Physicochemical Properties and Microstructure of Plaster Layer of Ancient Wall Paintings in Northwest China

Abstract

ABSTRACT Several grottoes were excavated in Northwest China between the 5th and 13th centuries, and the most famous relics of these grottoes are wall paintings. The plaster layer is the most significant layer in grotto wall paintings, and its characteristics are related to deterioration, treatment measures, and restoration materials during the conservation process. However, studies investigating and analyzing ancient plasters in grotto wall paintings are scarce. In this study, a series of laboratory experiments was conducted on ancient plasters of four grottoes in Northwest China to reflect their physicochemical properties and microstructural characteristics. The results reveal that the plaster is composed of sedimentary soil from the riverbed or floodplain near the grottoes mixed with vegetal fibers. The sedimentary soils were mainly composed of fine-grained soil mixed with a certain concentration of sand (0‒41%). In the plaster production process, lime was not artificially added, and the internal structure of the plaster was formed by the agglomeration of sand-clay fibers. Capillary pores in the plaster are the main channels for salt migration and accumulation, and the accumulation of salt destroys the cementation between the soil particles, rendering the plaster loose and porous. Therefore, attention should be paid during daily maintenance to prevent further damage to wall paintings due to the migration and accumulation of salts caused by rainwater and water vapor. Sedimentary soils from nearby riverbeds or floodplains can be used in restoration work to obtain better compatibility with the original plaster.