Altering is a wheel-throwing technique in which a piece is thrown round on the wheel as normal, then reshaped by hand once removed, combining the speed of the wheel with a final shape the wheel alone cannot produce directly. Common alterations include paddling a round form into an oval or square cross-section, faceting flat planes into a round wall with a cutting wire or tool, and pinching or pushing a rim out of round.
A piece is thrown normally, then altered at a specific stiffness, generally once it has firmed up somewhat from throwing-wet but is still soft enough to reshape without cracking, since fully leather-hard clay resists reshaping and is more likely to crack than deform smoothly. Paddling uses a flat wooden tool tapped against the outside wall, often with a hand or a support inside to keep the wall from collapsing, to push a round cross-section into an oval, square, or other flattened shape. Faceting cuts flat planes into a round wall with a wire or a flat blade, removing thin slices of clay to leave a many-sided form rather than reshaping the wall by pushing. A rim can also be altered on its own, pinched into points, pushed out of round, or folded over, without changing the body of the piece below it.
Altering timing matters more than with most other forming choices: too soon, and the piece is still too wet to hold the new shape or support itself once distorted from round; too late, and leather-hard clay cracks rather than reshaping cleanly under the same pressure. A wall that was thrown too thin for the planned alteration is also a common problem, particularly for faceting, which removes material and can cut through an already-thin wall; a piece meant for heavy altering is often thrown slightly thicker than a piece left round for exactly this reason.