Why glaze crazing happens and what a useful test can tell you
Crazing is a network of fine cracks in a fired glaze. It can appear immediately or develop later. The visible pattern is simple; identifying the cause requires a labelled comparison rather than a single recipe change.
- Published
- 19 August 2026
- Checked
- 31 August 2026
- Bench time
- 8 min read
Begin with glaze fit
After firing, the ceramic body and glaze contract as they cool. If the glaze ends up under too much tension relative to the body, it can crack into a crazed pattern. This relationship is called glaze fit. The same glaze may behave differently on another clay body or at another firing condition.
A thick application can make crazing easier to see or change the way a glaze responds, but thickness alone is not a complete diagnosis. Record the body, glaze batch, application, firing, and location in the kiln before changing anything.
Separate appearance from suitability
Some makers use crazing intentionally as a visual effect. Functional suitability is a separate question. Cracks can stain and can complicate cleaning; the safety of functional ware also depends on the complete glaze-body system and how the object is used.
Do not label ware food-safe from recipe ingredients or appearance alone. Use glazes and clay bodies according to manufacturer guidance, maintain good firing records, and use appropriate professional laboratory testing when a claim must be verified.
Create a controlled tile series
Make tiles from the same clay body and label them permanently before firing. Apply a measured range of glaze thicknesses, leaving part of each tile unglazed. Fire them together with witness cones. If comparing bodies, keep glaze and application as consistent as possible.
Inspect immediately, after several days, and after the kind of temperature changes the intended use may create. Tests should not damage household equipment or expose people to unsafe materials. Studio or laboratory protocols are preferable when durability matters commercially.
- Change one variable per tile series
- Include an unglazed area to identify the body
- Record measured application, not only coat count
- Keep failed tiles; date later observations
Choose the next change from the evidence
If every thickness crazes on one body but not another, glaze-body fit deserves attention. If only a very heavy application fails, application control may be the practical first fix. If results vary by shelf, investigate firing uniformity and heatwork before reformulating.
Recipe adjustment is ceramic chemistry, not a random substitution exercise. Consult a qualified glaze specialist and recalculate the formula when altering materials. A tidy test series makes that conversation faster and safer.