Wall Paintings Conservation at Mogao Grottoes
An understanding of the often complex causes and mechanisms of active deterioration through the integration of information on condition with analytical and environmental investigations was essential for the planning and design of an intervention strategy for Cave 85.
Analytical InvestigationThe analytical investigation had three primary objectives:
1. identify materials and techniques used in the creation of the paintings;
2. identify materials and techniques used in previous conservation treatments and assess their role in the deterioration of the paintings;
3. contribute to an understanding of the causes of deterioration
The analytical team's methodological approach included noninvasive examination methods, combined with sampling and instrumental analysis. In addition, the analytical investigation incorporated training of Dunhuang Academy staff throughout the project.
• an understanding of the late–Tang dynasty wall painting technique by looking at painting stratigraphy, together with characterization of earthen plaster, ground, mineral pigments, organic colorants, and binding medium;
• an extensive survey of soluble salts and their distribution throughout the cave;
• study of previous treatment materials and assessment of their role in the deterioration of the paintings;
• study of organic colorants and of the techniques useful for their analysis; and
• study of the light sensitivity of organic colorants and the implications for design of a lighting system for the cave.
Environmental InvestigationEnvironmental monitoring was undertaken over an extended period in order to understand the role of the environment in the deterioration of the paintings. The investigation included a moisture survey and a comprehensive program of meteorological monitoring of the site and the interior microclimate of the cave. Related parameters, such as air exchange rates, were also determined in order to develop preventive measures to mitigate ongoing deterioration and to establish a cave visitor carrying capacity. Training of Dunhuang Academy staff in the use of monitoring equipment and analysis and interpretation of data was a component of this work.
The environmental investigation included:
• role of water vapor in the deterioration of the wall paintings;
• microclimatic conditions of Cave 85, particularly humidity/temperature relationships diurnally and seasonally;
• air exchange rates of the cave; and
• climatic conditions of the site.
Summary of results of the diagnostic investigationResults from the conservation, analytical, and environmental work showed that the intrusion of exterior humid air into the cave during summer months was responsible for occasional high levels of relative humidity and the deliquescence of salts in the painted plaster, leading to both surface and subsurface deterioration.
Halite (NaCl) is the predominant salt species responsible for the ongoing deterioration of the painted plaster in Cave 85. The salt distribution survey carried out in Cave 85 (as part of the analytical investigation) correlated salt content in the painted plaster, both topographically and stratigraphically, with deterioration. The average soluble ion content in the plaster (expressed in weight percent) toward the rear west end of the cave where the condition of the paintings is poor is between 2.0 and 3.9 percent. On the east end of the cave, where the condition of the paintings is far better, the values are less than 1 percent.
In winter, the relative humidity (RH) at the Mogao Grottoes (situated in a desert climate) ranges between 10 and 20 percent. In the summer, the relative humidity increases to 40 percent or higher. Summer rain can raise the humidity to 80 percent or higher. These values are above the deliquescence equilibrium RH for halite (75 percent). Studies showed that the salts present in Cave 85 deliquesce at values below the equilibrium RH of halite because of the presence of other hygroscopic salts. Infiltration of humid exterior air into the cave significantly raises the relative humidity within the cave to levels high enough to cause deliquescence of the salts.
As is well known, salt-related deterioration of wall paintings results primarily from mechanical disruption by forces exerted due to crystallization from aqueous solution, or due to a hydration state change. Laboratory investigations attempting to simulate the behavior of salts within the plaster and salt movement through the conglomerate and plaster—with accumulation below and at the painted surface—are continuing.