By Francesca Piqué
Far to the west of Beijing, in northwest China, lies the oasis of Dunhuang, for a thousand years a major stop on the Silk Road. For travelers headed west, it was at Dunhuang that the Silk Road split in two. One route curved north and the other curved south, both moving along edges of the large and stark Taklamakan Desert. From the 4th century to the 14th century, Dunhuang was an important place for travelers to pray for a safe trek along this unforgiving desert—or to give thanks for having successfully arrived.
Along the Silk Road moved not only coveted goods but also ideas—among them Buddhism, which spread throughout China—and Mogao, on the outskirts of Dunhuang, became a site of Buddhist worship. Beginning in the 4th century, hundreds of cave temples were carved into one and a half kilometers of cliff face at Mogao, ranging from small decorated niches to large ornate chambers. The walls of the chambers were adorned with wall paintings, and many housed polychrome sculptures.
By the 14th century, safer and faster travel by sea—as well as political instability along land routes—caused traffic along the Silk Road to decline, and eventually the Mogao site was abandoned. Toward the end of the 19th century and into the early 20th century, the grottoes were "discovered" by explorers from the West and Japan. Today the site is managed by the Dunhuang Academy, established a half century ago and dedicated to the preservation and study of the grottoes.
The cave temples are now the focus of national and international tourism, attracted by Mogao's artistic, historic, and religious importance (see Conservation, vol. 14, no. 2). The site—a kind of archive of medieval Buddhist art—includes almost 500 painted caves with over 45,000 square meters (484,000 square feet) of wall paintings and over 2,000 sculptures. Having survived for centuries in Mogao's dry climate, this art nevertheless continues to require constant attention and research in order to address inevitable and ongoing deterioration.
Since 1989 the Getty Conservation Institute and the Dunhuang Academy have collaborated on conservation at Mogao, initially focusing on general site-related conservation issues. That effort resulted in improved conditions at the site, including a dramatic reduction in the amount of sand being blown into the caves from the cliff face above (see Conservation, vol. 9, no. 1).
Beginning in 1997, one aspect of the collaboration has concentrated on the conservation of wall paintings. The objective of the current wall paintings project is to develop approaches that would have wide applications not only at Mogao but also at similar sites on the Silk Road.
An Integrated Approach
The most effective approach, especially in a site the size of Mogao, is preventive conservation—which addresses the causes of deterioration—in addition to remedial conservation, which simply repairs damage. This approach requires an understanding of the causes and nature of ongoing deterioration, developed through a thoughtful integration of science and conservation.
Cave 85 at Mogao was selected as the collaborative project's model site for wall paintings conservation. This large 9th-century Tang dynasty cave was constructed for religious purposes and as a shrine to the local and powerful Zhai family. Within the cave are approximately 350 square meters of painted surface. The paintings have extraordinary artistic, cultural, social, and historical value, and they provide an illustrated encyclopedia of everyday life during the Tang dynasty. The cave was selected because its wall paintings suffer from many of the typical problems found throughout the site.
An essential and critical step in the project has been to assess the condition of the wall paintings through a detailed study of the cave to identify and record the type and distribution of deterioration. The identification of ongoing (or active) deterioration was helped by comparing historical photographs of the cave, taken by the Dunhuang Academy in the past 50 years, with the current condition of the wall paintings. Visible changes indicated areas of recent and, most likely, active deterioration. The assessment of the cave's condition shows that the detachment of the painted plaster from the conglomerate rock is an active and considerable problem, often resulting in the collapse and destruction of a portion of the paintings.
Understanding the causes and the mechanism of this ongoing deterioration requires interdisciplinary diagnostic investigations. These include the collection and study of conservation, analytical, and environmental information that is used to formulate hypotheses regarding the deterioration and to develop recommendations for interventions.
Diagnosis at Cave 85 began with the study of the active deterioration, the form it takes, and its distribution throughout the cave. Scientific investigations were necessary to understand the physical and chemical composition of the original materials and of the materials affecting the painting—in Cave 85, these were soluble salts in the rock and plaster and, in certain areas, polyvinyl acetate used in previous conservation interventions. Environmental monitoring carried out over the years provides an understanding of the fluctuations of humidity and temperature surrounding the paintings and at the site.
