Nineteen Hours to Leather-Hard, Twenty-Seven to Scrap

The loop tool went through the base trying to even out the foot ring. Six hours of work across two days. Piece is scrap. Honestly frustrated. Maybe salvaged the rim to practice glazing on? Or just total loss and start fresh.Pressed my thumbnail into the clay at 9:20 AM. Left a mark but didn’t indent easily—leather-hard, right in the window. By 10:15 the piece was upside-down on the wheel, centered with three coils of fresh clay. By 10:47 I’d cut completely through the base and the bowl I’d thrown yesterday was scrap.

Diane was trimming a mug at the next wheel. She heard the change in sound—the loop tool suddenly cutting without resistance—and looked over. Stopped her wheel with the foot pedal. “Did you go through?”

I lifted the tool. The ribbon of clay coming off the base was thinner than I expected, and then it was nothing, and then I was looking at the wooden bat through a hole in what used to be the bottom of a bowl.

“Yeah.”

She nodded. “How thick were the walls when you threw it?”

“Five millimetres on one side. Maybe seven on the other.”

“And you were trying to even them out by trimming the foot ring to match?”

“Yes.”

“Can’t do that. The interior curve is set when you throw. You trim the outside to match the inside, not the other way around. If the walls are uneven, the base thickness is uneven too. You cut down to match the thin side, you’ll go through before you reach the thick side.”

The piece had been drying for nineteen hours. Picked it up this morning and it weighed noticeably less than when I’d set it on the board yesterday—water evaporating, clay stiffening, moving from 20-25% moisture down to maybe 16%. The window for trimming is four to six hours wide depending on humidity. Too early and the piece deforms when you invert it. Too late and the tools chatter across the surface instead of cutting.

I’d arrived at hour nineteen, thumbnail test said leather-hard, looked correct. Centered it upside-down, secured it with coils of soft clay pressed against the rim, spun the wheel at maybe 50 RPM. The loop tool has a curved wire cutting edge, shaped like a ribbon. Hold it at 15-20 degrees to the surface, steady pressure, let the wheel bring the clay to the tool.

The first pass removed a clean spiral of clay about two millimetres thick. Second pass: thinner, maybe one millimetre. Compression rings formed where the tool tracked across the surface—concentric circles that you can smooth out or leave as texture. The base was still too thick, maybe twelve millimetres at the centre. Standard practice is to trim down to six or seven, leaving enough structure for a foot ring but removing the excess weight.

Third pass started well. Then the tool hit the section where the wall thickness dropped from seven millimetres to five. The base there was thinner than the rest—maybe eight millimetres instead of twelve—because I’d pulled the wall higher on that side during throwing. The tool went through.

Stopped the wheel. Lifted the piece off. The hole was maybe fifteen millimetres across, clean edges, right where the foot ring was supposed to be. Not repairable. You can’t add clay back to leather-hard and expect it to survive firing—the joint will have different moisture content, different shrinkage rate, guaranteed to crack when the chemical water is driven off during bisque firing around 450-600°C.

Diane pulled it off my wheel and set it on the scrap table. “This happens to everyone. Usually on their third or fourth piece, once they’ve figured out centering and think they can fix throwing mistakes with trimming.”

“Can you?”

“No. Trimming refines what you threw. It doesn’t correct it. If you throw a lopsided bowl, you’ll trim a lopsided bowl that weighs less.”

She picked up the ruined piece and pointed at the interior. “See how the curve is tighter on this side? That’s where you pulled the wall thinner. The base had to be thinner there too or you would’ve had a stress concentration—thick base under thin wall. That cracks in the bisque kiln. So the clay was already telling you the base was uneven. The loop tool just confirmed it.”

Put the piece back on the scrap pile. Six hours of work across two days. Forty minutes centering and throwing yesterday. Twenty-seven minutes trimming this morning before catastrophic failure.

Wedged a fresh lump of clay for three minutes—rams-head technique, impact-fold-rotate, checking for air bubbles with a wire slice through the middle. Centered it in nine minutes, which is faster than yesterday. Opened without going through the bottom. Pulled the wall to maybe ten centimeters, six passes, keeping pressure even on both sides this time. Rim stayed level. Diane checked it with a needle tool—six millimetres thick on all sides, consistent base depth.

“Better. That one might survive.”

Went back at 4:30 PM. The second piece had reached leather-hard—twenty-one hours this time, tested with thumbnail, left a mark but didn’t indent. Inverted it, centered, secured with coils. First trimming pass: clean ribbon, even thickness. Second pass: still cutting smoothly. Third pass: started shaping the foot ring, five millimetres high, curve matching the interior like Diane showed me.

Compression rings formed. Smoothed them with a rib tool. Cleaned up the edge where the foot ring meets the base. Trimmed the transition from wall to base so the curve was continuous. Undercut slightly so the piece will sit flat. Used the needle tool to check wall thickness: six millimetres, still consistent.

Lifted it off the wheel. Weighed it in my hand—noticeably lighter than after throwing. Maybe thirty percent less. The clay I’d trimmed away was sitting in grey ribbons on the bat, drying into brittle curls.

Set it on the bisque shelf. Diane has a kiln firing tomorrow night—twelve hours to reach 980°C, then thirty-six hours of cooldown. Won’t know if it survived until Thursday morning. Uneven heating, uneven cooling, differential shrinkage. The clay shrinks eight to twelve percent from wet to bisque. If the walls aren’t uniform the shrinkage won’t be uniform. S-cracks form from compression stress, same physics as railroad rails buckling.

The first piece is still on the scrap table. Looking at it on the way out: the hole goes clean through, the walls are still standing, the rim is level. Everything worked except the part I tried to fix.

Started grinding a telescope mirror two months ago and cracked the blank in twenty minutes because the grinding stand wasn’t level. Learned to check setup before starting. Apparently didn’t generalize that lesson to “check the piece you’re working on matches the technique you’re using.”

The second bowl is in the bisque queue. The first one is scrap. Difference is I threw the second one knowing trimming couldn’t fix mistakes, so I didn’t make mistakes that needed fixing.

Drove home with clay under my fingernails and the feeling that I’ve just been told something important about the difference between correction and compensation. Or maybe just that you can’t trim your way out of bad throwing. Both feel true.