Study On Moisture Retention Characteristics Of Wall Paintings At Mogao Grottoes

As an important cultural heritage on the ancient Silk Road in China,Mogao Grottoes are famous for its exquisite wall paintings and colorful sculptures.After thousands of years,wall paintings are being invaded by various deteriorations.The wall paintings of Mogao Grottoes are multi-layered,and the plaster layer is the vital one of that.Its moisture retention performance is related to the formation and development of mural deteriorations.Analysis of the moisture retention characteristics of plaster is the premise of studying the formation mechanism of salt deterioration and the method of desalination in wall paintings.In this dissertation,the plaster of Mogao Grottoes is used as the research object.Different types of simulated plaster samples are prepared based on the analysis of the plaster layer of Mogao Grottoes.The soil-water characteristic test was carried out to analyze the soil-water characteristic law of different types of simulated plaster samples in the full suction range.Through constant temperature moisture absorption-desorption test,the adsorption-desorption characteristics of plaster to water vapor were investigated.Using the designed device for capillary migration of wall painting plaster,the migration law of sodium chloride solution in different types of plaster was analyzed by capillary rise test.Combined with the analysis of microstructure and thermal constant,the following main conclusions are drawn:(1)SWCC of simulated plaster with different material composition in full suction range were obtained.The SWCC of different samples have some differences.However,compared with the coarse plaster and the fine plaster,the soil water characteristics are not much different.Based on the experimental data in the full suction range,the fitted SWCC of the highly accurate can be obtained,and the F-X model is more suitable to describe the SWCC of the plaster.According to the characteristics of SWCC in the high suction range of the plaster samples,it is recommended that the relative humidity in the cave should not be higher than 58%.Using the fitted SWCC and unsaturated permeability coefficient function,the unsaturated hydraulic conductivity change trend of plaster samples can be obtained.(2)The pore characteristics of different types of simulated plaster samples are different.The number of pores in the coarse plaster is the largest,and the pore distribution is also the largest.Most of the pores are in and around the wheat straw.There are many small pores(<100nm)in the fine plaster that are not in the remaining samples.These pores come from hemp fibers,but they have little effect on the moisture absorption and migration of the samples.There is little difference in the thermal conductivity of each type of sample.(3)The moisture absorption-desorption law of the wall painting plaster under high and low humidity environment is in the form of exponential decay.The characteristic of its moisture absorption and desorption rate is "fast first and then slow".After 50 h,the moisture absorption and desorption rate will obviously change.The moisture absorbed by the plaster cannot be completely desorbed.Under high humidity,it has a strong ability to adsorb moisture vapor,and this adsorption process has already completed about 60% of the total adsorption process only in 1/6 of the time.Extra attention should be paid to the increase in humidity in the cave.(4)The capillary rise rate of the simulated ground warfare samples has a tendency of fast first and then slower,and the coarse plaster sample is the fastest.the moisture content of the sample gradually decreases with the increase of height.The coarse plaster sample has a much higher moisture content than other types of plaster samples.The distribution of NaCl in the 0-7cm section of the sample is relatively uniform,and no accumulation will occur.A large amount of accumulation occurred in the 7-10 cm section,and deterioration and salt crystals appeared accordingly.(5)For the complex plaster sample,the moisture content distribution law and the salt distribution law are abruptly changed near the coarse-fine plaster layer interface,that is,there is a capillary barrier effect on the movement of capillary water at the interface.The capillary barrier effect puts forward a higher level of discussion on the formation mechanism of salt deterioration and the method of desalination.