## 3 Results and Discussion
### Before Aging: Paints Characterization
#### 3.2.1 Oil-Based Samples  
Two oil-based samples were studied: a model paint containing linseed stand oil (a pre-heated oil) mixed with PV16 (Oil_V), and a commercial oil paint by Schmincke containing PV16 (Oil_V_COMM). Comparing the IR spectra of the two different samples (Supplementary Figure S1), it can be observed that in the model sample, the peaks of the binder are more evident than the pigment ones, while in the commercial sample, the pigment peaks are more defined and intense in the fingerprint region with ones (see Supplementary Table S2). In both spectra, however, the typical absorption peaks related to oil are visible. Specifically, the symmetric and asymmetric _stretching_ of the methylene  
Figure 3: SEM images (300\(\times\)) of the paint samples before aging: (**a**) oil sample, (**b**) commercial oil sample, (**c**) alkyl sample, (**d**) acrylic sample, and (**e**) acrylic–styrene sample. Figure 2: Microscopic images of the paint samples before aging: (**a**) oil, (**b**) commercial oil, (**c**) alkyld, (**d**) acrylic, and (**e**) styrene–acrylic sample. group (CH\({}_{2}\)) in the range of 2916-2850 cm\({}^{-1}\), the _stretching_ of the carbonyl group (C=O) at 1739 cm\({}^{-1}\), and the symmetric _bending_ of the C-H bond at 1377 cm\({}^{-1}\) are present [44; 45]. In the model sample, additional oil-related bands are present, such as the C-H _stretching_ at 2954 cm\({}^{-1}\), the C-H asymmetric _bending_ at 1464 cm\({}^{-1}\), the C-O _stretching_ at 1178 and 1097 cm\({}^{-1}\), and the _rocking_ of the CH\({}_{2}\) group at 720 cm\({}^{-1}\)[44; 45]. \(\mu\)-Raman analysis confirmed the IR findings, identifying characteristic drying oil bands in both samples (Supplementary Figure S2): C-H _stretching_ (2925-2860 cm\({}^{-1}\)), C=O stretching (1748 cm\({}^{-1}\)), CH\({}_{2}\)_scissoring_ (1440 cm\({}^{-1}\)), and CH\({}_{2}\) in-phase _twisting_ (1305 cm\({}^{-1}\)) [46; 47]. The model sample showed an additional band at 1660 cm\({}^{-1}\), possibly corresponding to the C=C double-bond _stretching_[47]. THM-Py-GC/MS analysis confirmed that the binding medium in the two samples was a drying oil. Supplementary Table S3 lists all the compounds detected with their retention times (RT, min) and significant mass-to-charge (\(m/z\)) values. The obtained TIC (Total Ion Current) pyrograms (see Figure 4) show saturated monocarboxylic (such as caprylic, capric, lauric, myrsitic, palmitic, and stearic), dicarboxylic (such as suberic, azelaic, and sebacic), and unsaturated (palmitoleic, arachidonic, oleic, and linoleic) fatty acids (such as methyl and dimethyl esters, according to the derivatization) [48; 49]. In addition to the main fatty acids mentioned, both pyrograms revealed behenic acid (RT 19.74 min), a saturated fatty acid, which is detected both in linseed and sunflower oil [48]. This fatty acid is typically present in sunflower oil in higher concentrations; being detected in low concentrations in the samples under investigation suggests the absence of sunflower oil [48]. However, the coexistence of the two oils cannot be excluded with certainty. Considering the calculated molar ratio between palmitic and stearic acids, both paints seem to contain linseed oil [48; 50]. In particular, the ratio between palmitic and stearic acids (P/S) in the model sample is equal to 0.90, and in the commercial one, it is equal to 1.96. Moreover, the azelaic-to-suberic ratio (A/Sub = 3.08) confirmed that the oil used to prepare the samples has undergone a pre-heating treatment [48; 49; 50]. Whereas, the oil employed in the commercial  
Figure 4: Total Ion Current (TIC) pyrograms obtained from Oil_V (black) and Oil_V_COMM (light blue) samples before accelerated aging. paint may be identified as raw, as the A/Sub ratio is equal to 9.29 [48; 49; 50]. In both paints, three unusual peaks are present at the initial RT identified as glycine, N,N/dimethyl/methyl ester, [2-(N,N-Dimethyl)]-1,2-propanediamine, and phosphoric acid trimethyl ester, respectively.