Conservation Planning & Implementation
The conservation plan for Cave 85 included development and implementation of preventive conservation measures to inhibit further deterioration and remedial treatment interventions to stabilize the wall paintings and was based on a thorough understanding of the causes and mechanisms of deterioration.
Preventive Conservation Measures
In order to slow deterioration in the cave, preventive measures that mitigate and reduce the causes and mechanisms of deterioration of the wall paintings were implemented.
• better sealing of the outside doors and keeping the cave closed to visitation during periods of high humidity and rain to prevent the intrusion of exterior humid air; and
• use of an ultrasonic humidifier (i.e., Preservation Pencil) in conjunction with treatment procedures to reduce surface and subsurface salts in areas of high salt content.
Soluble salts have been established as the main cause of deterioration of the wall paintings in Cave 85. Therefore environmental control measures are the most effective means of slowing salt-related deterioration. A simple but effective strategy of keeping the outside door to the cave closed significantly reduces the air exchange rate of the interior microclimate with the exterior climate, during periods of high humidity, thus maintaining stable environmental conditions.
Reduction of soluble salts was also undertaken, although difficult given the fragile condition of the painted earthen plaster and the water sensitive nature of the materials. Surface salt reduction in limited areas was carried out with an ultrasonic humidifier in areas of high salt content to address salt accumulation at the surface and/or slightly below the paint layer. The ultrasonic humidifier projects a fine mist of warm water, which is used in conjunction with highly absorbent tissue to dissolve and absorb surface and subsurface salts.
Development of conservation interventions to stabilize the paintings follows principles of minimal intervention, compatibility, and retreatability and were based on understanding of the causes and mechanisms of deterioration. Particularly important was the realization that the soluble salts could not be completely removed and that mitigation of deterioration is only possible through control measures. Therefore all treatment materials were selected to exist in areas of high salt content and to withstand possible fluctuations in relative humidity.
Remedial treatments included:
• emergency stabilization: treating areas of exfoliation at the onset of the project through microgrouting using an earth-based slurry to provide both bulking and adhesion to the area behind the exfoliated layers;
• grouting: reattaching detached plaster to the conglomerate by injection of a fluid earth-based mixture with adhesive and bulking properties;
• fixing and consolidation: reattaching flaking paint layers and consolidation of powdering plaster areas with a 1 percent gelatin solution;
• plaster edging repairs: securing the edge of original plaster where loss of plaster had occurred with an earth-based plaster similar in composition to the original in areas.
A methodological approach to treatment development was designed and was applicable to all interventions. It included the following steps:
- Establish existing treatment conditions based on results of the diagnostic investigation and how they will impact the design of the treatment
- Assess treatment options and determine the most appropriate choices.
- Undertake a literature search of current research on treatment interventions selected.
- Specify desired performance characteristics of treatment materials indicating the long-term behavior of the material in its final state (which measures how a material will function over time).
- Specify desired working properties of treatment materials indicating the short-term behavior of the material in the state it is applied (which measures its practical ease of use).
- Select and formulate treatment materials based on desired performance characteristics and working properties
- Design testing protocol and procedures for laboratory and in situ testing to evaluate performance characteristics and working properties.
- Undertake testing.
- Evaluate results.
The implementation phase developed safe and appropriate methods of treatment based on the existing condition of the paintings and the cave, and provided training in all aspects of treatment to Dunhuang Academy conservation team members.
In a salt-laden environment, the use of water-based treatments can lead to damaging deliquescence and crystallization of salts on or below the painting surface. All treatment materials selected included water, and this therefore needed to be taken into account.
Several general principles were followed:
• Minimum amounts of water were used.
• Presses with highly absorbent layers were applied to the surface of the paintings after treatments so that salts could be absorbed. Absorbent layers were monitored and changed when damp. The press covered and maintained contact with the surface of the painting until the treated area was completely dry.
• Surfaces of treated areas were examined post-treatment for evidence of salt crystallization. In high salt areas, an ultrasonic humidifier (i.e., Preservation Pencil) was used to reduce salt on the surface after treatment.
• Treated areas will be monitored for salts and salt-related deterioration.
Specific application methods of treatment can also be altered to suit a particular condition or area. Grouting was carried out with syringes, combined with either needles and catheter tubes, depending on the size of the void and ease of access. Areas to be treated were studied and evaluated to plan the required sequence of treatments.
Consideration was also given during the implementation for a conservation approach that favors minimal intervention and future retreatability. In grouting of the wall paintings, the goal was not to fill every void but instead to do enough to anchor the paintings to the conglomerate by grouting key locations. This reduces the overall weight behind the paintings, a serious consideration for ceiling slope areas, and allows for future retreatability.
Final high resolution photographic documentation of the cave was undertaken by the Dunhuang Academy in 2005.
Last updated: January 2014