plaster is a powder obtained by grinding and heat-drying gypsum (calcium sulphate) which, when water is re-introduced, sets by chemical reaction to form a hard solid
Plaster can be used in certain circumstances to make moulds directly from a prototype but because it has no flexibility mould pieces cannot undercut at all if casts are made using a rigid-setting material, and it helps if both prototype and castings can flex a little to aid removal (for example polyurethane resin in its green stage before it hardens fully). Undercutting does not pose a problem however if plaster is used as a mould for latex or silicone rubber forms. Plaster is one of the least expensive materials if bought in its most common form, i.e. a 25kg sack, and one usually pays ‘over the odds’ for smaller amounts.
The more common uses of plaster in mouldmaking (as distinct from its use as a casting material) are either as a quick, cheap and versatile material with which to make mother moulds (the rigid jackets which help to keep silicone mould pieces in shape) or, more specialized, to make absorption moulds for clay slip or latex casting. For the former a hard-setting plaster is needed, so that the jackets can be strong without having to be too bulky but it also helps if it has a longer ‘working window’ between the onset of thickening and final setting so that this jacket can be more easily shaped. The best plaster for absorption moulds on the other hand is one which will be porous i.e. less dense and therefore not nearly as strong.
Mixing/working guide: the recommended ratio of plaster to water varies by type between 1:1 and 4:1 plaster to water by weight. Plaster is always distributed (sprinkled gently and evenly) into water, never the other way around. As a very rough average plaster has a 10min pot-life and will take 30mins to set, during which time it will become warm. It is best to wait until the set plaster has become cool to the touch before it is unpacked. As an equally rough average for mixing a specific amount of plaster; the amount of water measured out will roughly double in volume when the plaster is added and mixed i.e. 100ml of water will yield 200ml of plaster mix. This is based on experience of mixing relatively dense casting plasters (around 2.5:1 plaster to water ratio) so it’s likely that the cheaper, less dense plasters (around 1.5:1 plaster to water ratio) will yield a little less than double.
For more information on plasters generally and more specifics on cost and recommended sources please see the ‘plaster’ page in the casting section.