed in the Oil_V_COMM sample (Figure 8a). They are probably related to the vibrations of the sulfate group: specifically, the peak at 460 cm\({}^{-1}\) could be related to SO\({}_{4}\)\({}^{2-}\)_stretching_, the one at 603 cm\({}^{-1}\) to SO\({}_{4}\)\({}^{2-}\) group vibration, and the one at 1093 cm\({}^{-1}\) to SO\({}_{4}\)\({}^{2-}\) asymmetric _stretching_[63; 67; 68]. There are also new peaks related to phosphate, such as the bands at 624 and 637 cm\({}^{-1}\) of the PO\({}_{3}\)_bending_ and the band at 1120 cm\({}^{-1}\) of the asymmetric _stretching_ of PO\({}_{3}\)[33]. Moreover, there is a strong increase in the intensity of the peak related to the asymmetric _stretching_ of PO\({}_{3}\) (at 1026 cm\({}^{-1}\)), already present in the unaged sample. Finally, most of the bands related to the phosphate groups  
Figure 7: IR spectra of (**a**) Oil_V, (**b**) Oil_V_COMM, (**c**) Alk_V, (**d**) Acr_V, and (**e**) Sty_V samples. In black, the spectrum of the unaged sample; in green, the spectrum of the aged sample collected on the violet part; and in red, the spectrum of the aged sample collected on the grey part. of PV16 and that were also detected before aging are still visible, such as those at 565, 613, 628, and 884 cm\({}^{-1}\). Concerning sample Oil_V, the high-fluorescence drawback makes the interpretation difficult (Supplementary Figure S10a). However, a single new broadband at 1381 cm\({}^{-1}\) was detected, related to the vibration of the O-H bond of the carboxylic acids formed after aging due to the oxidation reaction that occurred in the binder [69]. In the alkyl sample (Figure 8b), the appearance of new bands related to the sulfate group is also visible: as for the Oil_V_COMM sample, the absorptions at 460, 607, and 1093 cm\({}^{-1}\) can be related to _stretching_, _vibration_, and asymmetric _stretching_ of the SO\({}_{4}\)\({}^{2-}\) group, respectively [63, 67, 68]. Other bands related to the sulfate group are those at 581 cm\({}^{-1}\), due to out-of-plane _bending_; and at 983 and 1146 cm\({}^{-1}\), due to symmetric and asymmetric _stretching_[62, 64, 68]. Furthermore, a shift is observed from 886 cm\({}^{-1}\) to 892 cm\({}^{-1}\) in the asymmetrical _stretching_ band of the PO\({}_{3}\) group [33]. According to [PERSON] et al.