Causes of Deterioration
Since 1997, GCI and Dunhuang Academy staff have worked in their laboratories and in Cave 85 to collect information and carry out tests to formulate and confirm hypotheses on the causes of deterioration. While the project is not complete—and some results need to be confirmed—a mechanism has been theorized as responsible for the ongoing deterioration in Cave 85. The active detachment is concentrated mainly on the cave's west wall and the west portion of the north and south walls and ceiling. Analytical study showed that, as expected, the distribution of deterioration corresponds to earthen plaster zones with large amounts of soluble salts in the plaster, mainly sodium chloride, naturally present in the conglomerate rock walls. The areas affected by deterioration contain almost 10 times more soluble salts than areas in good condition.
Although the environment at the site is generally quite dry, when it rains, the humidity rises and the salts in the walls absorb moisture in the air through a phenomenon called hygroscopicity. The resulting salt solution can move through the porous earthen plaster. When the climate returns to its typically dry condition, the salts crystallize, causing detachment of the painting or the formation of losses in the paint layer (depending on where in the painting stratigraphy the crystallization occurs). The physical history of the site lends support to this hypothesis, because most recent plaster losses, in this and in other caves, have occurred following periods of rain. This hypothesis is related to atmospheric humidity. Another salt activation mechanism may be linked to the migration of moisture vapor from the body of the rock.
To remove or to significantly reduce this cause of deterioration, the team has been working on a salt extraction process, combined with conservation intervention. Obviously the soluble salts present in the conglomerate rock cannot be completely removed; in addition, poulticing of salt can be dangerous to the earth-based and water-sensitive paintings. The team is also working on improving the sealing or closing of the cave entrance, in order to reduce fluctuations of the cave environment related to the rain by preventing humid air from entering the cave. The GCI and Dunhuang Academy team is currently studying this intervention and its consequences.
Understanding the causes and mechanism of deterioration is critical for development of effective conservation treatment. In particular, it is important to know if the causes of deterioration can be completely eliminated or only mitigated. For example, the intervention of grouting—defined as the introduction of material with adhesive and bulking properties into a void—was designed with a consideration of the substantial quantities of salts present in the plaster and the conglomerate rock. The reattachment of the earthen plaster rich in soluble salts must be done with materials compatible with the original, using minimal amounts of water, an appropriate application method, and, most important, an absorbing system to capture the salts mobilized by treatment. In addition, the set grout must have characteristics compatible with those of the original earthen plaster. After extensive laboratory and in situ testing, the project team developed a grout with the desired properties that has been used since April 2002 to reattach the paintings.
Following emergency stabilization of the paintings with Japanese paper and supporting presses, an important aspect of the treatment process has been the development of an absorbent system that extracts salts mobilized by the water and ensures that as little as possible remain in the paintings. Testing has included the use of simulated plaster panels artificially contaminated with salts to evaluate different absorbent systems and to assess the distribution of soluble salts before and after grouting combined with poulticing.
Although not completed, the collaborative project on the wall paintings at the Mogao grottoes has already provided important preliminary diagnostic results on the causes and the mechanism of deterioration, as well as on the stabilization treatment of the paintings. The combination of soluble salts in the plaster and rock and the humid air formed during rain events appears to cause the deterioration of the wall paintings. This mechanism is probably common to numerous sites on the Silk Road, and mitigation measures will therefore have wide applicability. Similarly, the methodology adopted to develop the adhesive grout mixtures for the salt-laden plasters in Cave 85 may be generally applicable to other similar sites.
This project is an example of the importance of combining conservation and science for diagnostic investigation with the planning of interventions. Unfortunately, a characteristic of in situ conservation is that the causes of deterioration cannot always be eliminated—only reduced. With deterioration still active, it is clear why, in the field of in situ conservation, a project is never really considered completed; regular monitoring and maintenance are required.
The preliminary results of the project will be presented at the international conference "Conservation of Ancient Sites on the Silk Road," to be held at the Dunhuang Academy in August 2003. On this occasion, Cave 85 will be open so that delegates may visit the conservation site, examine its problems, and view at close range the remarkable wall paintings that tell us so much about life in a distant time.
Francesca Piqué is a project specialist with GCI Field Projects and the head of the project conservation team at work at Mogao.