Information Gathering and Assessment
Research, compilation, and review of all relevant material on Cave 85 and the site to better understand the problems and the causes and mechanisms of deterioration. This includes the following main areas of investigation:
A bibliography of all relevant literature has been established and periodically updated since the start of the project. This bibliography will be a component of the Cave 85 project report (forthcoming, 2006). It is also being developed as a searchable database.
Research, Compilation, and Review of Background Information
Information relevant to the Cave 85 project that provided context for the assessments of significance, management, and condition of the cave and site, as well as information that served the diagnostic investigations, was researched, compiled, and reviewed. This comprised:
- physical description of Cave 85 and its wall paintings and sculpture, including information on the geology and hydrology of Mogao, construction, and painting techniques and iconography;
- historical information on the physical history and historical context of the site and Cave 85 through archival records, historical photographs, and oral sources; and
- conservation history—including previous interventions and information on the type and extent of past interventions at the site and in Cave 85—and general knowledge about conservation practices at Mogao.
Central to this component of the project was the assessment of cultural significance and the resulting Statement of Cultural Values and Significance. The significance assessment was an essential component of the project as it took into consideration the artistic, historic, social, and scientific values of the site guiding the overall decision-making process. The objective of this assessment was to inform and guide conservation in order that the cultural values and significance of the cave be preserved unimpaired.
The management assessment looked at all the relevant factors other than condition that may affect the future conservation and management of the site. The management assessment for Cave 85—done as a component of the master plan for the site—requires understanding and analysis of the management structure responsible for the conservation and maintenance of a site. It is the basis for determining what can be done and what needs to be improved to make a project successful and the results sustainable. The management assessment for the Mogao Grottoes included understanding:
- who is responsible for conservation and management decisions and for the day-to-day management of the site;
- the processes by which decisions are made and recorded;
- the security and regular maintenance of the site;
- the expertise, resources, skills, budget, and time available for the completion of the project, from both the Dunhuang Academy and the GCI perspective.
The aim of the condition assessment was to gain a comprehensive understanding of the state of the cave's preservation through identification and recording of the types and distribution of deterioration phenomena. The assessment included:
- detailed examination of the paintings;
- comprehensive color and black-and-white photographic survey of wall paintings;
- graphic documentation to map the types of deterioration and their distribution; and
- creation of an illustrated glossary in Chinese and English of condition terms.
The main deterioration phenomena observed in Cave 85 included surface deterioration (exfoliation and flaking), and subsurface deterioration (plaster detachment and disruption, powdering of the plaster, and areas of loss of plaster and painting). All are conditions common to other caves at Mogao, in varying degrees. The graphic condition recording showed that the distribution of deterioration—both surface and subsurface—increased in severity and extent to the rear (west) end of Cave 85.
The development of means to reduce or mitigate the causes and activation mechanisms and to formulate appropriate conservation treatments depends upon the diagnostic investigation. The diagnostic investigation in Cave 85 aimed to identify and understand the causes and mechanisms of deterioration of the wall paintings. Research and investigations were carried out by the conservation, analytical, and environmental teams. Focused research into the main types and location of active deterioration, original materials, painting techniques, salts, and environmental conditions provided the basis for formulating and testing hypotheses of deterioration.
Identification and determination of active deterioration Building upon the condition assessment, the main types of deterioration were categorized as active or historical in nature. Active deterioration refers to phenomena that are continuing. Historical deterioration refers to phenomena that are no longer active.
Active deterioration can be determined through review of historical records, such as past conservation reports and photographs, that indicate recurring deterioration and repeated treatment, as well as by in situ examination of the paintings.
Two principal types of active deterioration were identified in Cave 85:
- exfoliation: the lifting and resulting loss of paint, ground, and upper plaster layers. The severity of this phenomenon was often associated with previous treatments for flaking of the paint layer with extensive use of synthetic and impermeable fixatives. As the causes of deterioration remained, the flaking recurred in a more serious form as exfoliation.
- plaster detachment: loss of adhesion between the earthen plaster and the conglomerate. Detachment and subsequent collapse of the painted plaster has been a long-term problem in Cave 85 and in many of the caves at Mogao. Previous attempts at solving this problem—including the use of anchors to pin detaching plaster to the conglomerate—have failed.
The large scale of exfoliation and detachment, in particular, were considered the most urgent and severe problem affecting the wall paintings in Cave 85.
Understanding the causes and mechanisms of active deterioration
A better 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 the cave in Component Two: Testing and Development.
The analytical investigation had three primary objectives:
- to identify the materials and techniques used in the creation of the paintings;
- to identify the materials and techniques used in previous conservation treatments and to assess their role, if any, in the deterioration of the paintings; and
- to contribute to an understanding of the causes of deterioration.
The analytical team's methodological approach included noninvasive examination methods, combined with invasive sampling and instrumental analysis. In addition, the analytical investigation incorporated training of Dunhuang Academy staff throughout the project. Investigations included:
- developing an understanding of the lateTang 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 an assessment of their potential role in the deterioration of the paintings;
- study of organic colorants and of the techniques useful to their analysis; and
- study of the light sensitivity of organic colorants and the implications for design of a lighting system for the cave.
Environmental 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 measured in order to develop preventive measures to mitigate ongoing deterioration and to determine 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:
- possible role of water 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 investigation
Results from the conservation, analytical, and environmental teams concluded 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.
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.