Degradation of emerald green pigment in painted grottoes in Sichuan, China

Emerald green pigment was widely used in painted cultural artifacts during the 19th and 20th centuries. However, this pigment is inherently unstable, leading to a certain degree of degradation, thereby capturing the widespread attention of research scholars due to its degradation mechanisms. Previous studies primarily relied on the compositional analysis of individual samples, thus only observing the results of the degradation at a certain stage without reflecting the dynamic process of change. The Sichuan region in China hosts a large number of grottoes and cliff statues, many of which have undergone modern repainting. Focusing on the green pigment from these cultural relics, this research aims to capture the degradation products of emerald green at different stages. This approach promises a more systematic and comprehensive understanding of the degradation process, consequently advancing the exploration of its degradation mechanisms. In this study, 15 green pigment samples from five different painted grottoes in the Sichuan region were collected for analysis. Employing technological analytical methods, the presence of emerald green pigment (Cu(C₂H₃O₂)₂·3Cu(AsO₂)₂) was identified, along with several other substances containing Cu and As elements, such as Lavendulan (NaCaCu₅(AsO₄)₄Cl·5H₂O), copper arsenate (Cu₃(AsO₄)₂), alkaline arsenate copper compounds Cu₃(AsO₄)(OH)₃, Cu₂(AsO₄)(OH)·3H₂O and Cu(AsO₃OH)·2H₂O. The elemental compositions, chemical structures, and Raman spectra of these substances exhibit obvious connections and progressive relationships. This study, for the first time, elucidates the diverse degradation products of emerald green pigment from cultural artifacts within similar coexistence environments. It outlines a tentative degradation route, namely: (1) Oxidation and decomposition of Cu(C₂H₃O₂)₂·3Cu(AsO₂)₂ → Formation of Cu₃(AsO₄)₂, alkaline arsenate copper compounds and Ca(C₂O₄)·H₂O; (2) Transformation of Cu₃(AsO₄)₂, alkaline arsenate copper compounds→ Formation of NaCaCu₅(AsO₄)₄Cl·5H₂O. Throughout this process, the macroscopic color changes from green to blue.