Thursday, November 18, 2010

Lost Wax Casting of Bronze

The traditional process used to cast bronze, developed and used 4,000 years ago, is called lost wax casting. This method results in highly detailed, quality castings and has changed very little over the centuries. The method was used by craftsmen and artists from every culture from ancient Mesopotamia and Egypt up to today.

This process begins with the object that is to be cast. An artist creates a sculpture from clay or wax as these substances remain soft. These sculptures can be very intricate and finely detailed. A mold is then made of the sculpture, usually in two pieces, sometimes more when a sculpture is very large. Plaster is used on small pieces, but fiberglass is also used, especially for larger sculptures. After the molding material has been applied and it has dried thoroughly, the mold is then opened and the original sculpture is removed. Usually the original is destroyed in this process, as the molding material is very rigid. That is why the original sculpture is made of a soft material so that it can be removed from the mold.

After all of the original sculpture has been removed and the mold cleaned, a thin coating of wax is brushed onto the inside of the mold. This is done to capture every intricate detail of the mold. The mold is then put together, and wax is poured in the mold while it is being slowly rotated. After a layer of wax at least three quarters of an inch forms on the inside of the mold, the rest of the wax is dumped out. When the wax has thoroughly hardened, the mold is removed. The wax cast is then worked on by artisans to ensure that it is as perfect as possible.

The wax casts are then dipped into a mixture of plaster and sand. This is repeated many times. Each dipping has to dry thoroughly before the next. With each successive dip, the sand and plaster mixture gets more coarse. This results in a very strong shell being built up over the wax cast. After these shells have hardened, vents and 'runners' are added to allow the molten bronze to enter the shell, and for gases and excess metal to escape. The shells are then placed into a kiln and baked at very high temperatures. This heat causes the wax inside of the shell to melt away, leaving an exact image of the sculpture in the shell. This gives the process its name of 'lost wax casting'. After baking, the shell is now a mold ready for molten bronze.

Once the bronze has been poured and has cooled, the shell is then carefully chipped away, leaving a cast of the original sculpture in bronze. Any flaws are removed, the sculpture can be buffed and left its natural bronze color, or painted. This process is very labor intensive and expensive, but it results in such fine quality castings that it is still used after 4000 years.





1 comment:

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Worked in a steel mill for 30 years. Amateur chef, piano player, book reader, letter writer, gardener, peace advocate.