Analysis of the pigments for smoked mural by confocal micro-Raman spectroscopy

Smudging is one of the most serious damages of Mogao Grottoes' murals. In this paper, confocal micro-Raman spectroscopy is adopted to investigate the smoked murals in Mogao Grottoes. The exutive fragments from the smoked murals in 71 Grotto have been analyzed. The white pigment, which is covered by a layer of black, is identified as calcite, and the black is amorphous carbon. No any other chromatic pigments were found in all exutive fragments. From which, we may speculate that the white pigment (CaCO3) is stable undergoing smoking and other chromatic pigments may have been destroyed by smoking. Based on the analysis and the records of the Mogao Grottoes' murals, a representative smoking process was designed to explore the degradation of the pigments in an imitative mural. The typical pigments, including white (aragonite), black (carbon) and red (vermilion), are analyzed from imitated smoked mural, and the results show that aragonite is stable and vermilion is damaged. It is judged that the vermilion has been destroyed by the smoking process, in which the high-temperature smoke makes the vermilion easily react with oxygen and produce the mercury vapor and sulfur dioxide. The simulative smoking experiment is reliable and feasible for investigating the degradation process of pigments and is proposed to investigate more pigments which can supply significant and important information for the restoration and conservation of smoked murals in Mogao Grottoes. Copyright (c) 2017 John Wiley & Sons, Ltd.