A Multi-Spectroscopic Investigation into the Degradation Mechanisms and Lightfastness of CdS and Cd₁₋ₓZnₓS Yellow Acrylic Artists’ Paints Under Artificial Daylight: Competition Between Photocorrosion and Binder Degradation

This investigation evaluated the artificial aging dynamics of two acrylic artist's yellow paints: a Cd₀.₆₆Zn₀.₃₄S alloy (Cadmium Lemon) and a CdS (Cadmium Yellow Medium). By combining colorimetry, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), ATR-FTIR, micro-Raman spectroscopy, fluorescence micros-copy-spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF) analysis, and chemometrics, this study provides several key insights into heritage mate-rial preservation. The CdS paint demonstrated color stability ΔE*= 1.1, whereas the al-loyed Cd₀.₆₆Zn₀.₃₄S paint underwent severe alteration ΔE*= 15.52. Microscopy and pho-toluminescence mapping revealed pigment agglomerates throughout the paint layers. Photochemical activity and spectral properties varied significantly between regions of uniform pigment distribution and those surrounding these coarse clusters. ATR-FTIR and micro-Raman spectroscopy paired with chemometrics showed that while binder oxidation profiles are nearly identical for both paints, macroscopic color change is dic-tated by the photocorrosion of the inorganic pigment. This degradation of pigments is accompanied by lattice distortion and alterations in luminescence and absorption spec-tra. Electron-mediated photocorrosion leading to the reduction of Cd2+ to Cd0 likely in-duces the darkening of the paint film. Binder degradation is driven by photocatalysis rather than direct photolysis. Visible light with energies above the semiconductor bandgap λ< λedge ≈ 500 nm photoexcites the semiconductor pigment, thereby acceler-ating polymer oxidation.