From September 17-30, 1999, the GCI Mogao conservation team was in China for the fifth campaign in the Institute's wall paintings conservation project at the Mogao grottoes.
During the campaign, the team and its Dunhuang Academy counterparts examined the exfoliation and loss of paint that has recently affected the west portion of Cave 85, the focus of the project. Prior to the GCI team's arrival, Academy staff treated portions of the exfoliating area with polyvinyl acetate, a synthetic adhesive that has been used at Mogao since the 1950s. This material and its relationship, if any, to the deterioration process are under study. Therefore, the project team agreed to develop temporary emergency stabilization procedures, such as facing with Japanese paper, that can be used by Academy staff to secure paintings following events such as the recent one, which occurred shortly after rain at the site. The emergency stabilization will prevent paint loss while giving the team more time to develop and test an appropriate adhesive mixture to fix the exfoliating paintings. Gum tragacanth was recently identified at the GCI as the binding medium for the wall paintings of Cave 85. This natural product and others will be screened for suitability as adhesives.
A major component of the campaign was testing the working properties and performance characteristics of earth-based mixtures. The evaluation of these properties will allow the team to choose new mixtures to be tested, as well as to make a final selection of material for grouting (to reattach loose plaster) and plaster repair. The conservation team prepared 26 earth-based mixtures and tested their working properties, such as shrinkage, setting times, and water content. The Academy team was shown how to measure performance properties such as water vapor permeability, adhesion to the conglomerate, and strength; they will carry out these tests on the 26 mixtures after the samples have completely cured.
The conservation and analytical teams worked together to carry out core sampling in the rock conglomerate in the lower part of the cave where the painted plaster is missing. Samples were collected in six different locations at two to three heights, with depths up to 40 centimeters in 10 centimeter increments. A total of 64 samples were weighed immediately after extraction, then dried to determine the water content. The hygroscopicity of the samples will be determined at 60 percent and 80 percent RH. Qualitative and quantitative soluble salt analysis will be performed to obtain a profile of the distribution of salts within the lower part of the rock conglomerate. The analytical team identified sodium sulfate—a salt not previously found—in the conglomerate in the lower part of the cave.
Wall paintings conservator and project team member Zheng Jun of China's State Administration for Cultural Heritage was successful in training a computer specialist on the Academy staff in the transfer of graphic condition records in digital form. Zheng had learned this process during his working visit to the GCI in July 1999.
The next fieldwork is planned for March 2000.