Procedure

To cast a bronze sculpture, two different techniques are used. The casting usually is carried out with the help of Cire Perdue (lost wax) or sand casting. It can be made of clay, wax, plaster, stone or metal.

SAND CASTING
  1. Special processed casting sand is used to form a two part mould. The sand is placed around the model/pattern in a way so it won’t get stuck. The model/pattern that can be of a durable material, such as wood, plaster or clay.
  2. After the two sand parts are complete, the model/pattern is carefully removed so as not to damage the sand. You now have a precise void that represents the pattern.
  3. Vents are also made in the sand to create various channels for the hot bronze to flow into the mould cavity and for air to escape.
  4. Following the pouring of the molten metal into the cavity, the mould is cooled, the sand broken open and the metal sculpture exposed and ready for treatment and finishing, as for the lost wax method explained below.
LOST WAX

This is a much more complex and time related process.

  1. The model/pattern is made of wax and can be a lot more intricate and detailed as the bronze will replicate the finest of size and detail. Dependant on the sculpture size, the wax can be a solid form. Alternatively a series of thin shells that can be welded together and thereby save in the cost of the bronze casting metal.
  2. The wax model then has a ‘pouring cup’ and several sprues added that will allow the metal passage to all parts of the model and for air to escape. This wax shape is then coated with a ceramic shell, applied in many layers to form a ‘shell’ or 10 mm or more thick.
  3. After the ceramic has been dried the mould is then fired in an oven to melt and remove all the wax.
  4. The void formed within the ceramic shell is then ready to receive the molten metal, be it bronze or aluminium.
  5. Hot bronze with a melting temperature between 1030-1100°C is poured into the mould, and the bronze finds its way down through cup and the channels.
  6. The bronze then cools and hardens, allowing the mould to be  broken open and the bronze sculpture revealed with all its appendages.
  7. The cup and sprues are then removed and the sculpture fettled with files etc to remove imperfections and ensure the final shape and surface finish is what the artist wants.
  8. The completed bronze sculpture is then patinated. This is a process using heat and various acid combination to provide a ‘weathered’ look, all to provide a coloured finish that suits the artist’s expectations.
  9. The final step is a coating of wax to seal the patina.

Hopefully this brief overview of the two processes is enough to make you more interested in the methods of bronze casting.

Making the mould
Making the mould
Cleaning the cast
Cleaning the cast
Final sculpture
Final sculpture