## 3 Results
### FT-IR Spectroscopy
#### 3.5.2 Analyses After Coating Application and Removal  
To investigate the influence of the applied protective coatings and their subsequent removal, FTIR spectra were acquired on selected chromatic areas (red, yellow, blue, green), both before and after treatment, and after accelerated ageing.  
In all cases, the application of chitosan and nanocellulose-based coatings led to spectral changes primarily in the hydroxyl stretching region (\(\sim\)3300 cm\({}^{-1}\)) and in the 1200-850 cm\({}^{-1}\) fingerprint region. These variations are consistent with the presence of polysaccharide coatings at the surface, rather than with chemical interactions or modifications of the acrylic polymer itself. Indeed, both chitosan and nanocellulose are known to form hydrogen bondswith existing functional groups, but do not react covalently with acrylic matrices under mild conditions [92; 93; 94; 95].  
In the red and yellow marker samples, the appearance of broader O-H stretching at -3300 cm\({}^{-1}\) and new bands in the 1150-850 cm\({}^{-1}\) region can be attributed to characteristic vibrations of chitosan (C-O-C, glycosidic bond, C-O of primary and secondary OH), confirming their presence on the surface after treatment (Figure 4). Similar profiles have been described in polysaccharide-coated acrylic films and model systems [92; 94].  
The analyses carried out on the yellow-coloured area after the application of the coating revealed highly similar changes, including an increase in the intensity and broadening of the band at approximately -3300 cm\({}^{-1}\), as well as the appearance of new bands in the low-frequency region (Figure S8). These variations can be attributed partly to chitosan--due to the increased presence of hydroxyl groups and the vibrational modes associated with C-O-C linkages, the glycosidic bond, and C-O stretching--and partly to the nanocellulose used to reinforce the coating. The latter contributes further hydroxyl groups and exhibits characteristic low-frequency vibrations related to C-O bonds (from primary and secondary alcohols), pyranose ring deformations, and glycosidic bond vibrations [92; 94].  
Figure 4: FTIR spectra collected on red Posca after CS coating application (black line), after ageing process (red line), and after cleaning test on both non-aged (blue line) and aged (green line) coating.  
In the case of the blue marker, after treatment with chitosan and nanocellulose, a decrease in the intensity of the original C=O stretching peak at 1730 cm\({}^{-1}\) was observed, along with the appearance of amide I and II bands at 1650 and 1580 cm\({}^{-1}\), respectively, typical of chitosan (Figure S9) [92]. Multiple additional bands between 1400 and 900 cm\({}^{-1}\) were attributed to the overlapping spectral features of the coating materials. It should be emphasised that these spectral changes are not evidence of chemical interaction with the acrylic binder but rather reflect the spectral contribution of the coating over the pigmented surface, consistent with previous findings in layered systems [93; 94].  
The green marker area showed similar trends: an increased O-H signal and amide-related peaks (1650, 1580 cm\({}^{-1}\)), as well as new absorptions in the fingerprint region, indicating the presence of the coating without altering the chemical integrity of the underlying acrylic paint (Figure S10). Minor spectral differences between aged and non-aged samples were mainly attributable to superficial oxidative changes or moisture-induced reorganisation, rather than to degradation of the base layer.  
After enzymatic removal using \(\alpha\)-amylase, FTIR spectra showed a complete recovery of the original acrylic spectral features, with the disappearance of coating-specific bands. This confirms the effective reversibility of the treatments and the absence of chemical residues, a critical requirement in conservation protocols [71; 72; 73; 90; 91; 92; 93].