Sculpture's Unsung Heroes: The Role of Casting in the Fabrication Process

In a new venture, the Cass Sculpture Foundation will now be writing a periodic extended feature on topical issues affecting sculpture. This first feature seeks to demystify the production of sculpture through its explanation of 'expendable mould casting' and the role of the sculpture foundry.

Sculpture's range and diversity knows no bounds. As one of the 'plastic arts' (utilising materials that can be moulded, manipulated and modulated to produce three-dimensional works) its scope is incredibly extensive.

The production of contemporary sculpture often involves a plethora of people from a variety of disciplines working in a myriad of materials. However, despite the twenty-first century's incredible technological advancements, and the consequent use of new materials and production methods, many sculptural techniques that were used thousands of years ago are still being used today by the unsung heroes of the art world; the sculpture foundries and their artisans.

Whilst artists create preliminary forms from a variety of materials of which moulds are taken, it is the foundry that facilitates a model's transformation from wood, clay, wax, plastic or mixed media into an enduring metal casting. This essay seeks to examine the way in which many sculptures come into being by way of expendable mould casting.

The casting process is subdivided into two main categories: expendable and non-expendable casting. Expendable casting involves the use of non-reusable moulds, which are often made from plaster, sand, plastic, shell, foam or wax.





Lost-wax casting is the oldest, and one of the most reliable, casting techniques. Dating back to 6000 BC, this 'low tech, high skill' method involves taking a bronze cast of an artist's working model (that is often made from wax or clay).

The lost-wax method is known as investment casting. This refers to the casting of metal into a mould produced by surrounding, or investing, an expendable pattern with a coating, such as 'stucco,' that sets at room temperature. The expendable pattern, often made of wax or foam, is then removed through the use of heat prior to filling the mould with molten metal.

Lost-wax casting involves making a mould of an artist's working model. The mould has a rigid outer casing, often made from plaster or fibreglass, which supports a soft latex or rubber negative of the model inside.

Molten wax is then poured in and out of the mould until layers of wax accrete on the interior. This creates a hollow wax copy of the original model which is then removed from the mould and cleaned with a heated metal tool to remove any imperfections.

Most moulds comprise two halves that are shimmed (or held) together for the final stages of the hollow wax casting. Evidence of the parting line or flashing (excess material) that is created when the two halves of the mould come together is cleaned from the hollow wax copy. Moulds can be reused, depending on their durability, and can go on to make multiple hollow wax castings.





The hollow wax cast is then fitted with wax sprues - passages through which liquid materials can be introduced into the mould - which will be coated with ceramic-like stucco prior to firing. Sprues allow molten metal in and air out of the mould to prevent the final cast being filled with air bubbles. The wax cast is also fitted with a wax 'cup,' or stand, that enables it to be freestanding during the firing process.

The 'sprued' hollow wax cast is then dipped into stucco, a sand-like aggregate that is applied whilst wet and dries hard. This dipping process is repeated until the hollow wax casting is sufficiently covered prior to firing.

The stucco coated hollow wax casting - including sprues and cup - is placed into a kiln and fired. The firing process serves to further strengthen the stucco and melt away the wax inside leaving an exact negative of the artist's original model.

The hard stucco 'shell' is then reheated and buried in sand to prevent the shell shattering when molten metal is offered into it. Metal, often bronze, is melted in a crucible in a furnace and poured into the stucco mould and left to cool. The stucco is then hammered or sandblasted away from the new metal casting. The now metal sprues are removed from the new metal casting to be recycled and it is then chased to remove any signs of the casting process.

Lost-foam casting, or expendable pattern casting (EPC), involves the carving or moulding of polystyrene to create a low-density replica of a sculpture to be cast.

As with the lost-wax technique, the polystyrene replica is fitted with sprues and dipped into stucco. It is then placed within a flask (a frame for holding sand moulds) that is filled with sand to surround the foam. The flask is then placed on a compaction or vibrating table to ensure that the sand is compressed around the stucco covered foam. As molten metal is poured into the stucco mould within the flask, the polystyrene vaporises. When cool, the unbonded and compacted sand is removed from the flask, the stucco is removed and the final metal casting is complete.





Small and/or intricate sculptures can be produced much more efficiently with investment casting processes. By using inexpensive wax or foam for impression moulds, elaborate and expensive tooling can be avoided which also increases the cost-effectiveness of the process.

Similarly, the waste moulding of plaster is a useful and economical way of realising sculpture. It is commonly used as a means of postponing the costly process of casting or carving a sculpture until a patron, or funding, is found for a work.

This process involves the creation of a temporary mould which can be used prior to the fabrication of a bronze or as a pointing guide during the carving of a work in stone.

A plaster mixture, reinforced with a fibrous material such as burlap or sisal, is produced and placed over the damp clay of a prototype sculpture. When cured, this plaster mould is removed from the clay which can then be used to create a positive image of the piece in a more durable material than the clay original. This can then be refined, painted and waxed to resemble a finished bronze casting.

Sand casting is one of the oldest and most simple casting methods. Molten metal, typically iron, bronze, brass or aluminium, is poured directly into a mould cavity made from Green sand - an aggregate of sand, bentonite clay, pulverised coal and water. Sand casting is incredibly versatile as it can be used to make very large and/or very small parts or sculptures.

Plaster casting is very similar to sand casting but uses a plaster mould into which molten non-ferrous metals are poured. (Ferrous metals are not suitable for casting in plaster as they contain iron that reacts with the sulphur in the gypsum that comprises the plaster.)

Plaster, along with other materials such as concrete and resin, can also be cast using single-use waste moulds to create simple and inexpensive sculpture castings.

Expendable mould casting is only one facet of an extensive range of moulding methods and production techniques used to realise sculpture. The following features will address the many and various other practices employed by foundries.