Bronze Casting - Lost wax method

Elizabeth O'Kane uses a method of bronze casting known as lost wax.This method is complicated and labour intensive, requiring much interaction between the artist and the foundry. From start to finish there are roughly five stages. These can be described as positive, negative, positive, negative, positive.

1. The original sculpture is modelled by the artist in clay (although any material can be used such as polystyrene, wax, wood, stone, etc...). This is the first positive.

2. Lost wax casting usually requires a mould to be made of the sculpture. Let us take the example of a sculpted head. This would require a two piece mould - for the front and back of head (however more a complicated sculpture will require a mould consisting of several parts, and the sculpture may need to be cut into several pieces for access). The work is laid down face up. The back of the head is temporarily covered. Several coats of liquid rubber are applied to the surface of the sculpture to copy the face's exact features. When the rubber solidifies plaster is applied over it to hold the rubber in place and allowed to dry. The sculpture and mould are turned so the back of the head now faces upwards and this process is repeated. This mould is a negative of the sculpture. The plaster and rubber halves are then carefully removed from the original sculpture.

3. Liquid wax is brushed into each half of the rubber mould. The two halves of the mould are assembled and hot liquid wax is poured into an opening in the mould. This seals the two halves of the wax cast together, and excess wax is poured out. When the wax cast hardens the plaster and rubber are carefully removed to reveal a hollow wax cast, or positive of the original sculpture. The wax copy is refined and "chased" by the artist.

4. Wax rods, or "sprues", are attached to the hollow wax sculpture, and a cup area attached to the top of this: the sprues will be the eventual channels for pouring the bronze. The sprued wax is dipped in a liquid ceramic mixture (covering both the inside and outside of the wax) and dusted with silicone sand (inside and out) to strengthen the ceramic covering. This is allowed to dry. The process repeated about seven times. When dry it is fired in a hot kiln. This strengthens the ceramic and allows the wax to burn out (hence the name "lost wax") leaving a hollow "ceramic shell", or negative. This is allowed to cool.

5. Molten bronze is then poured into the hollow ceramic shell through the hollowed sprues. This is allowed to cool and the ceramic shell is hammered off and sand blasted to reveal the bronze cast, or final positive. The sprue bars, now bronze, are cut off and any necessary welding is done. Metal chasing grinds away any weld marks. The bronze is polished to the desired "finish".

6. The final stage is "patination" or colouring. Chemicals are applied to heated metal to obtain the desired colour or "patina". This is the speeding up of the ageing process that would occur naturally when exposed to different weather or soil conditions. The patina is very delicate and so the bronze is waxed to preserve the patina.

Care of your bronze

Your bronze can be protected by dusting it occasionally with a soft, clean, damp cloth. Occasionally you can apply a clear coat of wax (e.g. Clear tree wax) by rubbing it lightly over the bronze with a cloth. Leave for a few hours and polish it up to desired lustre. More frequent waxing may be necessary in very humid areas. Remember that the patina is very delicate and so do not use spray waxes such as Pledge, Brasso or any sort of metal polish.