Traditional method - make a mould
Before a cast can be made, it is necessary to make a mould of the object you wish to copy. The object could be a sculpture or another cast. A traditional mould is made of plaster pieces which together are called a "piece mould". The picture shows one that has been disassembled. To make such a mould, plaster is applied to the object, a small amount at a time, so that an interlocking three-dimensional jigsaw is built up around it. The pieces of the mould are separated by a thin layer of shellac lacquer to prevent sticking. The plaster pieces are then removed from the object and reassembled when it is time to make the cast. The inner surface of the plaster pieces are also coated in shellac before a cast can be made. Large moulds may take up to a year to make, but they can be re-used many times - some have been in use for over a hundred and fifty years.