Casting Process

There are seven main stages to produce a finished bronze casting.

Moulding  

 

 Artist's original work in any medium is coated with silicon rubber, which takes a negative impression of every detail of the form and modelling. The rubber, once set, is backed-up with a plaster jacket, which keeps the silicon in shape.

Wax Production

   

 Molten wax is carefully painted into the rubber mould to a thickness of approximately seven millimetres. When it has cooled and set, more wax is swilled around any joints between parts of the mould to create a sealed hollow wax replica of the original work. The mould is then gently pealed away from the wax, and any seams or defects are carefully worked with tools, paying close attention to the original model. The artist is then called in to check the replica and to make any necessary alterations.

 

Spruing   

 Spruing is the system by which solid wax "sprues" are welded to the main parts of the hollow wax and then brought together to a central point above the piece where a polystyrene cup is then joined with glue wax. They act as "feeders" for the molten bronze to travel into the hollow cavity remaining once the wax has been burnt out.

 

Ceramic Shell Coating  

 The sprued wax is then dipped in ceramic shell mixture, which has the consistency of creamy plaster. The first coat is allowed to dry thoroughly, before the process is repeated again, up to perhaps eleven times, with each coat drying thoroughly before the next is applied. The ceramic shell coating forms a negative impression of the wax and is built up to between 1cm and 2cm, depending on the weight of the wax. Larger shell moulds are strengthened with wire mesh, which helps to stand the force of the bronze when it enters the mould. When the mould is of a satisfactory thickness, it is cooked in a kiln where all the wax melts out and it becomes a hollow or "shell". It is then left to bake, ready for casting.

 

Casting

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 The hot shells are removed from the kiln and placed in a container. Molten bronze is then poured into each hollow cup from which it runs down the hollow tubes left by the sprues, filling the cavity left by the wax. When the moulds have cooled after about two hours, the shell is chipped away to reveal the bronze positive copy of the artists work.

Chasing  

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 The bronze is then worked by a craftsman, who removes all evidence of sprues and any other defects with fine "chasing" tools. All original detail is preserved and any abnormalities are corrected. Larger pieces are cast in sections, which are then welded together. These welds are then worked and blended in with the original modelling so that they become invisible and the piece appears to have been cast in one.

Patination  

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This is the final stage of the production. Bronze can be oxidised with a number of different chemicals and heat to produce a range of colours, from basic brown and black to varying shades of green, antique bronze, oranges, reds and blues. This a highly-skilled operation, as the craftsman has to work to the artists specifications and at the same time obtain an often complex chemical reaction on the bronze. When the desired colour is reached a silicon wax polish is applied with heat, which produces a quality sheen on the bronze and from this stage it should begin to "mature". The piece may then finally be placed on a base of the artists choice ready for collection.

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