On the afternoon of 14 December, the 5^th session of “China-Greek Dialogues on Heritage Research and Conservation” of the 9^th lecture on coordinated and innovative artefact preservation successfully took place online and was livestreamed via the WeChat official account: China-Greece Cultural Conservation. The event was organised by China’s Palace Museum and Greece’s Institute of Electronic Structure and Laser of the Foundation for Research and Technology-Hellas (IESL-FORTH) and supported by the China-Greece Belt-and-Road Joint Laboratory on Cultural Heritage Conservation Technology (Joint Laboratory). It was jointly hosted by Qu Liang, executive deputy director of the Joint Laboratory and director of the Palace Museum’s Conservation Standards Research Institute, and Dr Paraskevi Pouli from IESL-FORTH. The seminar was honoured to have professor Guo Hong from the University of Science and Technology Beijing to give a special report with the title of “Preservation and Conservation of Murals in Drepung Monastery in Tibet”. Professor Guo currently serves as the vice-chairperson of China Association for Conservation Technology of Cultural Heritage’s executive council and the director of the academic committee of the National Cultural Heritage Administration’s key scientific-research base for the preservation of colour-painted ceramics. He is committed to research on the preservation of murals and colour-painted artefacts.

I. Background

Professor Guo began by briefly introducing the murals in Drepung Monastery. Construction of the monastery began in the 14^th year of the Yongle Emperor of Ming (1416). The monastery is the first of the three major monasteries of the Gelug in Lhasa and its murals are invaluable materials for the research on the origin and development, social changes, religion and science and technology of Tibetan, and are of great historical, cultural and scientific value. Guo remarked that the preservation of cultural heritage began with value assessment and preservationists achieve value preservation through the preservation of materials.

Drepung Monastery’s buildings, murals and their damage.

II. Requirements

Professor Guo’s preservation and conservation of the murals in Drepung Monastery mostly focused on its main hall, especially the mural next to the inner circumambulation path in its rear left side. The wall was seriously damaged and was threatening the stability of the entire structure. It urgently needed to be reinforced, which required the extraction of the mural. Once the wall was reinforced, the mural was returned to its original place.

Image and analysis from the three-dimensional laser scan of the damaged wall.

III. Issues

The challenge that faced the extraction and reattachment of the mural next to the inner circumambulation path in Drepung Monastery’s main hall was mostly the lack of space for manoeuvre. Moreover, due to the complexity of the damage to the mural, including hollowing, peeling, cracking, salt and alkali dissolution and chalking, the extraction and reattachment of the mural were far more challenging. Other aspects concerning the preservation and conservation of the murals in Drepung Monastery included analysis of the damage, the choice of preservation and conservation materials and techniques and the application of preservation and conservation ideas.

Other issues concerning the preservation and conservation of the murals in Drepung Monastery.

IV. Practice

Professor Guo explained that the preservation and conservation of the murals mostly focused on the following areas: examination and assessment of the preservation condition of the murals, monitoring of the preservation environment, research on production materials and techniques, types of damage and analysis of their causes, selection of conservation materials and techniques and mural conservation.

Study of the preservation condition of the murals and research on production materials and techniques

The study and assessment of the preservation condition of the murals and the research on production materials and techniques took advantage of digital analysis, no-destructive testing and sample analysis. The main objectives of digital analysis included information preservation, later-stage follow-up and monitoring and pre-emptive preservation. The value of the murals is reflected by their pigments and information on their components were obtained and analysed using a portable X-ray fluorescence (XRF) spectrometer, Raman Laser Spectrometer and scanning electron microscope. Meanwhile, high-performance liquid chromatography was employed to analyse the cementitious materials found among pigments, and it was found that the materials were chiefly made up of cowhide glue. The walls could be seen where the paint on the murals had come off and they were built using traditional Tibetan construction techniques. Their frameworks were created using rectangular beams and yellow clay was added to the block stones. The base layer is a double layer, with the bottom layer consisting of coarse sand and clay and the top fine clay. The pigments are all inorganic minerals.

Types of damage to the murals and analysis of their causes

The types of damage to the murals may be divided into that to the supporting structures, base layers and pigments. Due to their large size and heavy weight, the walls would sink and become misshapen over time. Meanwhile, humidity is another critical factor in the occurrence of damage. As the soluble salts in murals interact with moisture and repeatedly dissolve and crystalise, the dissolution of the salts would weaken their surfaces and cause them to peel off. Moreover, moisture would cause the cementitious materials among pigments and in the base layer to repeatedly expand and contract, leading to damage such as peeling and hollowing and the loss of the cementitious materials, which would weaken the walls and corrode the wooden structures. Furthermore, earthquakes would lead to structural changes, causing murals to peel off and crack. There are also human factors, such as the holding of religious ceremonies, tourists touching the murals and the traditional Tibetan construction method known as da’aga, which all have a certain negative impact on the murals.

