Polychromic structures and pigments in Guangyuan Thousand-Buddha Grotto of the Tang Dynasty (China)

This paper presents a multi-analytical study of the polychromy in the Guangyuan Thousand-Buddha Grotto. Samples taken from 15 locations in No. 512 and No. 689 caves are prepared as cross-sections for the analysis by optical microscopy (OM) and scanning electron microscopy coupled with backscattered electron imaging (SEM-BSE). The cross section analysis indicates that some paintings were repainted in the past. The chemical composition of each painting layer is obtained by scanning electron microscopy coupled with energy-dispersive X-ray analysis (SEM-EDX). The main pigments for 15 samples including green, red, blue, white and black paint layers, are identified by the combination of polarized light microscopy (PLM), Fourier Transform infrared spectrum (FT-IR), Raman spectroscopy (RS) and X-ray diffraction (XRD) analysis. The integrated analytical results reveal that the green pigments are malachite and atacamite; the red color is attributed to minium, hematite and cinnabar; the blue pigments are lazurite and organic blue materials; the white color is ascribed to anglesite and gypsum; and the black surface of polychrome is the accumulation of longtime smudging by folk burning incense in the devotional practice or the soot deposition resulting from burning bonfires in the caves. Some arsenic-containing pigment is detected in the green samples. This case study also demonstrates the advantages and the limitations of every analytical technique for the pigment identification, confirming the necessity of the integrated analytical techniques approach. The present results are not only useful to assist in the authenticity of the used pigment materials and history of the polychromic in the past, but also aimed at guiding the conservation scientists in taking into account materials and methods utilized in the past.