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The History and Process of Lost Wax Casting
Lost-wax casting, sometimes called by the French name of cire perdue or the Latin cera perduta, is the process by which a bronze or brass is cast from an artist's sculpture; in industrial uses, the modern process is called investment casting. An ancient practice, the process today varies from foundry to foundry, but the steps which are usually used in casting small bronze sculptures in a modern bronze foundry are generally quite standardised.
Other names for the process include "lost mould," which recognises that other materials besides wax can be used, including tallow, resin, tar, and textile; and "waste wax process" or "waste mould casting", because the mould is destroyed to unveil the cast item. Other methods of casting include open casting, bivalve mould, and piece mould. Lost-wax casting was widespread in Europe until 18th century, when a piece-mould process came to predominate.
Casts can be made of the wax model itself, the direct method; or of a wax copy of a model that need not be of wax, the indirect method. These are the steps for the indirect process:
Mouldmaking. A mould is made of the original model or sculpture. The rigid outer moulds contain the softer inner mould, which is the exact negative of the original model. Inner moulds are usually made of latex, polyurethane rubber or silicone, which is supported by the outer mould. The outer mould can be made from plaster, but can also be made of fiberglass or other materials. Most moulds are at least two pieces, and a shim with keys is placed between the two halves during construction so that the mould can be put back together accurately. In case there are long, thin pieces sticking out of the model, these are often cut off of the original and moulded separately. Sometimes many moulds are needed to recreate the original model, especially large ones.
Wax. Once the mould is finished, molten wax is poured into it and swished around until an even coating, usually about 1/8 inch or 3 mm thick, covers the inner surface of the mould. This is repeated until the desired thickness is reached. Another method is filling the entire mould with molten wax, and let it cool, until a desired thickness has set on the surface of the mould. After this the rest of the wax is poured out again, the mould is turned upside down and the wax layer is left to cool and harden. With this method it is more difficult to control the overall thickness of the wax layer.
Removal of wax. This hollow wax copy of the original model is removed from the mould. The model-maker may reuse the mold to make multiple copies, limited only by the durability of the mold.
Chasing. Each hollow wax copy is then "chased": a heated metal tool is used to rub out the marks that show the parting line or flashing where the pieces of the mould came together. The wax is dressed to hide any imperfections. The wax now looks like the finished piece. Wax pieces that were moulded separately can be heated and attached; foundries often use registration marks to indicate exactly where they go.
Spruing. The wax copy is sprued with a treelike structure of wax that will eventually provide paths for molten casting material to flow and air to escape. The carefully planned spruing usually begins at the top with a wax "cup," which is attached by wax cylinders to various points on the wax copy. This spruing doesn't have to be hollow, as it will be melted out later in the process.
Slurry. A sprued wax copy is dipped into a slurry of silica, then into a sand-like stucco, or dry crystalline silica of a controlled grain size. The slurry and grit combination is called ceramic shell mold material, although it is not literally made of ceramic. This shell is allowed to dry, and the process is repeated until at least a half-inch coating covers the entire piece. The bigger the piece, the thicker the shell needs to be. Only the inside of the cup is not coated, and the cup's flat top serves as the base upon which the piece stands during this process. Prior to silica, a mixture of plaster and fire-proof material such as chamotte was used.
Burnout. The ceramic shell-coated piece is placed cup-down in a kiln, whose heat hardens the silica coatings into a shell, and the wax melts and runs out. The melted wax can be recovered and reused, although often it is simply burned up. Now all that remains of the original artwork is the negative space, formerly occupied by the wax, inside the hardened ceramic shell. The feeder and vent tubes and cup are also hollow.
Testing. The ceramic shell is allowed to cool, then is tested to see if water will flow through the feeder and vent tubes as necessary. Cracks or leaks can be patched with thick refractory paste. To test the thickness, holes can be drilled into the shell, then patched.
Pouring. The shell is reheated in the kiln to harden the patches and remove all traces of moisture, then placed cup-upwards into a tub filled with sand. Metal is melted in a crucible in a furnace, then poured carefully into the shell. If the shell were not hot, the temperature difference would shatter it. The filled shells are allowed to cool.
Release. The shell is hammered or sand-blasted away, releasing the rough casting. The spruing, which are also faithfully recreated in metal, are cut off, to be reused in another casting.
Metal-chasing. Just as the wax copies were chased, the casting is worked until the telltale signs of the casting process are removed, and the casting now looks like the original model. Pits left by air bubbles in the casting, and the stubs of spruing are filed down and polished.