warping
Leave a wet wooden board to dry in the sun on one side only and it curls into a shallow dish, bowing away from the side that dried first. A flat ceramic tile does exactly the same thing during drying. Warping is that unwanted change of shape — a bending, bowing, or twisting of a piece that was meant to be flat or true — caused not by any outside push but by the body shrinking more in one place or direction than another.
Warping is differential shrinkage that bends instead of cracks. If the top of a tile dries and shrinks before the bottom, the shrinking top face pulls itself shorter while the still-wet bottom stays long; the only way to reconcile a short top with a long bottom is to curl, top-face concave. The same happens across thick and thin sections, or between a rim exposed to moving air and a sheltered centre. Anisotropic particles add a subtler cause: platy clay particles laid down flat by extrusion or pressing shrink more across their stacked faces than along them, so the body shrinks unequally in different directions and warps or changes proportion even when it dries evenly.
Warping is the gentler cousin of cracking — the body relieves its shrinkage mismatch by moving rather than breaking — but a warped part is still a rejected part when flatness matters, as it does for tiles, tape-cast electronic substrates, and kiln furniture. The cures are the same as for cracking: dry slowly and symmetrically so no face or edge races ahead, support flat pieces evenly (often flipped or set on porous batts), and design forming so that platy particles are not all aligned one way. Warping that appears in drying can also be locked in or worsened later during firing.
A freshly cast flat tile left to dry on a solid, non-porous shelf bows upward at the edges: its top and sides lose water while the sealed underside stays wet, so the top shrinks first and the tile curls. Set the identical tile on a porous plaster batt that lets the bottom breathe too, and it dries flat.
Warping is shrinkage mismatch relieved by bending: the face that dries and shrinks first becomes the concave side.
Warping is often blamed on the firing, but much of it is set during drying and merely revealed by the kiln. If a part warps the same way every time, look first at how uniformly and symmetrically it is being dried and supported, not only at the firing.