Press moulding is the simplest way to repeat a form without a wheel. A slab of clay is pressed into or over a plaster shape, left until it firms, and lifted off. It is how most repeated dishes, tiles and shaped bowls are made outside a factory.
Slump and hump
A slump mould is a hollow the clay lies into, giving a bowl with the mould's inside shape. A hump mould is a form the clay lies over, giving a bowl with the mould's outside shape and a foot you can trim afterwards. Both do the same job from opposite directions, and which one suits depends on which surface needs to be exact.
Getting a clean result
Roll the slab evenly, lay it in or over without stretching it, and press it down from the centre outwards so air is pushed out rather than trapped. Trim the edge with a knife against the mould's rim while the clay is still soft, and leave it until it has firmed enough to hold its shape before lifting.
Timing the release
Too soon and the piece distorts as it is lifted; too late and it shrinks tight onto a hump mould and cracks. On a slump mould the clay shrinks away from the sides and releases easily, which is why slump moulds are more forgiving and are what most people should start with.
Making moulds without plaster
A bisque-fired clay form works as a mould and absorbs almost as well as plaster, which makes it a practical route if you already have a kiln. Bowls, dishes and found objects can be used directly as slump forms if lined with cloth, though nothing releases as cleanly as an absorbent surface.
Questions people ask about clay mould
What is press moulding?
Pressing a slab of clay into or over a plaster form to repeat a shape without a wheel.
What is the difference between a slump and a hump mould?
Clay lies into a slump mould and over a hump mould, so each gives an exact copy of a different surface.
When do I lift the clay out?
Once it has firmed enough to hold its shape. On a hump mould, before it shrinks tight and cracks.