Trimming is where a thrown pot becomes a finished one, and it happens in a narrow window of clay stiffness. Most trimming problems people blame on the tool are timing problems, and most of the rest are a blunt edge.
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Leather hard, and how to tell
Leather hard means the clay holds its shape firmly but still marks under a thumbnail and feels cool. Too wet and the pot distorts under the pressure of trimming or under the chuck holding it; too dry and the tool skates and leaves chatter marks. The window is wide enough to work in and narrow enough that a pot left overnight uncovered is often past it.
Loop, ribbon and fixed tools
A loop or ribbon tool has a thin metal band and removes material fast, which suits roughing out a foot. A fixed trimming tool with a shaped end cuts more precisely and gives a clean foot ring. Most potters use one of each: the loop for volume, the fixed tool for the final cut.
Chatter, and what it actually means
Chatter marks are a regular pattern of ripples, and they come from a blunt tool or clay that has gone too dry, not from your hands. Sharpen or replace the tool and check the clay; if the pot is too dry, wrapping it damp overnight will often bring it back into the window. Some potters induce chatter deliberately as decoration, which is the same physics used on purpose.
Holding the pot
A pot is trimmed upside down, held by a chuck, a hump of clay or three lumps pressed to the wheel head. It must run true or the foot will be off centre, and it must not be held so hard that the rim distorts. Centring the pot on the wheel before trimming takes a moment and saves the pot.
Questions people ask about ribbon tool
When should I trim a pot?
At leather hard, when the clay holds its shape but still marks under a thumbnail. Too wet distorts, too dry chatters.
Why do I get chatter marks?
A blunt trimming tool or clay that has gone too dry. Sharpen or replace the tool and check the state of the pot.
Loop tool or fixed trimming tool?
A loop removes material quickly for roughing out; a fixed shaped tool gives a cleaner final cut. Most potters use both.