Press molds and hump molds

Pressing soft clay into a plaster mold, or draping a slab over a plaster hump: the clay, the steps, and what to watch for.

Both are plaster molds, made the same way as a slip-cast one (see Pouring the plaster mold), but nothing is poured into them: you work plastic clay against the plaster. The press mold tool makes tiles, sprigs, dishes and small figures; the hump mold tool makes bowls, plates and dishes that open all the way up.

The clay

Use plastic clay, soft enough to take a thumbprint without sticking to your hand. Wedge it well: air in pressed clay bursts in the kiln. The pour sheet says how much one pressing takes, for a wall about 8 mm thick over the whole surface with a quarter to spare. Roll it between two guide sticks and compress both faces with a soft rib.

An open press mold

Lay the slab into the mold and press it from the middle out with a damp sponge, so no air is caught under it. Add small pieces where the form is deep and work them in.
Cut or scrape the clay level with the top of the mold.
Let it stiffen, 20 minutes to 2 hours, until the clay has pulled away from the plaster at the edge. A damp mold takes longer.
Lay a board on the mold, turn both over together and lift the mold off.

A press mold in pieces

Press clay into each plaster piece from the middle out and leave it standing 2 to 3 mm proud of the seam face. Scratch the proud edges and brush them with slip, then bring the pieces together on their natches and squeeze: clay coming out along the seam shows the join is full. When it has stiffened, lift each piece straight away from the clay, holding the form in your hand as the last piece comes off.

A hump mold

Roll the slab as thick as you set the clay in the tool: the hump was made for that thickness.
Stand the hump on its plinth. Carry the slab on a thin cloth, lay it over the hump and ease it down from the top outwards with the soft rib, so no air is caught and the clay does not fold.
Cut the slab off along the fine groove round the plinth, holding the knife square to it: that line is the rim.
Add a foot if you want one: score the base, add a coil or a ring of clay and smooth it in.
Take it off soon, after 15 to 40 minutes. Clay shrinks as it dries and the hump does not, so a piece left on the hump cracks. Lift it off as soon as it holds its shape, well before leather hard: lay a board on the base, turn hump and board over together and lift the hump out.

Fettle, dry, and press again

At leather hard, pare the edge or the seam line with a knife and smooth it with a damp sponge. Dry the piece slowly on a flat board, out of drafts. Let the mold dry between pressings until it no longer feels cold and clammy.

If something goes wrong

The clay sticks in the mold: it is too wet, or the mold is still damp from the last pressing. Wait longer before you turn it out, dust a damp mold lightly with dry clay, or let it dry between pressings.
Blisters or hollows on the face: air was caught under the slab. Press from the middle out, and add clay in small pieces where the form is deep.
The seam opens as it dries: the edges were not scored, or the clay was too stiff to join. Score and slip both edges, and close the mold while the clay is still soft.
The piece cracks on the hump: it stayed on too long. A sheet of newsprint between hump and slab lets the clay slide as it shrinks.