Selection of conservation materials and techniques

Later, professor Guo explained the selection of conservation materials and techniques based on laboratory research and on-site experiments. The materials of the base layers of the murals were analysed using X-Ray diffraction and XRF. Based on the experiment results and research on the modification of traditional mortar, aga clay (a traditional construction material used in Tibet) was eventually chosen as the conservation material to repair the damage to the base layers and as the cementitious material for the extraction and reattachment of murals followed by further research. The aga clay is siliceous limestone with between 70% and 93% calcium carbonate and between 7% and 30% silicon dioxide. Once roasted, it produces sticky air-hardening calcium oxide and hydraulic beta-calcium silicate gel and anorthite. A diffractometer was used to analyse changes to the main mineral components and their ratio in aga clay when roasted in different temperatures, and it was discovered that when the temperature was higher than 1,100ºC, the components would not change considerably.

Loess dolls, aga clay and three types of hydraulic lime from Europe were researched as reinforcement materials to be used to fill in the hollow parts in the walls, while experiments were also conducted in terms of drying shrinkage, mechanical strength, water stability and salt, alkali and freeze-thaw resistance. Research showed that when the roasting temperature was controlled, both loess dolls and aga clay could produce reinforcement materials with air-hardening and hydraulic properties that could reach relatively high mechanical strength by the fourteenth day.

Reinforcement materials that are often used to treat peeling murals include animal glues, Paraloid B72, polyvinyl acetate, Primal AC-33 and gelatin-alum water. Polyvinyl acetate was initially used during the conservation of murals in the Mogao Caves, and extensive research subsequently discovered that it is also able to achieve positive results in repairing peeling murals in grottoes found in China’s dry northwest region. The five materials were assessed based on their permeability, stickiness, breathability, discolouration and stability, and their effects on the colours of the murals were regarded as the most important indicators. Eventually, Primal AC-33 was chosen as the material to repair the peeling murals in Drepung Monastery.

Extraction and reattachment technology of the murals

Before their extraction and reattachment, the damage to the murals needed to be repaired and the pigments reinforced. A protective layer was then placed over the murals and marked with the contents of the murals to facilitate later reattachment. Once extracted, the murals were then preserved and conversed with methods commonly applied to museum collections.

In situ mural preservation and conservation

Depending on the types of damage to the murals, different materials and techniques are used for their in situ preservation and conservation. Regarding the murals in Drepung Monastery, hollowing and peeling were the most serious and challenging damage to repair. The process of filling the hollows with reinforcement materials was: dusting of the mural surfaces, drilling of holes for grouting or bolting, supporting the murals, laying of pipes for grouting, grouting, removal of the pipes and wainscoting, filling in the drilled holes and touching up and distressing the areas accordingly. The process of repairing and reinforcing the peeled areas was: dusting, softening the pigments, infiltration and reinforcement of the edges, rolling cotton balls over the surfaces and smoothing the edges and cracks.

V. Discussion

The Venice Charter, which was drawn up and promulgated in 1964, is a milestone in the conservation of cultural heritage and has today become a guiding international agreement in the preservation and conservation of cultural heritage around the globe. China’s “Principles for the Conservation of Heritage Sites in China”, which include the principles of not changing the status quo of artefacts, minimal intervention, re-treatability, compatibility of conversation materials and identifiability, were established while taking into consideration the Venice Charter and the actual situation of artefact preservation in China. The preservation and conservation of the murals in Drepung Monastery strictly adhered to these principles.

Finally, professor Guo mentioned that the emergency preservation and conservation of the murals in Drepung Monastery were completed in 2015. Practical and viable plans for monitoring the conservation results were devised and served as the basis for the murals’ pre-emptive preservation and daily maintenance. After the lecture, experts and scholars from the conference call and audience from the livestream all engaged in animated discussions with professor Guo. Qu Liang, director of the Palace Museum’s Conservation Standards Research Institute, remarked that the report on the preservation and conservation of the murals was of great reference value for the peers from both Greece and China. The Greek team had many new ideas and technologies regarding in situ analysis and damage monitoring and expressed the wish to engage in closer communication and collaboration with professor Guo in the preservation of and research on murals.