POSITIVE ORIGINAL TO NEGATIVE MOULD

The first stage of making a bronze is to sculpt the original. This can be done using different materials such as clay, plasticine or wax, depending on where and what is being sculpted. This is often built up over a steel and aluminium anatomical skeleton known as an armature. A silicon rubber mould is made over the original. The soft silicon rubber forms an exact negative of the positive original held in the right shape by a rigid plaster or fiberglass outer case. Multi-section moulds fit together with millimetre precision.

 

NEGATIVE MOULD TO POSITIVE WAX

Molten wax is slushed into the mould, poured out and the remaining skin allowed to cool; this forms a hollow wax positive approximately 4mm thick. The seam lines where the mould sections fitted together are then worked out and the sculpture cut up into castable sections. To this a series of wax pipes called runners and risers are fitted (known as sprues); these allow the molten bronze to flow in and the gases to come out. Each time an edition is cast another wax has to be made.

 

POSITIVE WAX TO NEGATIVE CERAMIC INVESTMENT

The ‘sprued up’ wax is then coated inside and out with liquid ceramic and grit, built up in layers to form a strong heat-resistant investment around the wax. This is then baked upside down in an oven, allowing the wax to be burnt out – hence the term ‘Cire Perdue’ or ‘lost wax’.

 

NEGATIVE CERAMIC INVESTMENT TO POSITIVE BRONZE

The negative space formerly occupied by the wax is now filled with molten bronze poured in at 1200ºC into the pre-heated ceramic investment. Other metals such as silver can be cast using the same method but at different temperatures. When the bronze has cooled, the ceramic shell is painstakingly hammered away and the sprues cut off. To remove the hard ceramic from the surface detail and deep undercuts, the bronze may be grit blasted or placed in acid, which further breaks down the investment.

 

CHASING

If the bronze has been cast in several pieces, it is now welded together using a Tig Welder and rods of bronze and chased. This is a highly skilled process recreating any surface detail. It’s easy to push fingers into soft wet clay, not so easy to reproduce in hard metal; the sign of a quality casting is not to notice the chasing. If structurally necessary, sculptures are fitted internally with stainless steel supports.

 

PATINATING

The sculpture is now ready to be heated up and applied with a wide range of chemicals, which form the finished patina.