Fine casting plaster is the grade most pottery moulds are made from: strong enough to survive repeated use and absorbent enough to draw water from clay and slip. The same material makes a plaster bat, which is the cheapest and most useful piece of reclaim equipment a studio can own.
What makes a casting grade different
The particle size and the water-to-plaster ratio it is designed for, which together set how porous and how strong the set plaster is. A mould needs both: too porous and it crumbles at the edges, too dense and it stops absorbing. Suppliers sell grades chosen for this balance, which is why a pottery supplier rather than a builders merchant is where mould plaster comes from.
The plaster bat
A flat slab of plaster, cast in a shallow frame, that reclaim slurry is poured onto. The plaster draws water out from below and the clay stiffens to a workable state in hours rather than days. It is the whole reclaim system for most studios, it costs a bag of plaster to make, and it works indefinitely if it is kept clean.
Making a bat
Cast it thick, at least four or five centimetres, on a level surface in a frame, and let it dry for several days. Keep it off the floor and out of the way of anything that might chip it, because chips end up in the clay and a plaster fragment in a body blows a crater out of a fired pot. Scrape it clean rather than cutting on it.
Looking after plaster equipment
Plaster and clay must not mix. Keep bats and moulds away from wedging surfaces and clay storage, inspect them for chips before use, and retire anything crumbling at an edge. That single discipline prevents the most frustrating fault in pottery, because a plaster inclusion only shows itself after the piece is fired.
Questions people ask about plaster bat
What plaster should I use for a mould?
A fine casting or potters plaster from a pottery supplier, which balances absorbency and strength for repeated use.
What is a plaster bat for?
Drying reclaimed clay slurry back to a workable stiffness. It is the cheapest reclaim equipment there is.
Why do chipped plaster bats matter?
A plaster fragment in a clay body absorbs water and expands after firing, blowing a crater out of the pot.