Ancient mural painting protection method

Country ancient mural painting has a long history. After a long history of the rise and fall of evolution and the vicissitudes of nature, many of the magnificent murals recorded in historical documents have long been gone. Survived to date a large number of successive buildings, grottoes and tombs in the exquisite murals have become one of China's valuable cultural heritage.

First, the storage of ancient frescoes and the existing problems

Mural painting refers to the paintings drawn on the walls and ceilings of buildings or painted on cloth, paper and other carriers, and then pasted on the walls of the drawings or patterns. Ancient Chinese frescoes are generally differentiated by the different places where they are painted, such as temple frescoes, temple frescoes, grotto frescoes, tomb frescoes, and residential frescoes. Whether it is cave murals, hall murals or temple murals, most of them have a history of hundreds or even thousands of years. Whether early murals or relatively late murals, are specific historical conditions of people's understanding of this form of painting art murals as well as the true record of the historical background, representing the highest level of mural art at that time, all have their important historical, artistic and scientific value.

For the protection and restoration of frescoes, we first need to recognize the mud layer structure of frescoes, the mud layer structure of frescoes and frescoes uncovering and restoration of the relationship is relatively close. The development of mural painting protection technology program, first of all need to understand the structure of the mud layer of mural paintings research. Buildings and grottoes within the mural paintings, generally have mud layer or gray layer. Burial mural, some directly painted on the stone wall; some brush a very thin layer of white clay powder or wipe a very thin layer of white ash.

Because the murals are located in the geographical environment, geological background and other natural environment is more complex, often by the rain, wind and sand erosion and the threat of bright light exposure, sometimes the earthquake will also bring devastating damage to the murals. Frescoes have experienced hundreds of years, the production of materials continue to age, the inherent structure of the frescoes have been damaged, resulting in the loss of bonding between the layers of materials, the frescoes become very fragile. In the above comprehensive factors **** with the role of the frescoes produced such as pigment layer armor, the battle of the ground crisp alkali, hollow drums, shedding, pigment fading, mildew, scratches and other diseases. How to prevent its damage and how to use modern science and technology and the choice of restoration materials is China's mural painting protection workers face important issues. [1]

Second, the ancient mural paintings of the uncovered technology

1, mural paintings of the preparatory work

Preparatory work, including the picture of the temporary reinforcement, block painting line, drawing the size of the block map, the production of uncovered with the customary board known as the front of the wall plate and the purchase of uncovered with the equipment, medicines and other items of work.

Temporary reinforcement of the picture: ancient murals stored to today, intact as before is not easy to find, the vast majority of more or less there are some of the symptoms of the disease as mentioned earlier. Among them, such as degumming, crisp alkali, cracks, etc. must be taken before uncovering the temporary reinforcement measures. The so-called temporary, is the use of reinforcing materials, must have a "reversible" in order to prepare for the repair that should be improved can be re-processed.

Dusting: general mural clay layer is more solid without degumming and serious crisp alkali, in the temporary reinforcement before, should first use a damp towel to paint the area soil gently wiped away, or vacuum cleaner to remove the picture floating dust.

Degumming: mural painting in the tomb, degumming phenomenon is more common, can be sprayed with glue alum water 1 ~ 2, humidity, infrared lamps can be used to dry, the temperature between 40 a 60 ℃, humidity is too high easy to make the color discoloration, the light distance from the surface of the mural should be 30 cm away, the use of agents in addition to the glue alum water, but also can be used in polyvinyl alcohol or polyvinyl alcohol butyral and other macromolecular materials.

Mud layer crisp alkali treatment: the materials used, methods and degumming treatment of the same, only the concentration of the material used to increase the number of times spraying. Individual cases such as Shaanxi Yongtai Princess Tomb in the top of the rain ceiling mural, the surface layer of a thin and brittle skin, the inside of the white ash is loose like tofu dregs, simple painting is not effective, this situation should be taken by the method of the needle, will be reinforced agent, a needle a needle injection into the loose layer. Work a square inch by square inch in order to reinforce the treatment. The agent used should be diluted first and then concentrated.

