Mould Making

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A mould enables wax replicas to be made of the original sculpture and makes possible the casting of multiple bronze editions of the same object. Moulds are composed of two parts:
A coating of rubber, which captures in negative the form and surface texture of the original sculpture, even the fine imprints of fingerprints and an outer coating of resin (‘the ‘jacket’), to support the flexible rubber. The number of sections in which a mould is made is dependent upon the size and form of each individual sculpture. After the mould is removed the original model is then retained for reference only and is no longer used in the casting process. Successful casting projects begin with the production of a quality mould to capture the form and detail of the sculpture.

Wax Positive

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Each section of the mould is carefully painted with hot wax to ensure a good reproduction of the texture of the original. The mould is then closed and filled with cooler wax. This wax is then poured out, leaving a thin skin on the interior of the mould, approximately 4mm thick. When the wax has cooled sufficiently, the mould is removed revealing a hollow wax replica of the original sculpture. Whether your work is small or monumental, lost wax casting allows you to capture the most intricate details of your work, right down to a finger print. The wax positive is then worked on by skilled technicians to remove the seam lines created where the mould sections are joined together. Wax technicians work closely with our clients to ensure all the details are of the highest quality. It is at this stage that both the foundry and artist add their respective stamps / signatures (edition number) to the piece. A system of wax tubes (sprues) is then attached to the wax pattern to create channels through which the bronze will eventually flow.

Ceramic Shell

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"Investment" is the process of building a rock-hard shell around the wax sculpture. Later in the process, when the wax has been melted out, the investment will serve as a mold for the molten bronze. For most of history, an investment consisting of plaster, sand and water was used to accomplish this task. In the last 15 years, a new technology called ceramic shell has become the industry standard. The sprue system and wax positive are coated with an initial liquid ceramic containing very fine powder and sodium silicate. The liquid combined with grades of molochite granules, starting with fine through to coarser grades are built up over several layers over the wax sculpture. The wax pattern is now placed into a drying cabinet in which the temperature and humidity are carefully controlled. Drying time varies and is dependent on the size of each sculpture. Once dry, this process is repeated until the required thickness is achieved. Prior to the ceramic shell being placed in the kiln, the wax cup is melted to aid the waxing.

The Pour

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The ceramic shell is then placed upside down in a kiln and heated at 725°C for approximately 1 hour. During this process the wax melts and pours out through the cup. It is collected at the base of the kiln and can then be re-used. Once the ceramic shells are cool enough (around 300 - 400°C) they are placed in a bed of sand to support them and facilitate casting. Once the metal reaches 1200°C it is ready to be poured into the ceramic shell. This is done by decanting the metal into a pre heated pot - a crucible - and then poured into the ceramic shell at a reasonable speed. The investments, now filled with bronze, are left to cool.

Fettling / Metal Chasing

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The investments are removed from the sand and the ceramic shell is removed from the bronze cast by gentle tapping on the cup or the spruing system, or by grit blasting the cast which disperses the ceramic shell whilst leaving the bronze intact. The spruing (gates) system and the cups are cut from the casts and then re used. Many sculptures are cast in several sections, especially large scale pieces. It is the job of the metalworker (chaser) to weld together these pieces to form a whole whilst removing any evidence of a join. He will do this by TIG welding all the sections together with wire matching the parent metal. Any excess weld can then be ground down carefully using a variety of tools ranging from air powered tungsten burrs to small chisels and matting tools. The metalworker skilfully blends the joint into the surrounding surface matching the texture exactly.

Patination

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The final process is patination. This is the chemical colouring of the surface of the bronze using oxidising agents. Bronze Editions, working closely with Janapatina to provide an endless array of colours from traditional classic bronze to more experimental finishes. Bronze Editions work closely with the artist and encourage the artist to learn about the process. This opens up the possibilities to really find what the artist want.