Conservation

Located in arid weather condition of a desert, many caves in Dunhuang and the relics inside have survived more than a thousand of years. However, various factors, such as natural environment, man-made environment, natural deterioration of painting materials, and the increased popularity of tourism have caused considerable damage to the micro environment in some of the caves and serious deteriorations to the murals have occurred.     

The Dunhuang Academy is responsible for the conservation of Mogao Grottoes, Yulin Grottoes and the West Thousand Grottoes. With the tremendous work by the staff of the Academy, we are now a pioneer in the conservation of murals in China.    

Started in 1943, when the Dunhuang Art Institute (the first predecessor of the current Dunhuang Academy) was established, the staff did their best to work on tasks they could undertake with the very limited resources, such as cleaning up the sand accumulated outside and inside the caves, temporarily reinforcing the passage way between the caves. After the Institute was renamed to the Dunhuang Relics Institute in 1950, a detail study was conducted on the deteriorations of the rock structure and the relics. The risk of cliff surface collapsing was high due to the cracks in the rock structure. In the 60s, reinforcement of the cliff surface was performed in time to prevent serious damage of the caves.  Large areas of detached murals were also treated and saved from permanent damage.

In 1984, after the Institute was renamed to the current name of the Dunhuang Academy, conservation of the Mogao Grottoes entered a new stage. The team was expanded to include trained talents, modern concepts and techniques were introduced in collaboration with other domestic and foreign institutions. Realizing the difficulty and complexity of the huge task ahead, a comprehensive approach was also adopted to include conservation of the environment around the caves in addition of the conservation of the relics inside; collaboration and support of other scientific disciplines; and scientific management.

In the past half century, many projects were conducted on the environmental monitoring, both outside and inside the caves; analysis of the colouring dye of the murals; research on the mechanism of the weathering of the rock structure, the mechanism of the deteriorations of the murals, conservation techniques and materials; prevention of deterioration, etc. Many of these projects were done in collaboration with foreign institutions, such as the National Institute for Cultural Properties, Tokyo, Japan; the Getty Conservation Institute, U.S.A.the Australian Heritage Council;  the Andrew W. Mellon Foundation, U.S.A.; the Northwest University, U.S.A.; Tokyo University of the Arts, Japan; Osaka University, Japan; etc.

Although we have made outstanding progress, the road ahead is still long and difficult. We must focus on preventive conservation which is the best method to slow down deteriorations. The “visitor load” analysis indicates that we cannot allow unlimited access to the caves. The digitization project (hyperlink) in progress will help us to achieve the proper balance between tourism and conservation.

Major Rock Structure Deteriorations:

• Weathering and rain erosion

The rock structure around the caves comprises of sand stone or conglomerate held together by clay. Thus, the rock structure is very loose, with low strength and high porosity, and disintegrates easily in water. The minerals in the cementing clay expand when they absorb water, contract when they lose water, thus causing weathering damage to the rock surface. Although it is typically dry with low precipitation in this area, heavy shower is common. Rain erosion is particularly significant during heavy rain storm when the cementing clay disintegrates in water.

After many years of research, we invented PS (high modulus potassium silicate), an inorganic material for reinforcement.  PS reacts with clay to change its structure, from loose and crystallized clay into a non-crystallized reticular gel which greatly increase the strength and water resistance. Using PS, about 1800m² of the cliff surface was treated and reinforced.   

P – Weathered rock surface.   P – Reinforced surface after PS treatment

• Cracks

Cracks were abundant in the rock structure. More cracks were developed due to re-distribution of stress by the construction of the caves. Under the influence of external factors, such as earthquake, wind and rain erosion, the cracks further develop and rocks are in risk of collapsing.  We also invented an inorganic grouting material called PS-F (PS: high modulus potassium silicate, F: fly ash). The material is easy to apply and has minimal shrinkage, and is an ideal grout for cracks in conglomerate. In 1999, we grouted the cracks in approximately 1600m² of the rock surface at the top of the cliff, and have successfully stopped rain leakage into the grottoes.  

P – Crack grouted with PS-F

• Wind and sand erosion

Blowing sands are damaging. In a wind tunnel experiment, wind and sand were blown at a speed of 20 m/s towards a piece of conglomerate rock of 5x5x5 cm in dimension obtained in Mogaoku. After 3 minutes, 2/3 of the rock was damaged.  In Mogaoku, sand storm with wind speed of 20 m/s is common. Wind erosion reduces the height of the rock structure above the caves allowing rain to leak into the grottoes. Sand blown into the caves may result abrasion on the paintings.  Large quantity of sand in front of the caves area also has an impact to the environment, tourism and daily operations.

