Mechanistic Study on Orpiment Pigment Discoloration Induced by Reactive Oxygen Species

Orpiment (As₂S₃), a yellow mineral pigment widely used in historical artworks, undergoes degradation that seriously threatens the integrity of Dunhuang murals. Reactive oxygen species (ROS) exist widely in air, which may be one reason for the color change of pigments. This study aims to investigate the degradation effects and mechanisms of four ROS-hydroxyl radical (·OH), singlet oxygen (¹O₂), peroxynitrite anion (ONOO^-), and hydrogen peroxide (H₂O₂)-on orpiment. By simulating chemical reaction systems, the interaction processes between different ROS and orpiment were qualitatively and quantitatively analyzed, and the degradation capacities of each ROS on orpiment were evaluated. The experiments show that all ROS can induce orpiment degradation, among which ·OH exhibits the strongest degradation capacity due to its high oxidation potential, while ¹O₂, ONOO^-, and H₂O₂ have relatively minor impacts on orpiment aging. It is the first time that a study has confirmed that ROS (especially ·OH) may drive orpiment degradation in environments, contributing to the increasing number of conservation strategies for artworks.