Why Did the Colors of Murals in Mogao Grottoes Change?

Why did the color of some murals change? According to a survey, the colors of the majority of murals and colored sculptures painted with lead pigments in the Mogao Grottoes have undergone changes over time, with some even turning completely black. This is because the element lead has various chemical valence states and can undergo a transformation into lead compounds under specific conditions. The use of orange-red lead and white lead pigments is prevalent in the murals and colored sculptures. The red lead pigment is highly susceptible to significant discoloration. Lead white pigments are prone to turn black when exposed to the wet alkaline surface of the underlayer. In addition, the reproduction and metabolism process of mold in damp conditions results in the release of hydrogen peroxide, which can oxidize and discolor red lead.

How to prevent discoloration of pigments on murals and colored sculptures. Measure 1: Preventing high humidity is crucial in protecting the color of murals and colored sculptures from alterations. Firstly, water from the environment must be managed to prevent infiltration and thus reduce humidity in caves. Secondly, caves must be adequately ventilated, or moisture must be absorbed to decrease the humidity within the caves. Measure 2: Caves should be scientifically illuminated, as light can result in the aging of the organic adhesive mixed with the pigments, which consequently leads to discoloration or diminished color saturation. Measure 3: The number of tourists should be regulated since excessive quantities of carbon dioxide and water vapor in the caves can generate acidic water vapor leading to pigment discoloration.