## 3 Results
### Artificial Ageing Protocol (AAP)
#### 3.1.2 Fourier Transform Infrared Spectroscopy (FTIR)  
The study of the samples by FTIR spectroscopy allowed for identifying the main absorption bands and peaks of the binders before applying the artificial ageing protocol. In the samples made on animal glue (samples P1 aCa(A) and P2 vCa(A)), the results show the characteristic groups of amide A at 3642 cm\({}^{-1}\) (P2 vCa(A)) and 3418 cm\({}^{-1}\) (P1 aCa(A)), amide I at 1623 cm\({}^{-1}\) (P1 aCa(A)) and 1618 cm\({}^{-1}\) (P2 vCa(A)), and amide II at 1565 cm\({}^{-1}\) (P1 aCa(A)), which are signed with arrows in Figure 6[36; 37]. For Arabic gum samples studied before ageing (P2 vGa(A) and P1 aGa(A)), the main functional groups were identified, such as the  
Figure 5: Samples before and after artificial ageing seen from above. hydroxyl group at 3425 cm\({}^{-1}\) (P1 aGa), the bands 1457 cm\({}^{-1}\) (P2 vGa(A)) and 1463 cm\({}^{-1}\) (P1 aGa(A)) attributed to C=C conjugated double bond stretching, and glycosidic linkage indicated by the band at 1015 cm\({}^{-1}\) (P1 bGa) and at 1009 cm\({}^{-1}\) (P1 aGa(A)) (pink shaded area in Figure 7a,c) [38; 39]. For linseed oil repainting samples before ageing (P1 aGa(B), P1 aCa(B), P1 aGa(C), P2 vGa(B), P2 vGa(B), P2 vGa(C), and P2 vCa(C)), the signals assigned to the lipid fraction are evident in the doublet bands at 2920 cm\({}^{-1}\)-2857 cm\({}^{-1}\) due to the long-chain aliphatic group bands, referring to long hydrocarbon chains, such as those found in triglycerides of unsaturated fatty acids (like linolenic acid), which make up linseed oil. Additionally, the characteristic band at 1742 cm\({}^{-1}\) of the carbonyl group has been also identified (green rectangles in Figures 6a-d and 7a,c) [40]. Following the artificial ageing protocol, FTIR analysis revealed significant chemical changes in the binders used in the simulated polychrome samples. For the animal glue binder, particularly in the aged vermilion sample (P2 vCa(A) aged), a marked reduction in the absorbance of the amide II band was observed, indicating partial denaturation or degradation of proteinaceous components. In the case of the aged azurite sample bound with animal glue (P1 aCa(A) aged), the response was comparatively more stable, although a reduction in both amide I and II bands was still evident, suggesting structural alterations of the protein matrix, albeit to a lesser extent [36; 37] (Figure 6). Figure 6: Samples before and after artificial ageing. Comparative ageing of polychrome plaster samples with animal glue and different pigments (azurite and vermilion): (**a**) (P1 aCa(A) before and after ageing and P1 aCa(C) before and after ageing; (**b**) P1 aCa(B) before and after ageing; (**c**) P2 vCa(A), P2 vCa(B), and P2 vCa(C)- before ageing; and (**d**) P2 vCa(A), P2 vCa(B), and P2 vCa(C) after ageing. In the case of Arabic gum, the P1 aGa(A) aged sample with azurite, a decrease in the hydroxyl group of 3425 cm\({}^{-1}\) can be observed, which may be attributed to dehydration or partial loss of a hydrogen-bonded structure. A reduction in the stretching vibrations of the methyl and ethyl groups v(CH3) and v(CH2) at 2988 cm\({}^{-1}\) (P1 aGa(A)) is also observed. The bands 1457 cm\({}^{-1}\) (P2 vGa(A)) and 1463 cm\({}^{-1}\) (P1 aGa(A)) attributed to C=C conjugated double bond stretching and glycosidic linkage at 1015 cm\({}^{-1}\) (P1 bGa), at 1009 cm\({}^{-1}\) (P1 aGa(A)), remain stable [39; 41]. However, in the aged vermion sample with Arabic gum (P2 vGa(A) aged), the results were inconclusive due to interference from the pigment, which hindered the detection of gum-related spectral variations (Figure 7). Regarding linseed oil, aged samples based on Arabic gum (P1 aGa(B) aged and P1 aGa(C) aged) showed a notable reduction in the carbonyl band at 1742 cm\({}^{-1}\). Furthermore, a decline in the alkyl group bands (2930-2855 cm\({}^{-1}\)) was recorded in aged samples with animal glue as the underlying binder (P2 vCa(B) aged and P2 vCa(C) aged).