Trimming a foot ring without going through the bottom
Trimming is the stage where a heavy, clumsy thrown bowl becomes a light one, and also the stage where you discover exactly how thick the bottom is by cutting straight through it. I did that twice this month. Both times the hole appeared with a little sigh of air and a feeling of complete inevitability.
The setup: the bowl, leather-hard, upside down on the wheel, centered by tapping it while the wheel turns slowly, held with three coils of soft clay pressed around the rim. For the first two bowls I had no idea how thick the base was, so I guessed, and I guessed wrong. The instructor showed me the needle trick afterward: before trimming, push a needle tool straight down into the base at the center until it hits the bat, and read the depth off the needle. Both of the ones I holed had about 6 mm of base, and I had trimmed 8.
The third bowl wobbled off. I had centered it by eye rather than by tapping, and the trimming tool caught on the thick side and lifted the whole thing off the clay coils. It landed rim down on the splash pan and survived with a dent. The fourth, centered properly, stayed on, but I trimmed the foot ring too narrow, about 4 cm on a 20 cm bowl, and it looks like it is standing on tiptoe.
The fifth. Needle in: 9 mm of base, so I have 5 mm to play with. Tapped to center with the wheel at its slowest, held with coils. Marked the foot ring at 8 cm outside and 6 cm inside with a pencil on the turning wheel. Trimmed the outside first, taking the wall down in long passes with the loop tool held at a shallow angle, then the inside of the ring down to about 3 mm deep, leaving a 6 mm base. Then softened the outer edge of the ring with a damp sponge, because a sharp foot chips in the dishwasher.
The bowl weighed 540 g before trimming and 390 g after. That 150 g was all in the bottom, and the bowl now sits level and lifts like something made on purpose. It is drying slowly under plastic, and if it comes through the kiln it will be the first thing I have made that I would serve food in. The two with holes are being saved for a mosaic that will never happen.