## 3 Results
### Artificial Ageing Protocol (AAP)
#### 3.1.3 Gas Chromatography Coupled with Mass Spectrometry (GC-MS)  
The chemical stability of the animal glue binder was further evaluated by Gas Chromatography-Mass Spectrometry (GC-MS), focusing on its amino acid composition. The samples P1 aCa(A) and P2 vCa(A), analyzed before and after artificial ageing, consistently revealed glycine (32-35%), proline (15-21%), hydroxyproline (19-24%), glutamine (5-7%), and alanine (10-13%) as the dominant amino acids (Table 4). These amino acids are characteristic of collagen-derived materials and are indicative of the binder's proteinaceous origin [44]. Following 1 week of artificial ageing, the overall proportion of amino acids remained relatively stable, averaging 22% across all samples. This suggests that, despite the FTIR evidence of partial protein denaturation (i.e., reduction in amide I and II bands) (Figure 6) [36; 37], the primary amino acid profile of the binder was largely preserved. This apparent contradiction can be interpreted as surface-level degradation or conformational changes in the protein structure, rather than a complete breakdown of amino acid chains. It is because it has been demonstrated that the main amino acids composing animal glues, such as alanine (Ala), glycine (Gly), leucine (Leu), and proline (Pro), remain stable against long-term ageing [45]. Figure 8: (**a**) Example of Arabic gum binder mixed with red pigment (vermilion) in original polychrome of plasterworks from the Oratory of the Madraza. (**b**) Example of animal glue binder mixed with red pigment (vermilion) in the original polychrome of plasterworks from the Oratory of the Madraza. (**c**) Example of linseed oil binder mixed with blue pigment (artificial ultramarine blue) in a repolychrome of plasterworks from the Oratory of the Madraza. The analyses were carried out by [PERSON], a member of the Ge Grupo Espanód de Conservación. In the case of Arabic gum binder, the components detected before and after ageing revealed the following: the P2 bGa(A) sample was galactose (75%), arabin (20%), rhamn (2%), and glucose (3%), while in the P1 aGa(A) sample, galactose accounted for 40%; arabin, 33%; rhamn, 23%; and glucose, 3% (Table 5). These monosaccharides are typical constituents of arabinolactan polysaccharides, which form the structural backbone of Arabic gum. Regarding ageing, a general trend was observed in which galactose remained the predominant monosaccharide, maintaining levels between 40% and 50% [46]. However, other sugar components, particularly arabin and rhamn, showed a reduction, suggesting selective degradation or loss of specific sugar units within the polysaccharide matrix. This pattern indicates that, while the overall polysaccharide framework remains relatively intact, certain labile components are susceptible to degradation under ageing conditions [47]. These findings align with FTIR results showing decreased absorbance in hydroxyl group regions (around 3425 cm\({}^{-1}\)), consistent with the dehydration or breakdown of hydroxyl-rich polysaccharide chains [41]. Finally, for linseed oil, there is a chemical change in its composition, with the reduction in monounsaturated fatty acid components, especially in the case of oleic acid. This reduction reflects the susceptibility of these unsaturated components to oxidative degradation processes. Concurrently, there was an increase in saturated fatty acids such as palmitic acid, as well as in dicarboxylic acids, including azelaic, suberic, and sebacic acids. The accumulation of these dicarboxylic acids is a well-established marker of oxidative cleav