Comprehensive Analysis of Polychrome Grotto Relics: A Case Study of the Paint Layers from Anyue, Sichuan, China

Abstract

There are more than 18,500 grotto statues in Anyue County, Sichuan Province, China. These grotto statues painted with diverse colors were first sculpted in the Sui Dynasty (581–618 AD) and play a significant role in the history of grotto art development in China. However, due to natural weathering and human factors, the grotto paintings are suffering from many problems, such as flaking and powdering. To study the composition, structure, and destructive factors at work in the paint layers, a set of effective detection methods were integrated, including Raman spectroscopy, scanning electron microscopy–energy dispersive spectroscopy, enzyme-linked immunosorbent assay, immunofluorescence microscopy, and others. The pigments, layer structures, binding materials, and elemental migrations of 22 polychrome samples from the Anyue grottoes are detected. The results show that the Anyue grottoes were painted with more than ten pigments. Casein, egg white, mammalian glue, and plant glue were also detected in samples as binding materials. The accumulation of sulfur and calcium on the surface of several samples was found, which was the main reason for the flaking of the pigments. This study provides not only a reference method but also basic information for the subsequent restoration design.