Lost wax casting is the process by which a solid object made in wax is transformed into metal: bronze (an alloy of copper, tin and other trace metals) or brass (an alloy of copper, zinc and other trace metals). Thousands of years ago, bronze-age people knew how to do this; making the shape of a piece of jewelry or a spearhead in beeswax and then surrounding it with sand and clay. Molten bronze was poured in replacing the wax, which was burnt off as smoke. Variants of this technology were known to the ancient Chinese, the Greeks and the Romans.
In the Renaissance, Italian sculptors perfected the process for bronze statues such as the magnificent bronze statue made in 1545 by Benevenuto Cellini, housed in the Loggia dei Lanzi in Florence; Perseus Holding the Head of Medusa.
The modern process was developed about 100 years ago principally to cast gold, silver and platinum for jewelry. Important developments were the invention of rubber molds, which could produce the same wax shape repeatedly, and the spinning or vacuuming of a flask, so that molten metal could thoroughly fill the space originally occupied by the wax. Improvements in rubber technology, particularly by rubber mixtures hardened by acid, and of new wax varieties, have increased accuracy and diminished shrinkage to less than 2%.
In the process of reproducing a piece of hardware, the original brass is supported in a small box by a small wax sprue. A sprue is a ribbon-like wax attachment to the brass, which will serve as a channel for the liquid wax and later as a channel for the molten brass. Liquid rubber is poured around the brass and the supporting sprue. After 2-3 days of set time, the rubber mold is removed from the box and is carefully cut in two and the original brass is removed. The remaining cavity in the rubber mold is an exact negative impression of the original. These rubber molds are very durable and can be used repeatedly for years and years.
The next step involves creating the wax positive of the piece of hardware. The rubber mold is carefully put together again and liquid wax is injected into the empty mold through the sprue. Once the wax has hardened, the mold is carefully opened again to reveal an exact wax replica of the original, but with an attached sprue. The waxes with their attached sprues are then attached to a thicker column of wax, which will finally form the channel through which the liquid metal can flow. The waxes, their sprues and the central spine look a bit like a Christmas tree. These trees are placed in round cans or flasks and are surrounded by a special plaster called investment. The cans or flasks next go into a kiln where they are baked for a number of hours until the wax has burnt way, leaving an exact negative impression of the wax tree in the plaster.
The next step introduces the molten brass into the cavity left in the investment after the wax is burned off in the kiln. The type of brass used in this process is called alpha-beta brass; also called yellow metal or Muntz metal. The molten brass is put into the empty mold either by a centrifugal (spinning) process or by a vacuum process. Small hardware, such as posts, small plates and bails are made using the centrifugal method and heavier items, such as large Sheraton knobs are made using the vacuum process. In the centrifugal process, a charge of molten metal is introduced into the can, which is spun on the end of a long arm, which is also revolving-thus multiplying the centrifugal force. Vacuum casting uses a pump to evacuate the air from a chamber in which the flask, charged with molten metal, is placed. This pulls the metal into every nook and cranny.
The metal solidifies and the investment or plaster is broken away and removed to reveal the brass object: the sprues are cut off the central spine and the sprues are carefully ground away from the brass piece, leaving an object in metal which is an exact reproduction of the original.
The use of the modern technique to make replicas of brass furniture fittings was very rare until the last quarter of the 20th century. This method has revolutionized one aspect of antique furniture restoration by allowing the use of period originals as the template to begin the process.