Casting Bronze Sculpture

I am full of admiration for the bronze caster who casts my work.

He is primarily a sculptor, but has a great depth of technical and practical knowledge of casting.

Some casters are using the ceramic shell method, but I have direct experience of the lost wax method, and that’s what I describe here.

The lost wax method
The process used in fine art bronze casting has been around for thousands of years and is called the 'cire perdu’ or lost wax method. In this process, the caster makes an exact replica of the original ceramic sculpture in wax, approximately 1/4 inch (0.5cm) thick. Plaster is applied inside and outside the wax, which is then melted and bronze poured into its place.

Creating a wax replica
A rubber skin is painted on to the surface of the ceramic sculpture, to pick up every detail of its surface. The ceramic is removed and the inside of the rubber mould is coated with wax to a thickness of about 1/4 inch (0.5cm).

When the wax has set the caster removes the rubber mould. The wax is now a perfect replica of the original sculpture - except that it is very light and completely hollow.

Applying the plaster
Sprues, for channelling the bronze, and airs for dispersing the displaced air, are applied to the wax, and then special 'investment' plaster is poured through a hole to fill the inside of the wax replica. The same type of plaster is applied to the outside.

Beforehand, the caster pierces the wax surface with special nails, to maintain the gap between the inner and outer layers of plaster after the wax has been melted out.

Then the piece is turned upside down and heated in a kiln to melt out the wax - hence 'lost wax'.

Pouring the bronze
The caster turns the plaster mould the right way up, and pours molten bronze into a cup shape that feeds into the sprues system. This stage of casting a bronze sculpture has to be done quickly, before the bronze sets.

The metal hardens fast and soon after pouring the plaster mould can be shattered to reveal the progress of the metal.

Fettling a sculpture
Once the bronze has cooled, the sprues and airs - now themselves made of bronze - are cut off, and the nails that went through the wax drilled out.

Then the highly skilled process of fettling begins, as the caster works on the bronze surface until the evidence of sprues, airs, nails and any other imperfections is no more.

All the plaster investment has to be removed from the inside of the bronze - somehow. Sometimes a piece is cut out of the bronze to make a hole where the plaster can be cleaned out. This piece is then welded back and must be finished off by the fettler.

When the bronze is nearly clean of plaster it is sandblasted and is then ready for patination.

There are innumerable technical details which improve the accuracy of the work or, if they go just slightly awry, can cause the bronze to be incomplete, to split – or even to explode!

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