Slip casting makes identical hollow forms by pouring liquid clay into a plaster mould. It is how most commercially produced pottery is formed, and at studio scale it is the route to repeating a shape exactly rather than approximately.
The sequence
A dry mould is assembled and secured, casting slip is poured in steadily and topped up as the level drops, and after a measured time the excess is poured out. The mould stands upside down to drain, the cast firms and shrinks away from the plaster, the mould is opened and the piece lifted out. Then the seam is fettled and the mould is dried before the next cast.
Controlling wall thickness
Time in the mould is the control: longer gives a thicker wall. It varies with the slip, the mould's dryness and the room, so it is established by testing rather than read from a book. Keeping a note of the time that gave a good result, for that slip and that mould, is what makes casting repeatable.
Why casts fail
A damp mould absorbs too slowly and the wall never builds. Slip that is too watery shrinks excessively and cracks. Slip left standing without stirring settles, so the first cast of a session differs from the rest. And a mould whose parts do not register leaves a step in the seam that no amount of fettling hides.
Is it worth it at studio scale
For a repeated form sold in numbers, yes, and the mould making is the real investment rather than the casting. For one-off work it is a slow way to make something a wheel or a slab would produce faster. Most studios that cast do so for one or two products and make everything else another way.
Questions people ask about slip casting
How thick will a cast be?
It depends on how long the slip stands in the mould, and it varies with the slip and the mould's dryness. Establish it by testing and write it down.
Why did my cast crack in the mould?
Usually slip with too much water in it, which shrinks excessively. Casting slip is deflocculated to flow with less water.
Can I cast in the same mould repeatedly?
Only once it has dried between casts, which is why casting studios keep several moulds in rotation.