## 3 Results and Discussion
### Linseed Oil  
The IR spectrum (Figure 7a) obtained in the violet area is very similar to that of the unaged sample, while the spectrum of the grey region shows some variations. In particular, from a semi-quantitative analysis (Supplementary Table S12), an increase in intensity can be observed in the region between 3600 and 2600 cm\({}^{-1}\), i.e., the bands associated with C-H _stretching_, with also the formation of a band around 3250 cm\({}^{-1}\). This may be caused by the absorption of water due to the high relative humidity content present during aging and/or the formation of new carboxylic acids and alcohols due to hydrolysis and oxidation reactions [48]. The esters of triglycerides can indeed react with water molecules to form carboxylic acids and alcohols [48; 75]. At the same time, sulfur dioxide is a good oxidant that can therefore favor oxidation reactions of the oil, resulting in the formation of aldehydes, ketones, alcohols, and carboxylic acids [49]. Evidence for these hypotheses is the formation of a new band at 1705 cm\({}^{-1}\) and a shoulder at 1633 cm\({}^{-1}\), probably related to the C=O _stretching_ of carboxylic acids and diketones in enol form, respectively [45; 76]. Moreover, the semi-quantitative evaluation highlights a decrease in the intensity of the carbonyl _stretching_ (1737 cm\({}^{-1}\)) and C-H _bending_ (1377 cm\({}^{-1}\)) bands due to oxidation reactions (Supplementary Table S12). Furthermore, a band appears at 1432 cm\({}^{-1}\), which could be attributed to the formation of metal soaps due to the presence of manganese [77]. Metal soaps are metal carboxylate salts, usually found in oil paintings because of the chemical reaction between metal ions present in the painting layers and free fatty acids from the lipidic binders [77]. In this specific case, the new peak can be related to the _bending_ of the CH\({}_{2}\) group of manganese azelate or the symmetrical _stretching_ of the COO' group of manganese palmitate [77]. Finally, a band at 2346 cm\({}^{-1}\) is visible, which may be associated with the _stretching_ of the O-H bond of a sulfonic acid group (-SO\({}_{3}\)H) [69]. SO\({}_{2}\) can react with pre-existing hydroperoxide groups bound to allyl carbons to form free radicals that initiate crosslinking reactions with a mechanism similar to that of normal autoxidation [78]. This leads to the insertion of sulfonic groups within the polymer chain. A further indication of possible sulfonation of the oil is given by the results obtained with THM-Py-GC/MS and listed in Table 3.  
From the polymerization induced by accelerated aging, an increase in the content of saturated dicarboxylic acids (suberic, azelaic and sebacic acid) and a decrease in unsaturated fatty acids (oleic and linoleic acid) would be expected [48]. In fact, the dicarboxylic acids, and in particular azelaic acid, are formed during the oxidative fragmentation of the \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Sample** & & **P/S** & **A/P** & **A/Sub** & **O/S** & **D/P** & **\%D** & **PE\%** & **PhA\%** & **BA\%** \\ \hline \multirow{3}{*}{**Oil\_V**} & UNAGED & 0.90 & 0.65 & 3.08 & 1.24 & 0.97 & 20.90 & & \\ \cline{2-9}  & AGED & 1.05 & 0.24 & 4.27 & 0.99 & 0.31 & 9.59 & & \\ \hline \multirow{3}{*}{**Oil\_V\_COMM**} & UNAGED & 1.96 & 0.55 & 9.29 & 1.16 & 0.63 & 22.75 & & \\ \cline{2-9}  & AGED & 2.12 & 1.13 & 6.44 & 0.64 & 1.34 & 42.30 & & \\ \hline \multirow{3}{*}{**Alk\_V**} & UNAGED & 1.48 & 0.48 & 7.17 & 2.43 & 0.58 & 14.33 & 12.01 & 16.44 & 12.05 \\ \cline{2-9}  & AGED & 2.34 & 0.22 & 6.82 & 1.92 & 0.25 & 10.07 & 1.90 & 29.42 & 14.62 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Calculated molar ratio for unaged and aged Oil\_V, Oil\_V\_COMM, and Alk\_V samples. For the Alk\_V sample, the normalized % in weight of the other principal constituents was reported (pentaerythrhitol, phthalalc acid, and benzoic acid). unsaturated fatty acid from the vegetable oil as a tertiary oxidation products [48].