Plaster casts

A plaster cast is a copy made in plaster of another 3-dimensional form. The original from which the cast is taken may be a sculpture, building, a face, a pregnant belly, a fossil or other remains such as fresh or fossilised footprints – particularly in palaeontology (a track of dinosaur footprints made in this way can be seen outside the Oxford University Museum of Natural History).

Sometimes a blank block of plaster itself was carved to produce mock-ups or first drafts of sculptures (usually relief sculptures) that would ultimately be sculpted in stone, by measuring exactly from the cast, for example by using a pointing machine. These are still described as plaster casts. Examples of these by John Flaxman may be found in the central rotunda of the library at University College London, and elsewhere in the University's collections. It may also describe a finished original sculpture made out of plaster, though these are rarer.

Plaster is applied to the original to create a mould or cast (that is, a negative impression) of the original). This mould is then removed and fresh plaster is poured into it, creating a copy in plaster of the original. Usually very elaborate moulds were made out of several to even dozens of pieces, to cast the more difficult undercut sculptures. Plaster is not flexible, therefore the moulds were made as 3D jigsaw puzzles for easy removal of the original and the cast from the mould. Later gelatine, rubber and silicone moulds were used, backed by plaster or polyester for support.

The practice of reproducing famous sculptures in plaster originally dates back to the sixteenth century when Leone Leoni assembled a collection of casts in Milan. He collected "as many of the most celebrated works… carved and cast, antique and modern as he was able to obtain anywhere". Such private collections, however, remained modest and uncommon until the 18th century.

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