Modeling for casting

The material most widely used for making positive models for casting is clay. A small, compact design or a low relief can be modeled solidly in clay without any internal support; but a large clay model must be formed over a strong armature made of wood and metal. Since the armature may be very elaborate and can only be altered slightly, if at all, once work has started, the modeler must have a fairly clear idea from his or her drawings and maquettes of the arrangement of the main shapes of the finished model. The underlying main masses of the sculpture are built up firmly over the armature, and then the smaller forms, surface modeling, and details are modeled over them. The modeler’s chief tools are his or her fingers, but for fine work he or she may use a variety of wooden modeling tools to apply the clay and wire loop tools to cut it away. Reliefs are modeled on a vertical or nearly vertical board. The clay is keyed, or secured, onto the board with galvanized nails or wood laths. The amount of armature required depends on the height of the relief and the weight of clay involved.

To make a cast in metal, a foundry requires from the sculptor a model made of a rigid material, usually plaster. The sculptor can produce this either by modeling in clay and then casting in plaster from the clay model or by modeling directly in plaster. For direct plaster modeling, a strong armature is required because the material is brittle. The main forms may be built up roughly over the armature in expanded wire and then covered in plaster-soaked scrim (a loosely woven sacking). This provides a hollow base for the final modeling, which is done by applying plaster with metal spatulas and by scraping and cutting down with rasps and chisels.

Fibreglass and concrete sculptures are cast in plaster molds taken from the sculptor’s original model. The model is usually clay rather than plaster because if the forms of the sculpture are at all complex it is easier to remove a plaster mold from a soft clay model than from a model in a rigid material, such as plaster.

A great deal of the metal sculpture of the past, including Nigerian, Indian, and many Renaissance bronzes, was produced by the direct lost-wax process, which involves a special modeling technique (see Casting and molding below). The design is first modeled in some refractory material to within a fraction of an inch of the final surface, and then the final modeling is done in a layer of wax, using the fingers and also metal tools, which can be heated to make the wax more pliable. Medallions are often produced from wax originals, but because of their small size they can be solid-cast and therefore do not require a core.

Modeling for pottery sculpture

To withstand the stresses of firing, a large pottery sculpture must be hollow and of an even thickness. There are two main ways of achieving this. In the process of hollow modeling, which is typical of the potter’s approach to form, the main forms of the clay model are built up directly as hollow forms with walls of a roughly even thickness. The methods of building are similar to those employed for making hand-built pottery—coiling, pinching, and slabbing. The smaller forms and details are then added, and the finished work is allowed to dry out slowly and thoroughly before firing. The process of solid modeling is more typical of the sculptor’s traditional approach to form. The sculpture is modeled in solid clay, sometimes over a carefully considered armature, by the sculptor’s usual methods of clay modeling. Then it is cut open and hollowed out, and the armature, if there is one, is removed. The pieces are then rejoined and the work is dried out and fired.

General characteristics of modeled sculpture

The process of modeling affects the design of sculpture in three important ways. First, the forms of the sculpture tend to be ordered from the inside. There are no external containing forms and planes, as in carved sculpture. The overall design of the work—its main volumes, proportions, and axial arrangement—is determined by the underlying forms; and the smaller forms, surface modeling, and decorative details are all formed around and sustained by this underlying structure. Second, because its extension into space is not limited by the dimensions of a block of material, modeled sculpture tends to be much freer and more expansive in its spatial design than carved sculpture. If the tensile strength of metal is to be exploited in the finished work, there is almost unlimited freedom; designs for brittle materials such as concrete or plaster are more limited. Third, the plasticity of clay and wax encourages a fluent, immediate kind of manipulation, and many sculptors, such as Auguste Rodin, Giacomo Manzù, and Sir Jacob Epstein, like to preserve this record of their direct handling of the medium in their finished work. Their approach contrasts with that of the Benin and Indian bronze sculptors, who refined the surfaces of their work to remove all traces of personal “handwriting.”