While moulds can be fashioned directly, for example by carving wood or stone, both mould and cast are usually made in a pliable or amorphous material, such as plaster of Paris, wax or clay. The model is encased in the chosen material, so as to hold an impression of its shape and surface in negative: the mould is then carefully removed and the hollow interior filled to make the positive cast . A piece-mould, a mould constructed in numerous sections, is used to facilitate removal, the small sections sometimes held in place by an outer ‘case’ mould. The modern process of casting has been simplified by the use of synthetic rubbers that can be peeled away from undercut forms and reused. Other, less versatile, flexible materials for moulds include wax, gelatin and latex rubber . Alternatively, a one-off cast can be made with a waste-mould. If the original form is modelled in soft clay, a plaster mould of few sections is easily removed, but if there are undercut forms the mould is ‘wasted’ or chipped away from the cast. Plaster, concrete and glass-fibre reinforced polyester resin (GRP or fibreglass) are frequently cast in waste moulds. In the lost-wax method of casting metal, a wax positive can be modelled direct (direct lost-wax method) or, as is now more common, cast hollow from a piece-mould taken from the model or master (indirect lost-wax method; for a full description of both techniques see Metal, §III, 1(iv) and (v)).
Casts are generally stronger, as well as lighter and cheaper, if hollow. This is true not only of casts in bronze and other metals but also of those in terracotta, plaster, concrete and GRP. Unless small in size, solid forms are liable to crack or distort owing to the uneven contraction when the cast material cures, cools or is fired. With cold-cast materials, some moulds are filled with a single form, while others are filled in sections, which are then assembled or ‘squeezed’ together with a joint of fresh material. Clay can be press-moulded or poured as liquid ‘slip’ ; larger figures may be first cast in pieces and then joined before firing. Most cast objects need to have their seams and surfaces made good after being removed from a mould.