THE CASTING PROCESS
Casting is a process of reproducing a 3-D image using a mould to produce a replica
in another material.
The foundry I employ (Bronze Age in London's Docklands) uses the lost wax and ceramic shell methods for bronze casting. For resin casting, I use Alec Ryman Sculpture Casting, near Newbury in Berkshire (Cornish tin miner)
The bronze casting process involves five stages.
This consists of a silicone rubber coating of the original, constructed in sections so that the mould can be easily removed, as well as providing access to the inside surface and retrieval of the wax (or resin) positive later on.
Alec Ryman's team laying on the first coat of rubber
The mould holds a negative image of the original. A strong fibre-glass jacket is layered on top of the rubber to support it. This part of the process would be the same for bronze and resin casting.
Building a section of the fibre jacket
A hollow wax positive of the original is cast from the mould by 'slushing' molten wax into it.
The thickness of the wax corresponds to the eventual thickness of the bronze. This wax positive is then worked on to make good any imperfections.
A network of wax rods (sprues) and a funnel like cup are fitted on to the positive. These become the channels (feeds and vents) through which the molten metal will travel later in the process.
The wax of Flame sprued up ready for ceramic shell
The sprue system and wax positive are coated with a special ceramic liquid which sets to form an extremely hard shell around the wax. The shell is built up using five or six more similar layers and backed up with fibre glass matting in the liquid, so that the resultant case is strong enough to take the heat shock it will receive later on.
The inside cavity of the hollow wax is filled with a plaster based mixture (core).
The wax is them melted out of the ceramic shell in a very hot kiln leaving negative space within the shell ready to receive the molten bronze.
With the wax melted out, the shell is then packed in sand with the sprues uppermost and the funnel just showing.
The bronze is heated in a furnace until molten and then poured into the ceramic shell mould to the brim of the funnel.
In this photograph it is possible to see the ceramic shell coated sprues, which are now feeds and vents
Once cooled, usually overnight, the ceramic shell and core are chipped away.
The bronze cast of baby Myles Boden being cleaned up before fettling and chasing
The sprues are cut off (fettled) and the metal surface worked on to repair any casting imperfections (chasing). Any fixings are also welded into place at this point.
'Naiad' in the metalwork shop being chased
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 then coloured (patinated) by applying various metal oxides, often using heat.
Mark at Bronze Age applying copper nitrate with heat over a rubbed-back black base patina
The bronze can then be waxed and polished to slow down the natural oxidizing process.