Why does my clay crack while it dries?

Clay loses perhaps a tenth of its size between wet and fired. Cracks are simply that shrinkage happening at different speeds in different parts of the same piece.

Step by step

1. Work out where the crack is.At a joint means the join was poor. At a thin rim or handle means uneven drying. Across a flat base means the base dried faster than the walls, often because it sat on a board.
2. Check wall thickness first.Even thickness of around a centimetre. A wall twice as thick in one place takes twice as long to dry, and the shrinkage difference tears the clay.
3. Score and slip every joint.Both surfaces keyed and wetted with slip. Almost every crack that follows a seam is a joint that was pressed together dry.
4. Compress the clay as you work.Rib or paddle the surface, especially the base and rims. Compression aligns the clay particles and substantially reduces cracking. It is the most underused technique in hand building.
5. Dry slowly under loose plastic.A day or two loosely covered, then uncovered in a room without direct heat, sun or draughts. Speeding up drying is the commonest cause of the problem.
6. Protect the fast-drying parts.Rims, handles, spouts and edges dry first. A strip of plastic over them slows them to match the body.
7. Lift pieces off the board.A flat base drying on a solid board dries only from above, while the walls dry from both sides. Standing the piece on a slatted rack or turning it occasionally evens it out.
8. Repair damp cracks properly.Open the crack slightly, work in slip made from the same clay, compress it firmly with a rib and cover the area to slow it down. Simply smearing slip over the surface does not work.
9. Accept that bone-dry cracks are usually terminal.There is no water left for a repair to bond into. Patched dry cracks nearly always reopen in the kiln.
10. Look out for S-cracks in bases.An S-shaped crack in a base usually means the clay was not wedged well or the base was not compressed. Both are prevention, not repair.

Tips

• Compress the base and rims with a rib. It prevents more cracks than any other single habit.
• Cover rims and handles with plastic strips so they do not race ahead of the body.
• A crack in damp clay can be saved; a crack in bone-dry clay cannot.

Common mistakes

• Drying a piece near a radiator or in the sun — The exposed surfaces shrink while the interior is still wet, so the piece is pulled apart from within. Slow covered drying costs a few days and prevents nearly all of it.
• Smearing slip over a dry crack — Slip only bonds to damp clay. On dry clay it sits on the surface, shrinks as it dries and falls out, and the crack reopens during firing.

If it doesn't work

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