08.15 Plaster Molds

Plaster molds use plaster to form the negative shell of the mold. Plaster molds are used in slip casting since the cured plaster readily absorbs water from the liquid slip leaving a thin cast shell of semi rigid clay.

Since plaster is a rigid material, plaster molds need to avoid undercuts or the mold and / or cast will be damaged since the plaster cannot flex to accommodate the undercut. Rubber molds have more flexibility and can cast small features and undercuts well.

Plaster should be precisely mixed according to the manufacture’s instructions. A plaster calculator can be helpful for mixing non standard amounts of plaster