## 4 Discussion
### Artificial Ageing Protocol (AAP)
#### 4.1.1 Colorimetric Analysis
Therefore, animal glue shows initial signs of chemical deterioration, evidenced by the loss of components such as amide B and amine III, along with the reduction in the amide A, I, and II bands. This is mainly attributed to exposure to heat and UV radiation during phase 3 of the artificial ageing process [51]. A physical alteration is also evident, as shown in Figure 10, where cracking and fissures caused by thermal and UV exposure can be observed. In the case of Arabic gum, Fourier Transform Infrared Spectroscopy (FTIR) analyses show a decrease in the hydroxyl group band at 3425 cm\({}^{-1}\), which was observed in the aged sample P1 aGa(A) with azurite. This reduction may be attributed to dehydration or to the partial loss of a structure stabilized by hydrogen bonding. Meanwhile, the bands at 1457 cm\({}^{-1}\) (P2 vGa(A)) and 1463 cm\({}^{-1}\) (P1 aGa(A)), associated with the stretching of conjugated C=C double bonds, as well as the glycosidic linkage bands at 1015 cm\({}^{-1}\) (P1 bGa) and 1009 cm\({}^{-1}\) (P1 aGa(A)), remained stable [39; 41]. The decrease in hydroxyl signal is mainly related to UV exposure during artificial ageing, since Arabic gum has demonstrated chemical stability under thermal ageing conditions [39; 41; 52]. However, in the aged vermilion sample with Arabic gum (P2 vGa(A)), the results were inconclusive due to pigment interference, which hindered the detection of spectral changes attributable to the binder (Figure 7). Gas Chromatography coupled with Mass Spectrometry (GC-MS) results showed that the main components identified before and after ageing in the sample P2 bGa(A) were galactose (75%), arabin (20%), rhamnose (2%), and glucose (3%), while in the sample P1 aGa(A), the composition was galactose (40%), arabin (33%), rhamnose (23%), and glucose (3%) (Table 5). These monosaccharides are typical of arabin galactan polysaccharides, the principal structural components of Arabic gum. Regarding ageing effects, galactose remained the predominant monosaccharide, with consistent levels between 40% and 50% [46], while other sugars, particularly arabinose and rhamnose, showed a decrease, suggesting selective degradation or loss of specific sugar units within the polysaccharide matrix. This pattern indicates that, while the overall polysaccharide structure remains relatively intact, certain components may be more susceptible to degradation, especially under UV exposure [46]. Taken together, the spectroscopic and chromatographic data suggest that Arabic gum exhibits chemical stability under accelerated ageing conditions [46], undergoing primarily physical changes, specifically, the loss of hydroxyl groups associated with hydrogen bonding due to the removal of hydration water used during its preparation. Thus, the observed alteration in Arabic gum is best interpreted as a drying process following step 3 of the ageing protocol, corresponding to the UV exposure phase [39; 41; 46; 52]. The results obtained from Fourier Transform Infrared Spectroscopy (FTIR) analyses indicate that linseed oil undergoes both chemical and physical changes following artificial ageing. Specifically, the samples aged with Arabic gum as the binder (P1 aGa(B) aged and P1 aGa(C) aged) showed a notable reduction in the carbonyl band (\(\sim\)1742 cm\({}^{-1}\)), suggesting a loss of functional groups characteristic of the triglycerides present in the oil. Additionally, a decrease in alkyl group bands (2930-2855 cm\({}^{-1}\)) was observed in samples with animal glue as the underlying binder (P2 vCa(B) aged and P2 vCa(C) aged). This reduction can be attributed to partial evaporation of the organic medium, resulting from the decomposition of triacylglycerlogycerols of unsaturated fatty acids (linolenic, linoleic) and the formation of volatile compounds under UV radiation exposure [36]. In contrast, other samples showed an increase in the 1750-1700 cm\({}^{-1}\) region, associated with carbonyl groups such as ketones, esters, and carboxylic acids.