A plaster mould is a shaped block of pottery plaster used to form clay, either by pouring liquid slip into it for slip casting or by pressing soft clay into or over it. The absorbent plaster draws water from the clay.
What Is a Plaster Mould?
Pottery plaster is a fine, hard-setting gypsum plaster. When it sets, it is full of tiny pores that soak up water, which is what makes it so useful. In slip casting, a layer of clay forms against the mould wall as the plaster absorbs water from the slip. In press moulding and drape moulding, the plaster helps soft clay release as it firms.
Moulds can be one piece for simple open shapes, or several pieces that fit together for more complex forms like jugs and figures. They are made by casting plaster around a model, and the joins, called seams, leave fine lines on the cast that are cleaned off later.
Plaster must be kept out of clay. Even small chips of plaster in a clay body can absorb moisture after firing and pop out of the surface, so mould work is kept away from reclaim and throwing clay.
Moulds need to dry thoroughly between uses and wear out over time, especially with harsh deflocculants. Store them assembled and dry, and they will last for many casts.
This pottery glossary entry from Le Studio Art Space is general educational content. Always follow the clay, glaze and kiln makers’ data for your own materials and equipment.