Crazing is a network of fine cracks that appears in a glaze surface — either right out of the kiln or weeks later. It can look like an intentional decorative effect, and sometimes it is. But for functional pottery it is a problem — crazes trap bacteria and liquids and indicate a glaze that does not fit its clay body. Here is why it happens and how to fix it.
What is Crazing?
Crazing occurs when the glaze shrinks more than the clay body during cooling. The glaze is under tension, and when that tension exceeds the glaze’s strength, it cracks in a network pattern across the surface. The opposite problem — glaze shrinking less than the clay — causes shivering, where the glaze flakes off in sharp fragments.
Why Glaze Fit Matters
Glaze fit refers to how well the thermal expansion and contraction of the glaze matches the clay body it is applied to. A perfectly fitting glaze compresses slightly as the piece cools — this compression actually strengthens the piece. A poorly fitting glaze either crazes (too much expansion) or shivers (too little).
Glaze fit is measured by the coefficient of thermal expansion (COE) — a number that describes how much a material expands when heated. Glazes and clay bodies need compatible COE values to fit well.
Causes of Crazing
- Glaze has too high a thermal expansion — the most fundamental cause. The glaze chemistry needs adjustment to reduce expansion.
- Glaze applied too thin — thin glaze layers are more prone to crazing than properly thick applications.
- Wrong glaze for your clay body — a commercial glaze formulated for one clay body may craze on a different one. Always test new glazes on your specific clay body before using on finished work.
- Cooling too fast — rapid cooling increases thermal stress. Let the kiln cool slowly, especially through the quartz inversion range around 1063°F (573°C).
- Refiring — pieces that have been refired multiple times are more prone to crazing as the glaze-clay bond weakens.
How to Fix Crazing
Adjust the glaze chemistry
To reduce thermal expansion in a glaze, increase silica (SiO2) or alumina (Al2O3) content, or reduce high-expansion fluxes like sodium and potassium. Use a glaze calculator like Glazy or Digitalfire Insight to model the changes before mixing a new batch.
Apply glaze thicker
A thicker glaze application is more resistant to crazing than a thin one. Check your specific gravity and increase if consistently crazing.
Slow the cooling
Program a slower cooling rate through the quartz inversion range (approximately 1100–900°F / 593–482°C) to reduce thermal stress on the glaze-clay interface.
For more on glaze chemistry materials, see our Glaze Making for Beginners guide.
