Shin Maekawa, Zhang Yongjun, Wang Baoyi, Fu Wenli, and Xue Ping
The mogao grottoes are located approximately 25 km southwest of the city of Dunhuang in Gansu Province in Northwest China, between the Gobi and Takla Makan deserts. Situated along the ancient Silk Road, it maintains one of the world's richest collections of Buddhist murals and statues dating back from the fourth to thirteenth century c. e. The caves were excavated in various sizes and shapes into a natural cliff approximately 30-50 m high, running roughly north-south. The rock consists of a soft, poorly cemented, coarse conglomerate. In the 492 decorated grottoes, mud plaster mixed with straw was laid directly on the conglomerate surface, and a white ground-paint layer was applied prior to the application of the succeeding paint layers.
Over the centuries, the priceless Buddhist artifacts, wall paintings, and clay sculptures in the grottoes were constantly subjected to looting or deliberate defacement. The Dunhuang Academy was established at the site in 1944 to protect the sites and conduct research on the history and culture of the Dunhuang region. The recent economic boom, political change, and widespread cultural awareness of the Asian region have facilitated a sharp increase in the number of tourists visiting the site (Fig. 1). The large number of visitors could change the microclimate of the caves, which can be deleterious to the murals and statues; continued high relative humidity levels produced by visitors may allow growth of fungi and bacteria, which could lead to permanent staining of the wall paintings (ashrae 1993; Torraca 1984:1-18). On the other hand, an extremely dry microenvironment in the caves resulting from the intrusion of dry desert air may desiccate the paint layers, resulting in flaking of the painted surfaces (Maekawa 1993:616-23). Fatigue failure of the paint layers may result if the wall paintings are subjected to large daily variations of temperature and relative humidity. Large numbers of visitors could also produce a high enough concentration of carbon dioxide in the caves to be unsafe for the visitors themselves (ashrae 1993). Furthermore, the combination of the high relative humidity and carbon dioxide could result in formation of carbonic acid, which could alter the painting surface.
Figure 1
Number of domestic and foreign visitors at the Mogao grottoes between 1980 and 1991.
Objectives
Before any conservation work on a monument can be planned, basic information on the natural and fabricated environments affecting the site is needed. In the case of the grottoes, a dry desert climate combined with the stable microenvironment of caves contributed to the survival of the wall paintings and sculptures. However, the constant increase of visitors may be altering the microenvironment. Thus, in addition to general information on the climate of the site and microclimate of the caves, information was gathered about the effects of visitation on the caves' microenvironment: changes in cave temperature, relative humidity (RH), and carbon dioxide content. The number of visitors and their length of stay in the caves, as well as the months of maximum cave visitation, were also important factors to be documented. The ultimate objective of the environmental monitoring was to develop strategies to reduce the effects of visitors on the microenvironment of the grottoes.
Model Caves
Two architecturally similar, medium-size caves, Caves 323 and 335 (Fig. 2), were selected to document effects of visitors on the microenvironment. Both are located at ground level (there are no caves below them), oriented east to west. As indicated in Figure 2, each cave consists of a main chamber of about 125 m3 with an alcove of approximately 25 m3 and an entrance area of approximately 24 m3, connected by a short tunnel approximately 2 m high, 1 m wide, and 2 m long. Each cave has been fitted with a wall made of aluminum sheets on which two hinged doors, approximately 1 m wide and 1.8 m high each, are mounted in the middle. Both the wall and doors are made with fixed-angle louver openings. In 1989, as part of the collaborative conservation project of the Getty Conservation Institute and the Dunhuang Academy, a filter material was installed over the openings of the caves to protect against the intrusion of airborne sand and dust. The material used was a nonwoven, spun-bound, lightweight
Geotextile. Thus, it was expected that air exchange rates would be significantly reduced. Cave 323 was normally open to visitors for eight hours each day, and thus became the test cave. Cave 335 was closed to visitors, and served as the control.
Monitoring System
Figure 2
Schematic of Caves 323 and 335.
Figure 3
Map of the Mogao grottoes and locations of monitoring and base stations.
Three autonomous, environmental monitoring stations (one meteorological and two cave stations) and a base receiving station were installed (Figure 3 shows their locations). The meteorological station for collecting area-wide climatic data was located on top of the 35 m high cliff near the center of the site and approximately 200 m from the base station, which was set up at the Dunhuang Academy's laboratory building on the site. Cave stations were configured to gather information within the environment of the individual caves, which are approximately 250 m north of the meteorological station. The retrieval procedure for the accumulated environmental and microenvironmental data consisted of periodically transmitting the data collected at each station via UHF radio to the base station, where they were transferred to microdiskettes. These diskettes were then mailed to the Getty Conservation Institute in California for statistical analysis.
World History
