Conservation processes
Artifact conservation is demanding work. A conservator must have aesthetic sensibility and artistic skills to match those of the original artist. He must be intimately familiar with all kinds of repair materials and how to prepare them. And more importantly, ancient artifacts are just like people: each one is unique, so the conservator has to approach each one accordingly. How should this piece be conserved? How far should I go with repair? These judgments require experience and intuition, and lie at the very heart of conservation work.
To ensure quality, all NPM conservation work is done in-house. The conservators must go through extended on-the-job training, and are sent to Japan and Britain to take courses. They practice their craft on everyday items, and before embarking on a conservation job they always test the object first and do little "dry runs" to make sure no problems will crop up. Only then does the actual conservation work begin.
One of the simplest jobs a conservator handles is cleaning the surface of wood, for example, but even here, he can choose to clean it with any of eight different kinds of cleaner, including water, alcohol, olive soap, and even spit. Which is the most appropriate? How long do you leave the cleaning agent on the surface of the wood to clean it without damaging it? It takes a lot of experience to answer such questions.
Conservation work is guided by a number of basic principles. Apart from the "less is better" approach mentioned above, the most important principle is that the materials and methods used in conservation must be reversible. It should be possible to remove or undo, with damaging the object, anything that was previously applied or done. For example, you don't weld bronzes or stick tape to paper.
As Yang walks me through his world, one built on hard work and diligent study, I begin to better appreciate just how much there is to know in his complex specialty.
(1) Conservation of bronzes
Bronze vessels require care on a regular basis. Most ancient bronzes still surviving from the Shang and Zhou dynasties spent two to three thousand years buried underground before their discovery. Some will have spent the millennia in direct contact with the earth and underground moisture, while others were enclosed in dark, dank tombs. It would be unusual for such bronzes to emerge unblemished.
Most such blemishes are harmless, and collectors prize certain kinds of discoloration. Moreover, verdigris formed by corrosion seals the metal below away from the air, thus acting as a protective layer that prevents corrosion from creeping deeper into the metal. The only destructive type of corrosion occurs when chlorides react with the metal to form cuprous chloride.
Yang stresses: "Cuprous chloride absorbs oxygen and moisture from the air and produces cupric chloride, a powdery substance that will just keep on eating away at the bronze unless you put a stop to it. We call it bronze disease." When the bronze is placed in a sealed case at a relative humidity of 45% or lower, the problem will not get any worse.
Yang points out that there are different ways to treat bronze disease. Some are very time consuming, some are expensive, some eliminate the discoloration but leave the bronze pitted, and some leave the bronze discolored, so each bronze piece must be considered on its own merits.
Yang most often employs the "BTA" method. He first uses a small probe, scraper, and brush to clear away the loose cupric chloride powder, then uses an organic solvent mixed with acetone and toluene to wash away any oils adhering to the surface of the bronze, and next soaks the bronze piece in a mixture of benzotriazole (BTA) and alcohol for about 14 hours. The resulting chemical reactions produce a stable protective film. After that, he dries the piece in a vacuum chamber. This method turns the bronze a bit darker and causes it to lose a bit of its luster, but does an outstanding job of conserving the metal.
(2) Conservation of porcelains
Fragile porcelain is the type of artifact most often in need of repair. Before repair, it must be cleaned. If the piece has previously been repaired using superglue, the surface of the break must first be treated with a cotton swab soaked in hydrogen peroxide to soften the old glue. Dirt and grime are removed by brushing lightly with acetone or alcohol, then the piece is soaked in water. After the old glue has been removed, the piece must be dried in a ventilated or heated chamber.
Next, it's time to put the "jigsaw puzzle" back together based on the decorative pattern, thickness of the pieces, and other clues. The larger shards are pieced together first, then the small slivers are painstakingly fitted in. To check whether a shard has been properly fitted, a light is shone on the piece from behind; if no light gets through, a proper fit is confirmed. After the whole thing is fitted together, it is time to apply glue.
