The famous lost-wax art of bronze casting is about 5,000 years old. Possibly invented in Sumer by the metalworkers of Ur. In its original simple form the object was first modelled in wax over a clay core. Tubes of wax were fixed to top and bottom and the statue was covered thickly with a mixture of plaster and clay. This was then fired in a kiln and the molten wax ran out of the tubes.
The now hollow mould was filled with molten metal. Metal filled the top duct and air escaped through the other now empty tubes or ducts. The mould was broken and the metal had taken the shape of the wax original. The metal duct was broken off and the statue polished.
Nowadays we have improved matters. Firstly we can make rubber moulds and so cast more than one bronze. Editions are usually restricted to a stated number. You paint the wax, to the required thickness, in the rubber mould and when cool you peel off the rubber, leaving a wax positive.
Another improvement is ‘ceramic shell’. The wax model is given ducts of wax and then dipped in the ceramic shell mixture rather like egg and breadcrumbs. It is dried and dipped several times, both inside and out side, forming a core and a strong coating. When completely dry it is placed in a kiln and the molten wax runs out of the main duct and air is let in through the minor ones. The ceramic shell has now baked hard and has to be broken off. Another innovation, sandblasting helps this process. Any complicated model is cast in more than one part and these are now brazed together, yet another new process.
Many ancient bronzes were painted with bright, and to our eyes, garish and distracting colours. Aping the patinas of bronzes found buried after hundreds of years, we use mixtures of chemicals and acids and sometimes a blowtorch to produce years of aging in an hour. These recipes are closely guarded. If the bronze is for indoors it is then waxed. Outdoor bronzes will take on their own colours or improve the ones they have been given.
So to sum up: Casting now takes 5 processes.
1. Model in clay or other workable material = Positive.
2. Rubber mould in pieces, backed with Plaster of Paris or fibreglass = Negative.
3. Wax painted into rubber mould = Positive.
4. Ceramic shell painted inside and out and fired = Negative.
5. Bronze poured into empty spaces = Positive
So although more than one bronze may be cast, there is a tremendous amount of work along the way. This explains one of the reasons for the cost of bronzes.