casting

Process of producing solid objects by pouring molten material into a shaped mould and allowing it to cool. Casting is used to shape such materials as glass and plastics, as well as metals and alloys.

The casting of metals has been practised for approximately 5,000 years using first gold, copper, and bronze, then iron, and now alloys of zinc and other metals. The traditional method of casting metal is sand casting. Using a model of the object to be produced, a hollow mould is made in a damp sand and clay mix. Molten metal is then poured into the mould, taking its shape when it cools and solidifies. The sand mould is broken to release the casting. Permanent metal moulds called dies are also used for casting, in particular, small items in mass-production processes where molten metal is injected under pressure into cooled dies. Continuous casting is a method of shaping bars and slabs that involves pouring molten metal into a hollow, water-cooled mould of the desired cross section.

Hollow casting Unlike traditional casting, where the mould is filled with molten metal or liquid plaster, in hollow casting the lining of the mould is covered with layers of the sculpture material being used, such as metal or clay. Hollow casting is the preferred technique for artists working in metal as they can ensure that no part of the sculpture is much thicker than any other. This is important because the metal shrinks when it cools, and tensions can occur where the material varies in thickness. Hollow casts also reduce the overall weight of the piece, which is useful when creating large sculptures.

Cire perdue, or lost wax casting Cire perdue (French ‘lost wax’) is an ancient technique used for casting in bronze. The earliest examples date from around 3000 BC, and are found both in ancient Egypt and Ur. In its simplest form, a model of the sculpture is made of wax and coated in clay and plaster, leaving a small hole in the bottom. When heated, the clay and plaster hardens and the wax melts and runs out of the hole. The mould can then be filled with molten bronze, and later cut away to reveal the bronze sculpture.

For larger or hollow pieces, the sculptor first makes a plaster core, roughly in the shape of the intended work. This is then pierced with iron rods, and a layer of wax applied, into which the sculptor carves and shapes the detailed, final design. Next the wax is coated with liquid clay, which is left to harden. When the mould is heated, the layer of wax sandwiched between the mould and the plaster core, melts away. Molten bronze can then be poured into the gap left by the wax. Finally the mould and core are removed, leaving an exact reproduction of the original wax sculpture.