Degradation mechanisms of surface coatings on polychrome clay sculptures from Zhen’guo Temple, Pingyao, Shanxi Province, China

The surfaces of polychrome clay sculptures in ancient temple complexes commonly exhibit deterioration phenomena, including oil exudation, accumulation of pollutants, surface darkening, as well as cracking, lifting, and detachment of the gold leaf. Using the polychrome clay sculptures from the Kṣitigarbha Hall of Zhen’guo Temple in Pingyao, Shanxi, as a case study, this research employs various analytical techniques to systematically investigate the stratigraphy, material composition, production techniques, and degradation mechanisms of the surface coatings, including both gilded and painted layers. Results indicate that the sculptures have undergone multiple gilding and repainting processes. The outer layer of boiled tung oil has undergone significant oxidative aging, leading to oil exudation, pollutant accumulation, surface darkening, and reduced stability of the gilded layers, thereby causing cracking and detachment of the gold leaf. These findings provide scientific evidence for understanding lipid-based material aging in ancient polychrome clay sculptures and for their conservation and restoration.