Defect · Cooling
Crazing
Also called: craze lines
Fine network of cracks in the glaze surface, visible immediately or developing days to months after firing.
- Appears at
- Cooling
- Severity
- Affects function
- How common
- Common
Causes
Most likely
- ·Glaze thermal expansion coefficient is higher than clay body
- ·Glaze shrinks more than clay body during cooling
Prevention
- ·Match glaze and clay body thermal expansion (aim for glaze slightly lower than body)
- ·Increase silica in the glaze to lower expansion
- ·Replace high-expansion fluxes (Na2O, K2O) with lower-expansion ones (CaO, MgO)
Can it be fixed?
Not reliably — this one is prevented, not repaired. A cracked or bloated pot is a test tile now.
Questions
- Why does crazing happen?
- Most often: Glaze thermal expansion coefficient is higher than clay body. Other causes: Glaze shrinks more than clay body during cooling..
- Can crazing be fixed?
- Not reliably once it has happened; the fix is upstream, in prevention.
- How do I prevent crazing?
- Match glaze and clay body thermal expansion (aim for glaze slightly lower than body). Increase silica in the glaze to lower expansion. Replace high-expansion fluxes (Na2O, K2O) with lower-expansion ones (CaO, MgO).
Further reading. The industrial treatment of this defect in the Ceramicware Defects Handbook on Wikibooks (CC BY-SA 3.0).
Sources. Compiled from Claybench's defect reference — the descriptions, causes and remedies that appear consistently across standard ceramics references and studio practice; no single source. It is the same reference the photo diagnosis tool reasons over. Where the Wikibooks Ceramicware Defects Handbook covers a defect it is linked above.
Not sure which defect you're looking at?
Claybench can read a photo of the pot against this same reference and rank the likely causes for your clay, glaze and firing.