Why Your Teapot Cracked in the Kiln: Diagnosing Firing Faults

Cracked pots are a quiet kind of disappointment. A teapot looks sound on the shelf, then the kiln opens on a hairline running up the belly, or a base sitting in two clean halves. What makes kiln cracks bearable is that they are rarely random. A crack has a location, a direction and an edge quality, and those three things point back to decisions made hours before the lid went on.

Read the Crack Before You Tidy the Shelf

Glaze hides a lot, so look at the break before you write the pot off. Sharp, clean edges with bright clay showing mean the clay was brittle and hot when it separated — usually a cooling fault. Edges that look rounded or dusty, or have glaze crept into the gap, suggest the crack opened earlier and grew as the kiln climbed. A crack spiralling around the base and lifting it away from the wall is dunting. A crack starting at the spout join and running down the body usually began at a thickness change. Photograph the break and note the kiln schedule while it is fresh; a firing log turns one bad pot into useful information.

Water That Never Left: The Range Below 600°C

Clay carries two kinds of water. The physical water that made it plastic leaves as steam below roughly 120°C. The chemically bound water leaves somewhere between about 350°C and 650°C, and it needs time to work its way out of the wall. Push through that range too quickly and steam builds pressure inside the clay faster than it can escape. Something gives, and it is usually the thickest section: the base, the root of the handle, the underside of the spout.

Candle the kiln — lid propped, elements low — for several hours or overnight so the pots are genuinely dry before the climb starts.
Hold between 90°C and 120°C if your controller allows it, then ramp slowly towards 600°C.
Give thick bases and handles a slower climb than a set of thin test tiles needs.
Never fire a pot that still feels cool and damp at the base, however dry the rim seems.

Uneven Walls, Uneven Shrinkage

A teapot is a collection of different thicknesses pretending to be one object. A wall running 4 mm at the rim and 12 mm at the base is two pots sharing a skin, and they will not shrink at the same rate. The thin section gives first, and the crack appears where the two meet. Compress your joins properly: a coil pressed only on the outside is still a seam on the inside. Wedge out air pockets that leave weak spots, and when you attach a spout or a handle, score, slip and press until it is clay again rather than two surfaces touching. If the handle feels light and springy on the pot, it is probably too thin for the body it sits on.

Fast Ramping and the Quartz Inversion

Around 573°C, free silica in the clay shifts its crystal structure and changes volume by roughly two percent. Every pot passes through that point twice, going up and coming down. Heating through it quickly is often survivable. Cooling through it quickly is where dunting cracks come from, and denser, more vitrified bodies tend to be less forgiving. The same caution applies lower down, below about 300°C, where other inversions can catch a fast-cooling pot off guard. Do not prop the lid at 700°C to hurry things along. Let the kiln cool on its own to below 200°C, ideally overnight to near room temperature, before you open it. That wait costs nothing; a cracked teapot costs a week.

Glaze Compression and the Squeeze on the Rim

Glaze fit is a balance of expansion. A glaze that ends up too large for the body on cooling sits in compression, and heavy compression pulls the clay beneath it into tension. That can shiver glaze off a rim, and it can also drive a crack through the wall, especially where glaze pools: inside the base, under a lid, in the curve where the belly meets the foot. Crazing is the opposite problem — a glaze in tension, stretched by a body that shrinks more than it does. It looks like a fine web of lines, and while it may not crack the pot immediately, it can weaken the surface and let water seep in over time. Neither compression nor crazing is a crack in itself, but both are warnings that the glaze and body are arguing. If you see a crack running from a crazed area, the glaze fit is a prime suspect.

The Lid, the Spout, and Other Stress Risers

Every appendage on a teapot is a potential fault line. The lid is a thin disc sitting on a thicker gallery; if the gallery is too rigid, the lid can crack as it tries to shrink. A spout pulled too thin at the tip and too thick at the root will crack where the two meet. Handles are worse: a strap handle that dries faster than the body because it is thinner will pull away at the joins during firing. The fix is to design for gradual transitions. Taper the spout wall slowly, keep the handle thickness within a millimetre or two of the wall it attaches to, and never leave a join that is only pressed on one side. If you can see the join line after the pot is leather-hard, it will likely open in the kiln.

Watch the Drying, Not Just the Firing

Many kiln cracks begin as drying cracks. A teapot that dried unevenly — rim dry, base still cool — carries internal stresses into the kiln. Those stresses release during the early ramp, and the crack appears as if by magic. Slow the drying under plastic, rotate the pot, and check the base with your cheek. If it feels cold, it is still wet.

Kiln Packing, Support, and Thermal Mass

How you load the kiln matters as much as the schedule. A teapot sitting on a shelf that heats faster than its neighbours will expand first, and the base can crack from the bottom up. Placing a pot too close to an element creates a hot spot; placing it on a cold shelf creates a shadow. Use kiln furniture to keep pots at least a finger’s width apart, and avoid stacking heavy pieces directly above delicate ones. If you are firing a mixed load, put thick, dense pots on the lower shelf where heat rises slowly, and thin, delicate teapots on the upper shelf. The goal is even heat, not fast heat.

A Firing Log That Pays for Itself

A firing log is the cheapest diagnostic tool you own. Record the date, clay body, glaze, thickness at the base and rim, and the full firing schedule — including holds and cooling rates. When a crack appears, you can compare it to previous firings. Did the last batch with the same clay crack? Was the kiln opened early? Did the glaze batch change? After a few firings, patterns emerge: a clay that always dunts if cooled faster than 100°C per hour, a glaze that always crazes on a particular body. That log turns guesswork into a short list of suspects.

Quick Diagnostic Checklist

Sharp edges, bright clay: Cooling crack — slow the cooling, especially through 573°C and below 300°C.
Rounded edges, glaze in the gap: Crack opened during heating — check drying and the 350–650°C range.
Crack at a join or thickness change: Uneven shrinkage — compress joins, even out walls.
Crack spiralling around the base: Dunting — cool more slowly, avoid opening the kiln early.
Crack under a pooled glaze or on a rim: Glaze compression — adjust glaze fit or application thickness.
Crack on a handle or spout: Stress riser — redesign the transition, dry more slowly.