A casting mould is a closed plaster mould that liquid slip is poured into. The plaster draws water from the slip against its surface, a wall of clay builds up, the excess is poured away and the hollow cast is left. It is how most identical hollow forms are made.
How the wall builds
Slip poured into a dry mould starts losing water immediately at the plaster face, and a layer of firm clay grows inwards. The longer it stands, the thicker the wall, so timing controls thickness and is learned by testing with your own slip and mould. Pour the excess out, let the cast firm, and it shrinks away from the plaster and releases.
Multi-part moulds and keys
Anything that is not a simple open shape needs a mould in two or more parts, with keys, meaning matching bumps and hollows, so the parts register in exactly the same position each time. Getting the keys right and the parting line in a sensible place is most of the skill in mould making, because a mis-registered mould gives a seam nothing can rescue.
Casting slip is a specific product
Slip for casting contains a deflocculant so it flows at a low water content. Ordinary studio slip, made by slaking scraps in water, has far too much water and will crack in the mould and shrink excessively. Casting slip is bought ready made or mixed to a recipe with weighed additions.
Fettling and drying moulds
The seam left by a multi-part mould is cleaned off with a fettling knife and a sponge while the cast is leather hard. Moulds must dry fully between casts, which in practice means a working set of several moulds rather than one used repeatedly, and a warm dry place to keep them.
Questions people ask about mould casting
What is slip casting?
Pouring liquid slip into a plaster mould, letting a wall build as the plaster absorbs water, and pouring the excess away.
Can I use ordinary slip for casting?
No. Casting slip is deflocculated so it flows with much less water. Ordinary slip shrinks too much and cracks in the mould.
Why does my mould need keys?
So the parts register in the same position every time. A mis-registered mould leaves a seam that cannot be rescued.