Investigation of the painting materials in Zhongshan Grottoes (Shaanxi, China)

This paper reports on the characterization of paint samples from polychrome sculptures in the main cave of the Zhongshan Grottoes, China. Optical Microscopy (OM), Environmental Scanning Electron Microscopy in combination with Energy Dispersive X-ray analysis (ESEM/EDX), Fourier Transform Infrared spectroscopy (FTIR) and Raman spectroscopy were carried out in order to study the stratigraphy of the sculptural polychromy and to determine the painting materials. Minium Pb3O4 and mercury sulphide HgS, cinnabar or its synthetic form vermilion were found as red pigments. Two mixtures were used to produce a rose color: lead white Pb3 (CO3)2 (OH)2 with minium and hematite added to gypsum. Yellow was attributed to an ochre. The green paint layer has been identified as botallackite [Cu2(OH)3Cl], an isomer of atacamite and paratacamite. Copper oxalate was also found in this green paint layer and calcium oxalate were detected in a numerous of paint layers without restriction to any specific colors. Pigments and their use as mixture or as overlapping different paint layers in Zhongshan Grottoes were identified on a selected number of samples. Over painted areas could be identified and two reasons could explained the blackening of the paintings: a loss of the fragilized colored paint layer, which make the underneath black paint layer visible and a darkening of the upper paint layer, due to the burning of the candles in the temple. Furthermore, FTIR analysis performed on the samples give the indication of an oily binder.