Technological Analysis of Pigment Layers in the Guangyuan Thousand-Buddha Grottoes
HE Xiang1，BU Hai-jun2，ZHANG Ning3, GUO Hong1*
1. Institute of Cultural Heritage and History for Science & Technology, Beijing University of Science and Technology, Beijing 100083, China
2. Chongqing Shenghuang Construction Company Limited, Chongqing 100029, China
3. Qianfoya Stone Carving Art Museum，Guangyuan 628000, China
Abstract：Guangyuan Thousand-Buddha Grottoes arethe largest group of cliff figures and temple grottoes in Sichuan Province. The grottoes are of great value, studying of which contributes to a better understanding of ancient politics, economics and culture. However, after a long period of weathering, they are suffering from various kinds of diseases and in urgent need of protection. We conducted phase and component analysis to elucidate the pigments and technologies of painting layers and provide evidence for conservation, with polychrome fragments that fallen from eight different grottoes as samples. The X-ray diffraction (XRD) analysis identified pigments such as iron oxide red (Fe2O3), cinnabar (HgS), minium (Pb3O4) and carbon black, as well as lead white (PbSO4), gypsum (CaSO4·2H2O), feldspar and quartz in preparation layer and support rocks. This result confirmed that deterioration of the rocks is the main reason of the pigment layer fallen. In analyses of microscopic examination on cross-sections and scanning electron microscopy with an attached energy-dispersive spectrometer (SEM-EDX), two different painting technics, i. e. preparation layer of lead white and painting without preparation layers were recognized. Because of its low concentration, the green-blue pigment could only be recognized using laser confocal Raman Microspectroscopy, and the result was langite (Cu4SO4(OH)6·2H2O). Langite is seldomly reported as a pigment. As an uncommon but widespread mineral, it is not likely to be separated from other copper minerals in ancient pigments. Manufactured basic copper sulphate pigment (Bremen green) is synthesized in the eighteenth and nineteenth century. As a result, the painting may have been redrawn in modern times if pure basic copper sulphate pigment is discovered. These results have provided scientific evidence for related conservation research and provided a new case of using langite as pigment．In addition, langite may also be helpful in studying the preservation history of grottoes.
Key words：Guangyuan Thousand-Buddha Grottoes; Ramon spectrum；Pigments; Langite
