Influence of Environmental Factors on Deterioration of Mural Paintings in Mogao Cave 285, Dunhuang

Abstract

The Mogao caves in Dunhuang, located in the inland desert region of China, are a UNESCO World Heritage Site. Within this site, cave 285 is one of the most important caves. As a great deal of deterioration has taken place in this cave, a large amount of research has been carried out on the environmental effects that caused these changes. The deterioration such as changes in color and cracks in the mural paintings are due to physical environmental factors. In this work, the influence of the hygrothermal and light environments on the deterioration of the mural paintings in cave 285 was examined through several experiments using simulated mural paintings, focusing on the time period when the entrance hall had collapsed. In the drying experiments, many cracks in the Bengal red, a few in the lapis lazuli, were observed but none could be seen in the others. White crystals appeared in all the paint layers mixed with salt, and a herpetiform swell appeared in the talc substrate to which the saturated NaCl solution was added. The UV radiation caused the chroma saturation changes in most of the paint layers, showing a significant change in the case of the organic pigments. To examine the influence of light on the deterioration of the mural paintings, the simulation of illuminance under natural lighting condition during the period when the entrance hall collapsed was conducted, and the calculated results were compared with the visual observation. It was found that many discolored paintings or color changes of the layers are related to the large amount of annually integrated illuminance. Previous researches show that the East wall has been least affected by moisture, solar radiation, and sunlight compared to the other walls and ceiling. However, the effects of deterioration, including scratches, detachment, and discoloration, are also seen on the east wall. Hence, we investigated the effects of adhesion and the collision of windblown sand as factors contributing to the deterioration of the east wall. We conclude that sand blown by high velocity wind has led to detachment, flaking, and losses including fading of the paintings. To examine the effects of the hygrothermal environment on the deterioration of the mural paintings in cave 285, simulations of heat, moisture, and salt transfer were conducted for the time before and after the fall of the entrance hall. It was shown that the cracks caused by drying shrinkage were mainly created soon after the production of the paintings. The cracks caused by thermal expansion are likely to keep increasing; biodegradation and destruction due to freeze-thaw are scarcely progressing currently but they probably occurred more frequently when the entrance hall had collapsed. A part of the paintings on the west wall deteriorated due to the crystallization of the salt contained in the paintings (or the soil wall) just after the production of the paintings. Regarding the relationship between temperature in the cave and insect excrement or secretion, the black-spotted soiling spread over the entire walls and the brown deposit seen in the cracks or peeled parts on the ceiling is considered to be the results of the excrement or secretion of insects on the wall paintings. The inside of the main chamber where temperature is stable throughout the year provides the insects a suitable space to live in.