Little new in bronze casting
Liu Guoqing, a lost-wax craftsman who works on contract to the National Palace Museum, ticks off a list of reproductions that must be delivered to the museum by the end of September. In all, the list runs to 486 pieces. The third of five brothers, Liu Guoqing took over his father's business along with second brother Liu Guoxian and fourth brother Liu Guozheng.
Their late father first started doing contract work for the museum over 20 years ago, having been referred by an acquaintance. Now that a government procurement system is in force, such contracts must be awarded through an open bidding process, but the Liu brothers are almost always awarded a contract in the bidding procedures held every one or two years because there are so few lost-wax artisans left these days. The contracts have always called for the production of anywhere from several dozen to several hundred pieces.
Liu Guozheng, who also does original art making bronze flowers, notes that "the National Palace Museum always waits until they have almost nothing left in stock before they hold a bidding procedure." Contracts are not awarded on any regular schedule, so the income is not steady. The brothers therefore do other types of metal work, as well. Among other items, they make saxophone mouthpieces, incense burners, and ashtrays using lost-wax casting.
Bronze is an alloy of 80% copper and at least 2% of either tin or lead (or both). Most bronzes are made by casting. Archaeologists have discovered the remains of bronze casting operations at three different locations in Henan Province, and have determined that ancient craftsmen made their molds from pottery clay, and melted the metal in crucibles. Today the molds are generally made of silicone while the metal is melted in an electric furnace, but two things remain unchanged-the metal must be melted and cast at 1200°C or higher, and those working the metal must endure infernal heat as they sweat away in front of an open furnace.
Strolling into the Liu family's workshop in Xinzhuang City, Taipei County, I catch a slight smell of wax. The brothers are scheduled to fire up the furnace the next day, and are busy making a reproduction of Zi Fu Xin, a jue (wine vessel) from the late Shang made by a Prince Xin in memory of his father; thus the inscription zi (son) fu (father) xin (Xin family) that appears on the rim.
Liu Guoqing pours molten wax into the gap between the polyresin outer mold and the silicone inner mold. A half-hour later, after the wax has cooled and become solid again, Liu Guoqing opens the mold and takes out the wax model and hands it off to his wife and sister-in-law, who look for blemishes in the wax. Pockmarks are filled with beeswax, but if there is anything wrong with the patterns on the wax model, repair is impossible and they have to start all over again. Being a traditional mom-and-pop operation, the Liu family has no computerized equipment to ensure a perfect process, so in addition to painstaking attention to detail, a bit of luck is also needed.
A wax model free of defects is covered with a coat of zirconium silica hydrogel, after which Liu Guoxian makes a sand casting mold by covering the model with a layer of coarse casting sand. This completes the preparations for the dewaxing.
Two or three days later, the three brothers transfer an entire rack of air-dried sand molds into a steam dewaxer, where they are evenly heated to 100-150°C, and the wax comes gushing out of the machine. After the wax is removed, the inside surfaces of the empty sand mold constitute a negative image of the Zi Fu Xin wine vessel.
To cast an imitation of an ancient bronze, the artisan must go through several steps that precede the pouring of the molten metal into the mold (left to right): (1) Preparation of a wax model. (2) The wax model is dipped into a slurry, which dries to form a sand mold. (3) Removal of the mold. (4) High-temperature dewaxing. The casting is then poured, and later removed from mold, resulting in a reproduction of the famous Zi Fu Xin (a jue-type wine vessel). The product is a strikingly accurate reproduction of the original.