Crack reinforcement method: depending on the degree of solidity of the mural and the size of the cracks, sand and mud walls in the building, in the case of the drier, only the larger cracks on the paper and cloth strips, fine cracks, is not a dense situation can not be dealt with. In order to ensure the safety of uncovering, in recent years, sand and mud walls are also used more than the whole piece of cloth method. White ash wall, especially the plain white ash wall without mixing materials, regardless of whether there are cracks need to be according to the size of the picture, the whole piece of cloth protection. Because of this kind of white ash wall, itself thin and brittle, more micro-cracks, uncovered in the process of vibration and easy to break, in case of larger cracks need to be first paste cloth, and then all paste cloth reinforcement. The top of the cloth to grow out of the picture 20 to 30 centimeters, in order to uncover the temporary fixed in the wall plate. Reinforcement of paper, must be white, so as not to contaminate the picture due to paper color loss. The surface of the paper needs to be smooth, toughness, generally baked shellfish paper is appropriate, reinforced with cloth, gauze is appropriate, warp and weft line dense cloth and mural surface layer often bonding is not firm, the effect is not good. Stickers and cloth adhesive, in the drier case, available table of national paintings with the dough powder paste; in the case of dampness in the crypt, usually with peach glue, but stickers or cloth need to be dried with infrared lamps. If both paper and cloth, you must dry the paper and then paste the cloth.

Painting wall haze treatment: this situation should depend on the degree of haze. Slightly, no surface cracks, not only do not need to deal with and still take one of the favorable conditions in the uncovering method introduced, but the haze area is large need to be reinforced, in the absence of paintings or no major lines of the place, digging small holes with dry mud or dry mortar squeezed into the back of the painting wall, so that it is temporarily glued to the wall, the method is called "dry mud riveted solid". This method is called "dry mud riveting". If there has been regional shedding of the wall, the application of dry hard mud or mortar along the edge of the shedding hook, so that part of the mud bar squeezed into the back of the frescoes so that it is temporarily glued. This method is customarily referred to as "edge reinforcement".

2, block painting line

Small area of the mural, the conditions can be taken out of the whole mural, this process can naturally be omitted. Usually encountered is a larger painting, the need to uncover the block, the work should consider the following issues.

Partitioning should consider both the artistic integrity of the picture, but also to consider the uncovering, repair, transportation and other work convenience. In general, due to the need to consider the integrity of the picture art, its size can not be fully standardized, should be based on the actual situation of the picture, the big on the big, the small on the small. According to experience, in many areas of our country at present under the existing conditions, the height of the chunk to 2 ~ 3 meters is appropriate, the width of 1 a 1.5 meters is appropriate, the maximum does not exceed 2 meters. The area of each painting is generally 2 a 4 square meters, the maximum does not exceed 5 a 6 square meters.

Drawing the line according to the picture, the general study of the size and location of the chunks can be drawn after the line work. With the powder line package in the picture on the white or cyan powder line, the initial decision to draw the size of the block. The general situation to be reviewed once or twice to finalize.

3, measurement and drawing

For murals to be uncovered, it is necessary to measure and draw beforehand. If the wall is straight, to measure the total size of the picture and draw the "block size map". In the event of uneven or curved picture, it is more important to measure these special circumstances accurately.

4, the method of uncovering murals

Commonly used methods of uncovering roughly divided into demolition, sawing, vibration and prying and other four methods. After analyzing and comparing and then determine the method of uncovering, often in a place not only one, but a mixture of several methods.

Demolition method: if the conditions permit the removal of the wall behind the mural, this is a relatively trouble-free, simple way to remove the mural. When the preparatory work is completed, with the wall plate will start after the wall protection to take, outside the wall will be the wall of brick or adobe blocks, top to bottom layer by layer to remove, every certain distance of about 50 to 70 centimeters to temporarily add a baffle in order to prevent the process of taking the fresco mud layer to the back of the dumping, when the demolition of the bottom side, should be quickly pushed down to the inside of the wall plate together with the fresco mud layer to put the level.

