Potters plaster and pottery plaster are grades of gypsum plaster chosen for mould making, and they are not the same as the building plaster sold in a DIY shop. The difference is in absorbency and strength once set, and using the wrong grade gives a mould that either will not absorb or will not last.
Which plaster to buy
Suppliers sell plaster specifically for pottery moulds, usually as a fine casting plaster or a potters plaster, in bags from a few kilograms up to 25 kg. Building plasters are formulated to stick to walls and set hard rather than to absorb water repeatedly, and a mould made from one performs poorly and breaks down quickly.
Mixing it properly
Weigh water and plaster rather than guessing, because the ratio controls absorbency and strength. Sift the plaster into the water rather than pouring water onto plaster, let it slake undisturbed for a couple of minutes, then mix by hand under the surface to avoid whipping in air. Pour steadily from one point so air is pushed ahead.
Setting and drying
Plaster sets in minutes and warms as it does, and it is workable only during that window, so everything must be ready before mixing. Setting is not drying: a set mould is still full of water and must dry for days in a warm airy place before it will absorb anything. Rushing that is the most common reason a first mould fails.
What not to do
Never pour plaster waste down a sink; it sets in the trap and blocks it permanently. Let waste set in the bucket and break it out into the bin. And keep plaster away from clay: a fragment of plaster in a clay body absorbs water, expands after firing and blows a crater out of the pot, sometimes weeks later.
Questions people ask about potters plaster
Can I use building plaster for a mould?
Poorly. Pottery plasters are formulated to absorb water repeatedly; building plaster is not and breaks down quickly.
How do I mix plaster?
Weigh water and plaster, sift the plaster into the water, let it slake, then mix under the surface without whipping in air.
Why must plaster never get into clay?
A plaster fragment absorbs water, expands after firing and blows a crater out of the pot, often long afterwards.