Multi-Spectroscopic Investigation into 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 artists’ 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 microscopy-spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF) analysis, and chemometrics, this study provides several key insights into heritage material preservation. The CdS paint demonstrated color stability ΔE* = 1.09, whereas the alloyed Cd0.66Zn0.34S paint underwent severe alteration ΔE* = 15.52. Microscopy and photoluminescence 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 dictated by the photocorrosion of the inorganic pigment. This degradation of pigments is accompanied by lattice distortion and alterations in luminescence and absorption spectra. Electron-mediated photocorrosion leading to the reduction of Cd2+ to Cd0 likely induces 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 accelerating polymer oxidation.