Glaze defects feel like bad luck, but almost every one has a traceable cause — in application, the glaze recipe, the clay fit, or the firing. Here are the most common problems, what causes them, and how to fix each.
Open KilnLog — log defects & fixes →Crawling is when glaze pulls away from the clay, leaving bare spots and thick beads. The usual culprit is poor adhesion: dusty or oily bisque, glaze applied too thickly, or high shrinkage in the raw glaze. Fixes: wipe bisque clean and dust-free before glazing, apply a thinner, more even coat, and if the recipe is prone to it, calcine part of the clay content or add a touch of binder.
Crazing is a web of hairline cracks caused by a glaze-to-clay fit problem — the glaze shrinks more than the clay on cooling, so it's under tension. Fixes: lower the glaze's thermal expansion (more silica or boron, less high-expansion flux like sodium and potassium), make sure the body is fully vitrified, and try a slower cool. On functional ware, crazing can harbour bacteria, so it's worth correcting.
Shivering is crazing's opposite: the glaze is under compression and chips off, especially at rims and handles — sharp and dangerous on food surfaces. The fit is too tight. Fixes: raise the glaze's expansion slightly (a little more high-expansion flux, or less silica) until it sits in balance with the body.
Pinholes are tiny craters; blisters are larger broken bubbles. Both come from gases escaping through the molten glaze — often from under-bisqued ware, a too-fast glaze ramp, or firing too hot. Fixes: bisque hotter (cone 04) and slower so carbon and sulfur burn out, slow the final glaze ramp, and add a short hold near peak to let the surface heal over.
Dunting is a crack through clay and glaze, caused by thermal shock on cooling — usually cooling too fast through quartz inversion (~1063 °F / 573 °C) or opening the kiln too early. Fix: program a controlled, slower cool and don't crack the lid until the kiln is below ~300 °F (150 °C).
Most defect fixes are about changing one variable and seeing what happens. If you log the glaze, the bisque cone, the ramp, the hold and the result each firing, the pattern behind a defect — and the change that cured it — becomes obvious instead of mysterious.
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