The Lost Wax Method of Casting

Unlike stone, wax or clay - Bronze has to be cast in a molten state using a series of techniques that date back to approximately 2000 BC. There are several methods of casting bronze, however, the main technique used today is the ‘lost wax’ method known as ‘cire perdu’. The Egyptians were using this process as early as 1570 BC.

The Casting Process

The principal three methods of casting in bronze are; Investment, Ceramic Shell and Sand Casting.
The investment and ceramic shell methods both use the lost wax process. The ceramic method, as the name implies, consists of dipping the original form to be cast into a ceramic mixture to build up a thin layer or shell this differs from the investment method which uses plaster to form the mould. The mould is then baked in the kiln, which hardens the ceramic and burns or loses out the wax creating a cavity. The empty cavity, which represents the original form, is then filled by pouring molten bronze into the cavity. Once filled it is allowed to cool. The pouring process, using molten bronze at approximately 1,150 degrees centigrade, is a dangerous and specialist task and any errors could   ruin a piece entirely. Sand casting consists of forming a mould in a sand mixture in two half boxes, one side of the mould impressed in each box. When the impressions are complete both boxes are clamped together and the empty cavity, which now represents the original shape or form, is filled with molten bronze. This is the quickest method of casting but it only suits particular geometric forms.

For very large sculptures, the bronze is cast in several segments and welded together. Most if not all small bronzes can be scaled up but size and thickness are restrictions and dictate the methodology used. This scaling up process requires the expertise of professional foundry specialists to ensure that the final scale is representative and true and the finish is acceptable.

The Finishing Process

After allowing the molten bronze to cool the mould is removed. This leaves a very rough bronze cast, which is sandblasted to remove all plaster or ceramic, and excess bronze no longer required. The piece is then further fettled and chased before patination.

The patination process is the actual colouring of the finished bronze. There are many formulae and techniques for colouring metals. Mostly used, is the brush-on method which involves warming the bronze with a blowtorch and brushing on a chemical solution which slowly builds up the depth of colour. When the desired colour is achieved, it must be fixed. Applying a heated beeswax mixture to the sculpture does this. When cool the sculpture can then be polished to as high a finish as required. The finished work will have acquired a patination in whatever colour is desired. This can range from pure colour to an antiqued or aged oxidation similar to that acquired naturally by outdoor pieces over time.

Caring for your sculpture

Caring for your sculpture is quite easy and the more a piece is handled the more natural oil it retains on its surface. This accentuates certain areas and like fine furniture it acquires its own individual colour character and unique patination over time. Highly polished non patinated bronze areas of sculptures can be maintained in pristine condition by using ‘Brasso’ or ‘DuraGlit’ and a soft dry cloth to buff. Bronze may also be mounted on marble, wood or metal to further enhance its beauty. Sculpture is three-dimensional and directional spotlighting will raise highlighted areas of the piece to stunning effect.

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