In 1990, wind fence was constructed above the grottoes, sand blown into the grotto area have been reduced by 80%. Between 1992 and 1999, two belts of shrubs were planted to the west of the wind fence. These shrub belts are now 2 km long. They reduce wind speed, consolidate the sands around the shrubs and thus diminish further the amount of sand blown into the cave area.

In 1992, geomembrane and geotextile provided by the Getty Conservation Institute were used together with PS to reinforce the rock structure above Cave 460, thereby preventing further water leakage through the ceiling of the cave.

P – Wind fence      P – Treated rock structure and sand above cave 460

Major Mural Deteriorations:

Before discussing the deteriorations of the murals, we need to understand how the rock surface was prepared before painting.  

First a coarse plaster layer (grass mixed with mud) was applied followed by a fine plaster layer (cotton mixed with mud).  Finally a thin white powder ground layer was applied to create a smooth surface to paint on.

• Plaster disruption

Plaster disruption is the most serious deterioration of the wall paintings. The deterioration loosens the plaster layer, and causes adhesion loss of the ground layer or paint layer from the plaster. The paint layer is then lifted in pieces or in the form of punctate eruption. When the deterioration is serious, large pieces of plaster and painting may fall off. It spreads rapidly, causes the worst damage to wall paintings and is the most difficult to treat. Plaster disruption may reoccur within a few years of treatment. For treatment method, please see ‘Flaking’.  

Plaster disruption is caused by the movement of soluble salt in the plaster layer or the adjacent rock mass to the surface of the plaster layer. When the humidity in the cave is high, the salt dissolves and contracts, causing the plaster layer to shrink. When the humidity is low, the salt crystallizes and expands. The salt dissolves, crystallizes, then dissolves and crystallizes again according to the change of temperature and humidity. This cycle of contract and expand repeatedly causes serious disruption deterioration to the wall paintings.

Since the root cause of plaster disruption is the movement of soluble salt due to change of temperature and humidity, the best treatment of the deterioration is to maintain a stable low humidity environment inside the caves. This will slow the spread of the deterioration and reduce the risk of reoccurrence after treatment.

P – Flaking due to Plaster Disruption deterioration  (酥碱起甲)

P – Punctate Eruption (点状脱落) due to Plaster Disruption deterioration

• Flaking

When the paint layer or paint layer together with the ground layer are lifted in small pieces (like fish scales) from the plaster layer, the mural has flaking deterioration.

Flaking can be caused by plaster disruption as described above; or due to bonding loss between the ground and paint layer, or between the plaster and the ground layer. Bonding loss may be caused by natural aging or inappropriate amount of bonding material in the colour dye.

There are six steps to treat murals with flaking deterioration:

a)       Clear the dust from the surface of the painting and under the lifted paint layer. 

b)       Inject glue underneath the lifted paint layer. 

c)        Press the lifted paint layer using a stainless or wooden knife.

d)       Press the treated area using a cotton ball.

e)       Spray glue to the treated area.

f)         After the sprayed glue is about 70% dry, cover the treated area with a white silk cloth, then roll slowly with a soft rubber roller. 

Finally, desalination is performed to reduce the risk of plaster disruption in the future. If there is no plaster disruption, the treatment has been very successful. Murals treated in the 60s of the last century are still in good condition.

P – Mural with Flaking deterioration, before treatment    

P – Mural with Flaking deterioration, after treatment

• Detachment

Detachment deterioration occurs when the paint layer detaches from the plaster layer in the form of blister, or when the plaster layer detaches from the rock surface due to adhesion loss as shown in the cross-sectional view below.

Detached murals can be grouted and anchored as follows:

a)       Drill grouting holes in non-critical areas of the painting.  Then insert a grouting tube into each hole.

b)       Install wood board to support the detached area to be treated. 

c)        Grout is injected into the grouting tube using a small syringe. 

d)       After the grout is completely hardened, remove the wood board and cover the grouting hole with the same material as the plaster layer.

e)       To treat large area detachment, small rods made of wood or bamboo are inserted into the grouting holes to provide additional anchoring support.    

Then desalination is performed to reduce the risk of plaster disruption in the future.

• Discolouring and fading

For the paintings in Mogao Grottoes, the blue and green colour dyes are more stable than the red colour dye which consists mainly of cinnabar, terra rossa or red lead. Terra rossa is the most stable and is not affected by humidity. Cinnabar turns to dark when exposed to light for a long time. The worst discoloured red dye is red lead which oxidizes and darkens quickly when exposed to light and high relative humidity above 70%.

Colour fading is due to the aging of the dye when the original lustre is lost. Also, when the adhesive in the dye aged, the dye is pulverized. Colour fades when the pigment particles start to fall off.     

P – Discoloured mural