莫高窟壁画地仗层水汽吸附特征及其影响因素

The characteristics and influencing factors of the moisture adsorption in the earthen plaster of murals in Mogao Grottoes

敦煌莫高窟是我国乃至世界上最重要的佛教遗址之一。洞窟壁画产生的病害大多与水盐运移有关,而作为导致病害的水汽来源之一,洞窟空气中水分主要通过吸附作用进入地仗层孔隙结构中。该文利用试验方法探究莫高窟地仗层土体的吸湿机理,并分析吸湿过程的影响因素。结果表明,在较低与较高的相对湿度下,地仗层土体吸湿的主要驱动力分别为短程吸附作用与毛细凝聚作用。对于莫高窟地仗层来说,由土颗粒比表面积与黏土矿物含量等因素影响的短程吸附作用较弱,因此低湿度条件下吸湿量较小。但在较高湿度条件下,地仗层中由土体中孔径分布影响的毛细凝聚作用明显增强,吸湿量显著增大。莫高窟地仗层平衡吸湿量开始明显增大的临界湿度大约在80%。此外,地仗层中澄板土与砂的比例是影响地仗层吸湿量的主要因素。试验结果为莫高窟壁画病害机理研究及预防性保护提供理论依据。

The Mogao Grottoes in Dunhuang is one of the most important Buddhist sites in China and even in the world.Most deteriorations on the murals in caves are related to water-salt transport,and the moisture in the ambient environment in caves,which is one of the water sources that lead to the deteriorations,could move to the pore structure of the plaster and form the bound water by the adsorption process.The moisture adsorption mechanism of the earthen plaster is researched by experimental methods,and the influence factors of the moisture adsorption process is also analyzed in this paper.The results show that the leading driving force of moisture adsorption in earthen plaster is short-range absorption and capillary condensation under lower humidity condition and higher humidity condition respectively.The hygroscopic capacity is relative small under lower humidity condition owing to the small surface area and low content of clay mineral of the Dengban soil particles lead to a weak short-range adsorption;however,the capillary condensation which is influenced by the pore distribution in earthen plaster is enhanced obviously and hygroscopic capacity of earthen plaster dramatically increased under higher humidity,and the critical humidity for the hygroscopic capacity of earthen plaster increase obviously is approximately 80%.In addition,the ratio of Dengban soil and sand in earthen plaster is the main factor to influence the hygroscopic capacity of earthen plaster.The conclusion can offer a theoretical foundation for the research on the deterioration mechanism and preventive protection of murals in Mogao Grottoes.