When a porcelain piece is reinforced, glued, and restored to its original color, the choice of chemicals, filler, and pigment will all have a huge impact on the quality of the conservation work. In the old days they used plaster, or perhaps lime and egg whites. More recently, however, isinglass and B72 have generally been the adhesives of choice. No matter what kind of adhesive method is used, the most important thing is to avoid damaging the artifact; quality of conservation is secondary.
Recreating the coloring of the glaze is the most important aspect of the conservation process, and the most difficult. The piece must be restored to its original color, and the color has got to be convincing. It's not a job well done unless it is impossible to tell the difference with the naked eye. If the glaze layer is relatively thin, it is enough just to apply some color to the surface, but if the glaze layer is thick then a bit of re-glazing is in order. Glazes from different dynasties are different in color, moreover, so you will show yourself up as an amateur if you use a Ming color on a piece from the Song Dynasty.
According to Yang, the NPM often uses non-water-soluble pigments, mixing in an appropriate amount of binding agent (similar to paint baking colors), then using a small spray gun to spray an even coat onto the porcelain. Afterwards, the piece is baked for two hours in a heated chamber. After that, the technician dips a calligraphy brush into binding agent and pigment and carefully paints on the appropriate pattern. It may not sound too difficult, but the conservator must have a fine aesthetic sense and well honed technique, otherwise the decorative elements will retain none of the charm of the original.
(3) Conservation of lacquerware
Lacquerware is one of the most unique classes of items in the NPM collection. Lacquer itself is a liquid that must be brushed onto a solid material before its beauty becomes evident. The solid material to which it is applied is called a "base" and may be made of wood, bamboo, cloth, copper, or paper.
Lacquer does an outstanding job of protecting the base beneath. It is extremely adhesive, and thus does not fall away easily. It is resistant to acids, alkalis, and bacteria, and thus highly impervious to degradation. There is an old saying that you can "bury a single drop of lacquer in the soil, and it will not break down in a thousand years." As long as lacquerware is not exposed to big changes in sunlight or temperature and humidity, which would cause the underlying wood or bamboo base to expand and contract, it will usually remain in good condition for a very long time.
Ancient lacquerware generally gets dirty over the years, and needs to be cleaned, but water is absolutely the last thing to do the cleaning with. Lacquer is waterproof, to be sure, but a very old piece usually has cracks in it.
"If water gets through the cracks and underneath the lacquer, it will break down the undercoat, which would damage the base through expansion and warping, no matter what it is made of."
Yang takes out a feather duster and a wool brush, the best tools for cleaning lacquerware.
(4) Conservation of paper
Chinese painting and calligraphy from the Han and subsequent dynasties were most often executed on silk paper, but by the Song Dynasty cotton paper had become the medium of choice. The biggest shortcoming of cotton paper is that it grows brittle with age, and is thin and prone to wrinkling. Silk paper, too, is very apt to wrinkle if there is uneven tension on the warp and weft threads. Everything in the NPM collection is on cotton paper, which ages with time. The protective silk borders and backs are also easily damaged, and a scroll that is frequently rolled and unrolled, or hung on a wall, is susceptible to cracking and creasing.
Lai Qingzhong, who has worked in paper artifact conservation at the NPM for almost 30 years, points out that paper and silk are quite delicate. They are easily damaged by changes in temperature and humidity, and therefore require painstaking care and regular attention. If there is a crease mark, for example, it must be supported from the reverse side. If the head on the end of the scroll spindle has fallen off or broken, it has to be fixed. If there is insect damage, the holes must be filled.
Because cotton paper is light and thin, it is supported on the reverse by lining paper to afford greater toughness and make the painting lay more evenly. Then silk backing and borders are added. Even with all these protective measures, however, the backing and borders must be removed once every 100 years and remounted. But if the lining paper is dyed a special color, then care must be taken not to touch it, because removing it would change the look of the painting. The backing must be removed with extreme caution, one small piece at a time, beginning in the center and working toward the borders. Traditional methods are then used to dry the painting in a dark place, after which a new backing is applied with a specially prepared paste.
The porcelain dish on the right looks flawless to the naked eye, but an ultraviolet light (left) reveals that the upper part of the dish has actually been repaired in the past.