## 4 Discussion  
The analysis carried out on the original banknote by the artist [PERSON], alongside the study of experimentally painted samples, enabled a comprehensive investigation into the morphological and compositional characteristics of UniPosca acrylic markers. The samples prepared on zero-euro banknotes were consistent with the original artwork in both substrate and average paint thickness (0.120 mm \(\pm\) 0.062), validating their suitability for accelerated ageing and conservation testing. EDXRF analysis revealed silicon and aluminium in all colour fields. However, FTIR and Raman spectroscopy confirmed that the main colouring agents were organic in nature, with inorganic components such as silicon functioning as extenders or dispersants. This finding aligns with established practices in commercial coatings, where silica is often added to improve pigment dispersion and opacity [108; 109]. The observed discrepancy in aluminium concentration, low in painted areas and high in unpainted regions, suggests a masking or substitution effect, corroborating similar findings in the field of banknote diagnostics [110]. Vibrational spectroscopy enabled detailed classification of the pigments. FTIR spectra from untreated areas exhibited characteristic bands of the acrylic binder--C=O stretching (1730-1725 cm\({}^{-1}\)) and C-O-C vibrations (1145-1075 cm\({}^{-1}\))--consistent with copolymer systems based on methacrylates and acrylates. Variations in these bands among colour fields indicate differences in binder composition, suggesting formulation variability among marker hues. Post-ageing spectra revealed a significant attenuation and downshift of the C=O band (to \(\sim\)1715-1710 cm\({}^{-1}\)), indicative of photooxidative cleavage and carboxylic acid formation, consistent with literature reports on polymer degradation [71; 72; 73; 85; 86; 90]. The most severe degradation was noted in blue fields, which showed diminished FTIR signal intensity and band resolution, suggesting both binder breakdown and pigment destabilisation, potentially due to aggregation or changes in chromophore dispersion. Raman spectroscopy complemented this analysis, revealing strong, pigment-specific vibrational features: phthalocyanines in blue and green (680, 740, and 1320 cm\({}^{-1}\)), naphthol-azo in red (1350-1450 cm\({}^{-1}\)), and diarylide markers in yellow (1380 and 1600 cm\({}^{-1}\)) [70; 90; 111; 112; 113; 114]. Following ageing, the blue pigments exhibited decreased Raman intensity and minor shifts--indicative of molecular rearrangement or pigment-binder dissociation. Although phthalocyanines are inherently photostable, these effects are attributed to dispersion state and particle size--critical parameters in photostability models involving electronic excitation and non-radiative decay on reflective substrates [108; 113]. Combined FTIR, Raman and multispectral imaging consistently highlighted the chromatic instability of the blue and black fields after UV ageing. Visual and analytical evidence--such as surface cracking, colour shifts (\(\Delta\)E > 5), and binder recession--were linked both to photodegradation processes and to the interaction with the complex ban-knote surface, which includes metallic inks and optically variable features that influence pigment adhesion and behaviour [88; 90; 110]. The application of biopolymer-based protective coatings demonstrated a substantial stabilizing effect. Chitosan and nanocellulose-containing films reduced colorimetric variation (\(\Delta\)E < 3), confirming their ability to mitigate both chemical and mechanical degradation under UV stress, as supported by the recent literature [90, 91, 92, 93]. Morphological analysis showed that pure chitosan produced homogeneous, transparent films, while higher CNC content (e.g., in CS/CNC6) induced microscale surface irregularities and UV-activated aggregate formation. SEM imaging confirmed CNC agglomeration, though no detachment or cracking occurred, even after artificial ageing.