Sawing method: this is when the back of the mural wall can not be removed when a method. The basic method of operation is to use a suitable saw blade to saw the mud layer of the mural, away from the original wall.

Shock to take the method: uncovering the conditions are basically similar to the saw to take the method. When the mural mud layer and the wall is particularly strong bonding is not easy to saw the case, the use of this method. The use of tools such as drills, from the mural mud layer at one end of the mud layer inserted into the mud layer deep chiseling, chiseling process around the mud layer by the vibration of the gradual separation from the wall or rock wall.

Prying method: when most of the mural mud layer and wall detachment, that is, there is a large area of empty haze, the use of this method is the most favorable. With a kind of "flat shovel", press on the long wooden handle, from the side of the painting from top to bottom, gradually prying to make the mud layer of the mural paintings and the wall all detached to achieve the purpose of uncovering.

Other uncovering methods: Tang Dynasty and Liao, Jin Dynasty tombs in the part of the tomb, generally not bricklaying, only in the earth wall plastered whitewash, painting murals on the situation, although also then saw the gray layer and the earth wall saw, but basically sawing and prying and the general sawing situation is different. This case can also be used "wooden box set to take" method, that is, on all sides of the ledge plus boards, made of a box, to be uncovered fresco set, from the back of the fresco even soil with shoveling or sawing, loaded into the box.

In addition, under certain conditions, can also be used to "take the method of adhesive tape". That is, in the cloth first coated with chemical materials, such as polyvinyl acetate or polyvinyl butyral and other polymer adhesive, and then the cloth is glued to the picture to be removed, the mural color layer glued to the cloth to be removed down.

Three, ancient mural restoration techniques

1, remove the back of the mud layer

Remove the back of the mud layer, the first picture down, placed flat on the restoration of the table, the need to take away the mud layer, sawed into a small square with a knife, about 3 ~ 5 cm square. Then use a small shovel to shovel one piece at a time, so that the back of the mural is basically flat, but be careful not to shovel broken picture. If the surface layer of the mural for the gray layer, the bottom layer for the mud layer, the mud layer should be shoveled clean, leaving only the gray layer, the back of the concave and convex too high, slowly scrape off the protruding part of the scraper, so that the gray layer to reach a basic level.

2, reinforcing the surface layer

The back of the mud layer shoveled to the basic even after the reinforcement of the surface layer, in the back of the frescoes with a row of brush brush or spray with a sprayer to spray the reinforcement material, the main thing in this process is the choice of reinforcement materials. The purpose of restoring the fresco is to better protect the fresco and the most important is to protect the picture. Reinforcement of the surface layer directly affects the picture, so the choice of reinforcement materials must be carefully considered.

The requirements of this material: waterproof performance; to be absolutely no damage to the fresco color material; can only hope to choose a longer aging period of the material, the length of the aging period is sometimes not necessarily a very strong emphasis, but need to be particularly stressed is the decomposition of the aging of the reinforcement material can not be damaged frescoes in color and mud layer or gray layer of the structure,

For example, the use of the traditional glue alum water reinforcement, because it is on color No damage, can be used as a direct contact with the color layer of the material, that is, can be sprayed on the front, can also be sprayed on the back. This material is water-soluble, waterproof performance is poor, but consider the storage conditions after repair, are greatly improved than the original environment, once the failure, can continue to spray, or after a period of time, there is a reliable material and then the second reinforcement. Whatever the future is what reinforcing material, it can not have a reaction to the glue alum water, because the ancient Chinese mural painting on the use of glue alum water, if the glue alum water with a reaction, this material can not play a role in protecting the color layer of the mural.

Again, in order to enhance the mechanical strength of the surface layer of the mural, most of the current use of polyvinyl acetate emulsion, polyvinyl butyral or polymethylmethacrylate butyl acetate and other polymer materials.

