VIS-Light-Induced Degradation of Street Art Paints and Organic Pigments

Contemporary murals are highly susceptible to rapid color fading due to outdoor urban exposure. This study investigates the photodegradation mechanisms affecting synthetic organic pigments (SOPs) and commercial acrylic–vinyl paints under simulated visible light exposure. Artificial aging experiments were conducted on two types of systems: (a) pigment pellets, composed of pre-fixed pigments on mineral bases, as supplied by the manufacturer, and (b) commercial paints applied on glass substrates. Both systems were aged under controlled and uncontrolled relative humidity (RH) conditions. Colorimetric analyses revealed significant color fading, particularly in pigments PR112, PO5, and PV23. Vibrational spectroscopies highlighted the reduction in pigment-related bands after aging, indicating SOPs’ vulnerability to photodegradation. In paint mock-ups, FTIR-ATR measurements indicated binder degradation and an increase in signals from inorganic fillers. A superficial layer enriched in inorganic components was investigated non-invasively by Micro Spatially Offset Raman spectroscopy (Micro-SORS) in the beamsteer modality. It highlighted a gradient of calcite to pigment with depth. These findings indicate that color fading in mural paints results from the combined degradation of both pigments and binders.