Casting the Bronze........
The process of manufacturing bronze sculpture has changed very little in essence since its earliest days some 3500 years ago.
In this section we explain the surprising number of procedures involved in manufacture, each of which requires a high degree of hand finishing.
This illustrates the fascinating point that no two sculptures are identical.
Elephant in Wax: Original sculptors model
The original model is made in a variety of materials including clay, wax and types of plasticine. Because there is very little reduction in size during the manufacturing process, the model is scaled to the required size.
Mould in Rubber: Mould Making
A mould is taken from the model using specific grades of silicon rubber depending on the degree of strength required. This material cures overnight and can be used next day in the process of producing wax replicas of the model.
Liquid wax is poured into the mould and allowed to set hard. Once this is achieved, the wax model can be drawn from the mould. Care has to be taken to ensure the wax is not broken. Where the mould joins, a seam is created which has to be removed using a knife or scalpel.
Years ago the actual casting of bronze was done in sand pits but this tried and trusted method has been replaced by encasing the model in layers of liquid ceramic. Each factory has its formula for this mix which tends to be jealously protected. Between each layer of ceramic material, time has to be allowed for setting and curing.
Finished Ceramic Coat
The figure shown has been covered by three layers of ceramic mix and is ready for bronze casting. The wax rods seen attached to the model facilitate the flow of bronze and help the air and gas to escape. Detailed knowledge is required to know where to position the rods.
Removal of Wax
The wax inside the mould now has to be heated and removed to allow a cavity to be formed into which the liquid bronze can be poured. The wax is usually collected in the process and re-used.
Bronze is an alloy of copper and smaller quantities of tin. It is heated to 1250c and poured into the handling vessel illustrated. At this stage the heat is intense and two operatives are required to take the vessel to the moulds.
Casting the Mould
The moulds are placed in a sand filled container to prevent the ceramic container injuring the operatives in the unlikely event of a shattering. The vessel then fills the moulds to the required depth.
The liquid bronze can just be seen in the moulds which will be left for 24 hours to cool. Patience is required to prevent the process being hurried and the model damaged.
Breaking Open Ceramic Mould
Once the ceramic case is cool the operator can break it using a variety of tools. Often favoured is the hammer which has a vibrating effect, shattering the comparatively delicate ceramic shell. The bronze inside is extremely hard and not usually marked in this process.
The bronze figure once removed from the case requires chasing, the removal of any seams, lines or blemishes. It takes time and skill to reinstate the model to the sculptors original. A high speed drill and selection of chasing tools can be seen on the worktable.
This procedure finishes the bronze to the required colour and is considered to be one of the most skillful jobs in the foundry. If an unsuitable colour or finish is selected by the operator, a beautiful model can be made to look ordinary. The process of applying the appropriate acids and subsequently burnishing them takes much training since the effects can only be seen when the whole process is complete.
Here, the full effect of the patina can be appreciated and the deep chestnut colour suits the subject perfectly. Final waxing of the model enhances the colour and produces the reflective sheen. With care and skill, many finishes and colours can be achieved.