Polyvinyl alcohol and polyvinyl acetate emulsion in -17 ℃ easy to lose the cementing effect, and therefore should not be used in colder environments, and ROY-6260 and AC-3444 has a better resistance to freezing and thawing. The diluent in the polyvinyl acetate emulsion day and polyvinyl acetate emulsion is prone to precipitate after standing for a longer period of time. Therefore, in the use of the resin should pay attention to the precipitation, the use of the need to oscillate and shake well, as far as possible to do while using the preparation, so as not to affect the repair effect.ROY-6260 in the water after 24 hours of immersion, the film detached from the slide, and more than 80% of the polyvinyl alcohol was dissolved, so it is not suitable for use in the humid environment. [2]

Poly(vinyl alcohol), poly(vinyl acetate) latex has a strong resistance to light, they are very resistant to aging mural restoration adhesive under the current weak light in the cave. In addition, they have the ability to adapt to different humidity, the use of water solvent polyvinyl acetate latex mural restoration, can make the mural. Self-coloring gesso hanf color on the surface of the fresco. Skimming out the bottom color of the fresco. It does not have a discoloration effect on green and other colors, but can make blue and other colors to increase color saturation. The overall effect is that the frescoes become vibrant and darker in color after restoration. [3] This effect for mural restoration is to be considered carefully.

The aging of fresco reinforcement materials has a lot to do with their own aging resistance efficacy, and more importantly, it depends on the conditions under which the materials are aging, i.e., the environment in which the frescoes are located. Polyvinyl alcohol and polyvinyl acetate emulsion mixture of reinforcing materials, at present in the usual natural environment is not an excellent reinforcing materials, especially in the humidity of the environment, its performance is worse, poor permeability. However, in the particularly dry environment of Dunhuang and the wall with large porosity, the permeability of the aqueous solution of polyvinyl acetate emulsion and polyvinyl alcohol is still relatively good. More than thirty years of field reinforcement test proved that regardless of the reinforcement effect, or reinforcement process, polyvinyl acetate emulsion is a more ideal reinforcement material. At the same time, its heat oxidation resistance and UV aging resistance is also stronger.

Paraloid is currently more commonly used in the conservation of cultural relics. It is a very good mural reinforcement material, especially in the high humidity environment, its aging resistance is better than polyvinyl acetate emulsion, and permeability. [4]

3, make up the substrate

The back of the mural to make up the substrate, need to have a certain mechanical strength, such as compression, folding, vibration, impact resistance. In addition to the length of drying time, economic conditions, the source of the material must also be considered.

Early liberation period to make up for the substrate, the general use of gypsum, the method of operation is the picture down, the original layer of mud shoveled clean, in the back of the gray layer, according to the size of the paintings on the wooden bar around the frame, the frame is given to the first to pave the wavy coarse lead wire, and then use water to mix the gypsum evenly poured into the wooden frame, and then immediately with the sheared hemp uniformly pressed in the gypsum slurry, with the wooden board or spatula plastered flat. After the plaster solidifies and dries, reverse the picture and repair the concave residual edges. After this repair fresco general thickness of 10 centimeters or so, the advantage is easy to operate, economic, fast, but not suitable for more serious broken frescoes and frescoes with a larger arc. This method is limited by the effectiveness of gypsum, has been rarely used.

In recent years, the rapid development of domestic polymer materials manufacturing industry, to the repair of murals using modern materials provide a reliable source. Thus, the recent restoration of murals, in addition to the mud layer, such as the surface layer of white ash layer, in its back spray polyvinyl acetate Vu L liquid or polyvinyl alcohol a, and then make up the bottom layer of smear, still smearing white ash, but instead of polyvinyl alcohol butyl aldehyde or polyvinyl acetate emulsion ash, dry with epoxy resin paste cloth a layer. The total thickness of 1.5 a 2 cm, much lighter than with the weight of the sand clay, due to the continuous reduction in the price of polymer materials, the cost and the previous use of lacquer clay cost is similar.

4, fresco support and its choice of materials

Display exhibitions, for ornamental research need to formulate suitable materials for the support of the fresco.

