Why Clay Cracks When It Dries Too Quickly

Every crack in an unfired pot is really a conversation about water. Clay is millions of tiny platelets suspended in moisture, and when you throw or handbuild, those platelets pack down in a particular direction. As water leaves the body, the clay has to shrink, and it can only do that evenly if the moisture can travel from the centre of the wall to the surface at a steady rate. In practice, the outside always dries first. The skin tightens and pulls in while the middle is still soft and generous, and that difference sets up a strain the clay simply cannot stretch through.

Some bodies forgive more than others. A coarse grogged stoneware will shrug off a rushed drying now and then; a smooth porcelain or a fine white earthenware will punish you for it within the hour. Thick bases, narrow necks, handles and joins are all weak points where the strain concentrates. The good news is that the remedy is not complicated — it is just slow, and a little bit fussy.

Getting the Wrap Right

As soon as a piece is finished, get it under cover. A supermarket carrier bag works at a push, but a proper sheet of 150-gauge polythene, sold by the metre from most pottery suppliers, gives you far more control. Wrap the pot loosely rather than vacuum-tight, tucking the plastic under the base so the whole thing is enclosed, and leave a small gap at the top for a little air exchange.

If you have several pieces on the go, a damp box is a better home for them. A plastic storage crate with a couple of centimetres of plaster of Paris set in the bottom, kept damp with a splash of water, will hold a whole shelf of work at a steady humidity for weeks. Add a lidded bucket of water inside if you need to push the humidity higher still.

• Freshly thrown: wrap immediately, no exceptions, even if you only stop for a cup of tea.
• Leather-hard: keep it wrapped but open the plastic a little more each day.
• Trimming stage: a short spell uncovered while you turn the foot is fine, then back under cover.
• Handbuilt slabs: dry them between two boards with a sheet of newspaper to slow the surface.

Turn Every Day, Without Fail

Moisture pools. Left alone, a pot dries more on the side facing the draught and less on the side resting on the board, which is how you end up with one wobbly rim and one that is still soft a week later. Turn the piece over once a day, or at least rotate it a quarter turn, and swap it to a dry board if the one underneath has gone dark with damp.

This daily check is also your best early warning system. Run a finger along the rim and feel for cold patches — that is water still sitting in the wall. If one area feels noticeably cooler, wrap the pot back up and give it another day or two. It is far easier to slow a pot down than to rescue one that has already split. A light misting of water over the surface, followed by re-wrapping, can bring a stubborn dry edge back into line.

Rims and Bases: The Last to Dry

The rim of a pot is thin, exposed and constantly losing moisture, which is why it will often go leather-hard while the lower wall is still happily plastic. The base is the opposite problem: thick, sitting on a board, and shielded from moving air. If you let the rim dry hard first, it locks the shape while the rest of the pot is still shrinking underneath, and the crack appears right at the lip.

Keep rims covered for longer. A strip of plastic laid over the top of the pot, or a small piece of polythene draped over the opening of a bowl, will hold the rim back while the walls and base catch up. For bottles and narrow-necked forms, a twist of cling film over the neck does the same job. Turn the pot so the base gets a turn in the open air once the rim has firmed up enough to take it.

Judging Bone Dry and Moving On

Bone dry means bone dry — no cold feeling against your cheek, no darker patches, and the pot should feel noticeably lighter in the hand. Rush this stage and you will lose work in the bisque firing, when the last of the chemically bound water turns to steam and forces its way out of the clay.

A good habit is to leave finished pots in a cool, draughty-free corner of the workshop for a fortnight after they feel dry, turning them once or twice. Cold workshops slow drying, which is fine; warm dry workshops, radiators and sunny windowsills will crack a pot faster than anything. If you are in a hurry, dry under a cardboard box with a small ventilation hole. Patience here costs you nothing but a bit of shelf space, and it is the cheapest insurance you will ever buy for your pots.