## 3 Results and Discussion  
### Before Aging: Pigment Characterization  
With the ATR-FTIR and the \(\upmu\)-Raman analysis performed on the pigment powder, it was possible to identify the main bands of PV16 (Figure 1a,b and Supplementary Table S1).  
As shown in Figure 1a, PV16 has two typical IR bands related to the _stretching_ of the ammonium group (NH\({}_{4}\)\({}^{+}\)) at 3203 and 3066 cm\({}^{-1}\)[31]. Moreover, peaks attributed to the _stretching_ of the PO\({}_{3}\) group (1233, 1183, and 1072 cm\({}^{-1}\) in the IR spectrum; 1194 and 1161 cm\({}^{-1}\) in the Raman spectrum [32; 33]) and of the P-O bond (1031, 992, and 905 cm\({}^{-1}\) in the IR spectrum [25]; 884 cm\({}^{-1}\) in the Raman spectrum [34; 35]) are noticeable. Additionally, a peak around 760 cm\({}^{-1}\) in both spectra can be associated with the symmetric _stretching_ of the P-O-P bond [31; 33]. Absorptions due to _bending_ of the O-P-O bond at 591, 564, and 490 cm\({}^{-1}\) (Figure 1a) [25] and at 561 and 507 cm\({}^{-1}\) were detected as well (Figure 1b) [33].  
In addition to the main functional groups of PV16, both ATR-FTIR and \(\upmu\)-Raman analysis reveal the presence of silicates, possibly ascribable to kaolinite (Al\({}_{2}\)Si\({}_{2}\)O\({}_{5}\)(OH)\({}_{4}\)). This latter compound is often added as a whitener and/or filler in the pigment formulation [36]. In particular, in the IR spectrum, the presence of kaolinite is highlighted by the peaks at 3691 and 3618 cm\({}^{-1}\) (_stretching_ of O-H bond); at 695 cm\({}^{-1}\) (Al-OH bond); and at 479 and 441 cm\({}^{-1}\) (Si-O bond _bending_) [37]. In the \(\upmu\)-Raman spectrum, on the other hand, the typical absorption bands of kaolinite were detected at 432, 247, and 221 cm\({}^{-1}\)[38].  
Additional identification of the constituent materials in the pigment powder was provided by XRPD results. The XRPD results in Figure 1c confirm the presence of NH\({}_{4}\)MnP\({}_{2}\)O\({}_{7}\) and kaolinite, in agreement with previous studies carried out on this pigment [16; 31]. However, by considering the crystal structures proposed by [PERSON] et al. [39] for the \(\upalpha\)-NH\({}_{4}\)MnP\({}_{2}\)O\({}_{7}\) and \(\upbeta\)-NH\({}_{4}\)MnP\({}_{2}\)O\({}_{7}\) polymorphs, the comparison between the XRPD pattern and the simulations allows to identify the pure \(\upalpha\)-NH\({}_{4}\)MnP\({}_{2}\)O\({}_{7}\) phase as the only polymorph present in the pigment investigated (Figure 1c). Therefore, this result differs slightly from that of [PERSON] et al. [16], who also detected the \(\upbeta\) polymorph and the stewartite mineral. This is probably due to a change in the commercial formulation during the years [16]. The \(\upalpha\)-NH\({}_{4}\)MnP\({}_{2}\)O\({}_{7}\) crystalline structure is characterized by a space group \(P2_{1}/c\) in which a single type of distorted Mn\({}^{3+}\)O\({}_{6}\) octahedron is present [39].  
SEM-SE images (Figure 1d) revealed that the selected PV16 is a fine-grained pigment with an average particle diameter of approx. 2 \(\upmu\)m. Observing Figure 1d, kaolinite grains are mixed with those of the violet pigment, with an average particle diameter of approx. 9 \(\upmu\)m. Indeed, kaolinite usually exhibits anhedral particles with an average diameter of 10 \(\upmu\)m [40].