The earliest fresco support, is made of wood strips into a frame, epoxy resin for the adhesive to the fresco bonded to the wooden frame, time-tested, the wooden frame is subject to external gravity, temperature, humidity, the impact of deformation is very easy. Bonded to the

Wooden frame on the mural also with the deformation of the wooden frame distortion, warping, and ultimately lead to damage to the mural. In addition, insect infestation can also jeopardize the wood frame support.

Aluminum alloy type material for mural support, aluminum alloy type material with its light weight, flat, not deformed and other advantages, in recent years is highly favored. Most of the domestic use it for mural support. The use of aluminum alloy material does solve the problem of easy deformation of the wooden frame. But recently found part of the mural and aluminum alloy Converse cracking problem. After research and study that: aluminum alloy support and fresco bonding process because of its shrinkage coefficient difference is too large and cause part of the separation phenomenon.

Honeycomb panels acid, alkali, salt, and flat, light weight, and mural bonding area is large, mural and support the phenomenon of separation should be said to be minimal, honeycomb panels themselves do not deform and mural bonding, mural deformation is also impossible. At the same time, acid, alkali, salt-resistant honeycomb panel on the air of carbon dioxide, sulfur trioxide resistance to enhance, improve and extend the protection of the mural time cycle. So: honeycomb panel as a mural support, the solution to make up for the defects and shortcomings of wood frame, aluminum alloy profile frame support. In addition, the mural and honeycomb board bonding, acrylic as an adhesive, acrylic material after drying, good flexibility, strength, although slightly worse than with epoxy resin, but flat and uniform honeycomb board and the mural of the adhesion area is sufficient to carry the weight of the mural. [5]

5, the picture of the reinforcement and repair

Mending the defective gray layer or mud layer: the picture of the original lack of mud layer or gray layer, in the reinforcement of the surface layer of the temporary first pad of clay, back reinforcement at the same time in the repair of the picture, shoveling the first pad of clay, and then the original mural shoveled down the old soil or old ash with new soil or new ash is easy to crack, the tone of the original paintings are not the same, and the system method should be the same as the original mural. This process had initially in the reinforcement of the surface layer before the patch, the results in the restoration process often occurs in the complex, inequality and other ills. On the picture of other too small concave point, fill in the effect is not good, in general, there are a few *** with the provisions of compliance:

1 residual hole depth of less than 0.2 centimeters do not make up, such a situation, make up is not easy to be solid.

2 characters of the face, in general, do not make up, especially the eyes part of the best not to make up.

3 near the edge of the painting of the missing pieces, such as the sub-pieces of the mural will be linked into a whole display, can be left until later to make up for the general situation of a single piece should be made up to wipe the whole.

4 Where to fill the mud, do not allow more than the original edge of the mutilation, so as not to damage or cover the lines and colors of the original painting.

Restoration of the picture: fill in the mud after drying, by the artists based on the copy and the picture, fill in the lines and colors. When the pieces are linked into a whole, the seams still need to be made up by the artists to make up the mud, make up the line, make up the color. The overall tone requires the whole old as old, faithful to the original work, not allowed to re-create recovery.

Picture reinforcement or sealing: in the new material does not come up with reliable scientific information, generally still use the traditional glue alum water for spraying, but should not be too thick and too thick, the general requirements of the concentration of some dilute, spraying 2 to 3 times can be.

Four, conclusion

With the progress of science and technology, the repair of cultural relics of the increasing importance of research in-depth, the protection and repair of murals have benefited from the significant development in the selection of protective materials at the same time, we have to pay attention to a problem is that the reversibility of repair materials. In the protection of historical and cultural heritage at the same time, so that the frescoes are still able to meet the public's appreciation and long storage down.

Notes:

[1] Uncovering and restoration of ancient Chinese murals

[2] Experimental testing of the physical property index of several mural restoration materials

[3] Research on the application of polyvinyl acetate and polyvinyl alcohol in the restoration of murals

[4] Selection of Dunhuang mural reinforcing materials

[5] Comparison of the effects of different methods used in mural restoration

[5] The use of different methods of restoring murals. Comparison of the effect of different methods