Trimming is a finishing technique in which excess clay is cut away from a piece, usually its base, once the piece has firmed up to leather-hard. It is most closely associated with wheel-thrown work, where a piece is commonly returned to the wheel to have its foot and base evened out, but the same basic cutting technique applies to a handbuilt piece with excess clay to remove as well.
A leather-hard piece is set back on the wheel head, centered by eye or with the help of centering marks, and held in place, often with soft clay pads or a chuck rather than being fixed directly to the wheel head. As the wheel spins, a trimming tool (a loop tool, ribbon tool, or similar) shaves away thin curls of clay from the base and lower wall, evening out wall thickness that pulling by hand rarely leaves perfectly uniform, and cutting a foot ring, a raised ring left standing proud of a recessed center so the piece contacts a shelf or table only along that ring rather than across its whole base. Trimmed clay curls are usually dry enough to be reclaimed rather than discarded (see reclaiming clay).
Trimming happens at leather-hard specifically because the clay is firm enough to hold its shape and not deform under the trimming tool's pressure, but still soft enough to cut cleanly rather than crumbling or chipping the way fully dry (bone-dry) clay does. Trimmed too early, while still too soft, a piece can distort under light pressure from the tool or the centering process itself; trimmed too late, once too dry, the clay resists the tool and is more likely to crack. A piece that is not well centered before trimming ends up with an uneven wall or a foot ring that runs visibly off-center once glazed and fired, which is usually more noticeable than it was on the still-wet, still-forgiving clay.