Casting is the process of reproducing a 3D image using a mould to produce a replica in another material e.g. bronze. The process for these sculptures uses the lost wax and ceramic shell methods for bronze casting. This involves the following five stages;
A silicon rubber coating of the original. Rubber coating is constructed in sections to enable the mould to be removed from the original, and access to the inside surface and retrieval of the wax positive later on.
A strong fibre glass jacket is layered on top of the rubber to support it. This mould holds a negative image in rubber of the original.
A hollow wax positive of the original is cast from the mould by slushing hot wax into it. The thickness of the wax corresponds to the eventual thickness of the bronze eg.10mm. The wax positive is worked on to make good any imperfections e.g. seamlines, air bubbles. A network of wax rods (sprues) and a funnel- like cup are fitted onto the positive. These become the channels for the molten metal to travel to the job later in the process.
The sprue system and wax positive are coated with a liquid whose ingredients include sodium silicote. The liquid is combined with molicite granules to form an extremely hard shell around the wax when it is dry. The shell is built up using five or six layers like the above, and backed up with fibre glass matting in the liquid, so that the resultant shell is strong enough to take the heat shock it will receive later. The inside cavity of the hollow wax positive is filled with a plaster-based mixture which is called core. The wax is then melted out of the ceramic shell using a high powered blow torch heated up in a kiln leaving a negative space in the shell ready to receive the molten bronze.
The bronze is melted to a high temperature in a crucible using a furnace. The bronze is then poured from the crucible into the shells, which have been set into packed sand for extra strength. Leaving ten hours for the bronze to cool, the shell is then ready to be chipped and washed off the metal casting. The cast is a replica of the wax including the sprues and funnel cup.
Metal finishing and Patination
The sprues are cut off (fettled) and the metal surface worked on to repair any casting imperfections (chasing). Large pieces that are cast in sections are welded together, and the welds chased back to blend in with the form. The finished bronze is coloured (patina) by applying various metal oxides to the surface often using heat. The bronze can then be waxed and polished to slow down the natural oxidising process of the bronze or left natural