You pull a piece from the kiln, it looks perfect, and then over the next few hours you hear faint pinging and see a fine web of cracks spread across the glaze. That's crazing, and it's the single most common glaze defect potters run into. The good news: it's also one of the most fixable, once you understand what's actually happening.
Crazing is a network of fine cracks in the fired glaze surface. It happens when the glaze shrinks more than the clay body as the piece cools. The glaze is stretched into tension it can't withstand, so it splits to relieve the stress. In technical terms, the glaze's coefficient of thermal expansion (CTE) is higher than the body's. It's not a firing accident so much as a fit problem between two materials.
On functional ware, crazing isn't just cosmetic. Those cracks let liquid seep into the body and can harbor bacteria, so a crazed mug or bowl shouldn't be used for food or drink even after washing. On decorative pieces, some potters chase crazing on purpose and call it a crackle glaze.
Work through these in order, changing one variable at a time so you can tell what worked:
| Fix | Why it works |
|---|---|
| Add silica to the glaze | Lowers the glaze's expansion so it shrinks less on cooling |
| Increase alumina (more clay/kaolin) | Also lowers expansion and stiffens the melt |
| Apply a thinner coat | Thick glaze crazes more easily; thinner sits under compression |
| Fire to full maturity | An underfired glaze hasn't bonded; hitting the right cone helps |
| Let the kiln cool fully | Avoids thermal-shock crazing and dunting from cracking the lid too soon |
The most reliable prevention is matching a glaze to the clay body it was formulated for. Commercial glazes are tested against specific bodies and cones, so using them outside those parameters is where most crazing starts. If you mix your own, keep notes and adjust silica and alumina rather than guessing. And resist the urge to crack the kiln lid early to "peek" or speed things up, since that fast cool is a crazing (and cracking) trigger on its own.
One more habit worth building: fire a small test tile of any new body/glaze pairing before you commit a whole batch. It costs one shelf slot and saves an entire kiln load.
A programmable controller and a properly maintained kiln make dialing in cooling schedules far easier. Not sure which kiln or controller fits your work? Call or text me, Spencer, at (801) 839-5882 and I'll point you the right direction.
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