Reading Glaze Defects Like a Detective
Glaze defects are frustrating, but every defect has a cause, and once you know the cause, you can prevent it. The three most common defects in studio pottery are crawling, crazing, and pinholes. This lesson covers crawling and crazing.
Crawling
Crawling is when the glaze pulls back from the clay surface during firing, leaving bare patches of unglazed clay surrounded by thick glaze edges. It looks like the glaze ran away from parts of the pot.
Causes:
- Glaze applied over a dusty or oily surface. The glaze has nothing to grip.
- Glaze applied too thickly. The layer cracks as it dries, and those cracks widen during firing.
- Glaze applied over an under-fired bisque with too many impurities in the clay.
Fixes:
- Wipe the pot down with a clean, barely damp sponge before glazing.
- Apply glaze in thinner, even layers rather than one thick coat.
- Make sure your bisque firing is complete before glazing.
Crazing
Crazing is a fine network of cracks in the glaze surface, like a shattered windshield. The glaze has cracked after firing and cooling.
Causes:
- The glaze and clay body have different thermal expansion coefficients: they expand and contract at different rates as the kiln heats and cools.
- If the glaze contracts more than the clay during cooling, it shatters into a craze pattern.
Fixes:
- Use a glaze formulated for your specific clay body.
- A slow, controlled cool-down can sometimes reduce crazing.
- If a glaze consistently crazes on your clay, it is not the right fit; switch glazes.
Defect Diagnosis Flow
When something looks wrong:
- Bare islands with thick glaze edges → crawling
- Fine crack network only in glaze → crazing
- Tiny craters with raw clay showing → pinholes
Naming the defect correctly is the first step to fixing it.
Dig Deeper
Both crawling and crazing are well-documented glaze defects with specific causes rooted in chemistry and physics. Crazing in particular is driven by mismatched thermal expansion rates between the glaze and the clay body, a concept that applies to all ceramic materials from earthenware to porcelain.
Common Mistakes and Quick Fixes
- Guessing the defect name: Identify pattern first, then choose correction.
- Applying thicker glaze to “cover” defects: Thickness often worsens crawling and running.
- Skipping surface prep: Dust and oils are major adhesion failures.
- Trying one fix once and stopping: Defect control requires staged testing over multiple firings.
Practice Exercise
Make a defect board with small tiles labelled crawling, crazing, and pinholes using intentional test conditions. Keep it near your kiln as a visual diagnosis reference for future loads.
