Experimental Study on Capillary Migration of Water and Salt in Wall Painting PlasterA Case Study at Mogao Grottoes, China

The plaster of the wall paintings at Mogao Grottoes is all made of earth, and the migration of water and salt in this kind of plaster is one of the most important factors that cause the deterioration of the wall paintings. However, the characteristics of the migration in the earthen plaster, which are quite crucial for the research on the formation mechanism of deteriorations and the conservation treatments, have not been fully comprehended yet. In order to study the migration rule of water and salt in the plaster, different types of simulated plasters were made according to the material composition of the plaster at Mogao Grottoes. Through a series of experiments and analyses, pore-size distribution, microstructure and capillary migration rules of the plasters were characterized. Due to the difference in capillary performance, a capillary barrier effect will occur at the interface between the coarse plaster and the fine plaster. After the capillary migration, the salt is more likely to accumulate on the area of the capillary front, which will directly damage the paint layer. And the salt-water migration process inside the plaster will make the plaster structure loose and porous. Therefore, it is important to prevent rain infiltration and the rising damp after rainfall for wall painting conservation.