Bronze Casting

Bronze casting is the process of making sculptures by using liquid mixture of bronze, an alloy of copper and tin, which is (usually) poured into a mold containing a hollow cavity of the desired shape and then allowed to solidify. Bronze is the most preferred metals for casting, as it has a peculiar property of expanding before setting, thereby filling all the finest details of a mould. The strength and ductility of the metal is yet another factor for which it is preferred over ceramic and stone materials. This feature is especially helpful in the preparation of action oriented statues. To know more about the technique of bronze casting, read on.

Bronze Casting Process

The first step is sculpting the portrait in clay. After the clay dries out, a lacquer is painted over the sculpture, but not dried out.
The next step is making of rubber mould. A master mould enable hollow wax casts to be made, to produce a copy of the bronze.
Next, the sculpture is covered with a clay blanket, the thickness of which decides the thickness of the rubber mould.
A thick plaster jacket is levied on the mould. After the plaster covering dries out, a section is taken off and clay blanket is removed, replacing the plaster section.
After this, liquid black poly-sulphide rubber is poured, through the pourer, over the mould.
With the help of a chisel, the plaster section is lifted and the rubber mould is carefully peeled off the sculpture. The rubber mould is then placed into the plaster jacket, to hold the shape.
Next step is to make a wax cast. For this, molten wax is poured into the rubber mould. Through the aperture, cooling molten wax is poured in and out of the mould. After it cools, the mould is removed and the rubber is peeled off to reveal the wax cast.
After this, runners, risers and core pins are applied to the wax.
To melt the wax inside, it is necessary to encase it in a mould, which is made up of plaster and grog. This liquid mixture is painted on the surface of the wax.
To form the core, the above liquid mixture is poured inside the wax.
Now, the finished mould is placed in the kiln and fired up to 700^oC. It is then cooled and removed from the kiln, further strengthened by a coat of scrim soaked in plaster.
When the mould becomes strong enough, it is filled with molten bronze, an alloy of copper and tin.
After it is cooled, the mould is broken apart, revealing the raw cast sculpture.
The bronze cast is then thoroughly washed to remove all the residues of ceramic mould.
After this, the sculpture is chiseled to give it the final shape.
The next step involves bronze chasing, which gives it the original shape.
Patinating is the next process, wherein the bronze surface is colored chemically using oxidizing agents.
After coloring the sculpture, with the desired color, a protective coat of wax is applied on the surface and buffed up.

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