The Influence of Two Natural Reinforcement Fibers on the Hygrothermal Properties of Earthen Plasters in Mogao Grottoes of China

Murals in Mogao Grottoes consist of three parts: support layer, earthen plasters and paint layer. The earthen plasters play a key role in the preservation of murals. It is a mixture of Dengban soil, sand, and plant fiber. Two different natural fibers, hemp fiber and cotton fiber, were reinforced to earthen plasters in the same percentage to evaluate the influence on hygrothermal performance. The two types of earthen plasters were studied: one containing hemp fiber in the fine plaster (HFP) and the other containing cotton fiber in the fine plaster (CFP). Specific heat capacity, dry thermal conductivity, water vapor permeability, and sorption isotherms were investigated. The results showed that the difference between two natural fibers has much more impact on the hygric properties (water vapor permeability and sorption isotherms) of earthen plasters than on their thermal performance (specific heat capacity and dry thermal conductivity). The CFP with higher density has higher thermal conductivity than the HFP with lower density. But no significant differences of specific heat capacity were observed. Compared with HFP, CFP used in murals can reduce the rate of water transfer and prevent salt from transferring water to the mural surface. The overall findings highlight that all these features of CFP are beneficial to the long-term preservation of murals. The study of the earthen plasters in Mogao Grottoes is of general significance, and the measured properties can be used to obtain coupled heat and moisture analysis of the earthen plasters and to dissect the degradation mechanism of murals.