Bronze casting

At a time when contemporary art is criticised for lacking craft, bronze casting is a refreshing antidote. Each sculpture takes on average four months to be sculpted, moulded and then cast into bronze. It is a highly skilled, labour-intensive process. Hamish’s sculptures are cast in England by the Lockbund Sculpture Foundry.

He has a 20-year history of working with this foundry building a crucial relationship between sculptor and founder; together rising to various technical challenges, from casting intricate feather detail to a life-size cheetah supported on one leg. Whatever the sculpture, the finish is always of museum quality. ‘You sculpt what you want and we’ll work out how to cast it.’ Simon Allison – Lockbund Sculpture Foundry. “I love the fact that a finger print left in the clay original comes through into the bronze. I take full advantage of the technical capabilities of a good foundry.” Bronze’s tensile strength allows compositions with minimal supports, unthinkable in materials such as marble, wax and plaster from which sculptures are initially created. This results in tactile bronze sculptures that will last many centuries. Bronzes are made by pouring molten bronze into a ceramic investment – known as the ‘Cire Perdue’ or ‘Lost wax’ Method. The same technique dates back 5000 years. The skill of transforming one material into another is to preserve all of the detail of the original.

Positive original to negative mould

The first stage of making a bronze is to sculpt the original. Hamish uses different materials such as clay, plasticine or wax, depending on where and what he is sculpting. This is built up over a steel and aluminium anatomical skeleton known as an armature. A silicone rubber mould is made over the original. The soft silicone rubber forms an exact negative of the positive original held in the right shape by a rigid 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 is placed in acid, which further breaks down the investment.

Chasing

If the bronze has been cast in several pieces, it is now welded together and chased. This is a highly skilled process recreating any surface detail. “It’s easy for me to push my 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. Hamish is one of few sculptors who do their own patination, as he considers this to be as important as the colour of paint on a canvas.

Crucible being pulled out of the furnace prior to bonze pour.A Chaser removing spreus and weld lines.Reinforcing a silicone rubber mould with glassfibre and steel.Bronze is cast in multiple sections depending on the size of the crucible.