## 4 Discussion
### Artificial Ageing Protocol (AAP)
#### 4.1.1 Colorimetric Analysis
This increase, identified in P1 aCa(C), P2 vCa(B), P2 vCa(C), and P1 Ga(C), is indicative of oxidative polymerization processes, in which ester bond formation plays a fundamental role in the chemical transformation of linseed oil [42]. The appearance of these groups can also be attributed to the formation of carboxylic and dicarboxylic acids (e.g., azelaic acid), which are typically generated during linseed oil oxidation [36]. This chemical evolution was further confirmed by the presence of new hydroxyl-related bands in the 3550-3423 cm\({}^{-1}\) region, linked to advanced oxidation products. These bands were observed in multiple aged samples, including P1 aCa(C), P1 aGa(C), P2 vGa(C), and P1 aGa(B), demonstrating the progressive oxidation of the oil (Figures 6 and 7) [36; 42; 43]. From a compositional standpoint, Gas Chromatography coupled with Mass Spectrometry (GC-MS) analysis revealed a decrease in monounsaturated fatty acids, particularly oleic acid, along with an increase in saturated fatty acids, such as palmitic acid, and in dicarboxylic acids, including azelaic, suberic, and sebacic acids. These dicarboxylic acids are well-established markers of oxidative degradation in drying oils, confirming a substantial chemical transformation of the binder during the ageing process [53; 54]. Therefore, the combined findings from Fourier Transform Infrared Spectroscopy (FTIR) and Gas Chromatography coupled with Mass Spectrometry (GC-MS) analyses confirm that linseed oil undergoes a progressive chemical deterioration. These chemical processes have direct implications for the physical behavior of the paint material. The advancement of oxidative polymerization and the accumulation of degradation products ultimately lead to physical alteration, manifested as cracking and craquelure on the paint surface. These findings emphasize the complexity of binder ageing, where molecular rearrangements and loss of structural integrity can occur without significant compositional loss, and underline the importance of employing both spectroscopic and chromatographic techniques for a comprehensive evaluation. In addition, the results of the ageing process have a direct impact on the selection and application of cleaning treatment. For all of the three binders analyzed, the changes they have undergone, both chemically and physically, as can be seen in Figure 5, where cracking, fissures, and even losses of the polychrome layer have occurred have a direct impact on the proposed cleaning treatments. For linseed oil, the chemical changes that occur during ageing result in modifications to its solubility and polarity properties, which in turn affect the range of compatible solvents that can aid in its removal [17]. In the case of animal glue and Arabic gum binders, denaturation and drastic drying make them more sensitive to cleaning treatments, requiring careful control over the application time and the vehicle used to apply these treatments. Hence, it is importance to understand these changes in order to select appropriate cleaning treatments--in this case, solvents compatible with the removal of aged linseed oil and vehicles such as gels that allow control over the action of these solvents to ensure the preservation of the original polychromy.