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For most sculptures a master is made, from which a mould is produced, in which the concrete is cast. When producing the master it is important to bear in mind that the mould will have to be removed from the master and subsequently from the finished concrete so the shape should not have any ‘returns’ that will prevent this.
There are many materials that can be used for the mould, including: plaster of Paris, timber, fibreglass, expanded polystyrene, concrete and rubber. Porous surfaces should be sealed, for example with varnish. Whichever material is used it is important to get the mould material well in contact with every part of the master. The mould will need support to keep it rigid during concreting and the separate pieces of the mould need to be held tightly together. The shape of the sculpture, together with any lifting and turning that may be needed during installation, will dictate the need for reinforcement.
The concrete should be left in the mould for at least 12 hours. However, if it is left too long it may be difficult to remove the mould.
For information on sculptors who use concrete see: Link1