Why does glaze crack before firing?

Glaze cracking before firing, also known as “crazing,” is a common ceramic phenomenon that can occur for several reasons, often related to differences in shrinkage and thermal expansion between the glaze and the underlying clay body.

Thermal Expansion

Glaze and clay have different coefficients of thermal expansion, meaning they expand and contract at different rates during temperature changes. This disparity can lead to tensile stress, resulting in cracks on the glaze surface.

Shrinkage Mismatch

During drying and pre-firing stages, the clay and glaze may shrink at different rates. If the glaze shrinks more than the clay, it can cause tension and cracking on the glaze layer.

Glaze Thickness

Excessive glaze thickness can lead to uneven stresses as the piece dries and fires. Thicker glazes are more prone to cracking, especially if applied over an uneven or textured surface.

Surface Tension

Surface tension can develop if the glaze is too fluid during application. As the glaze dries and contracts, surface tension can create cracks as it pulls away from the edges.

Prevention and Solutions

To minimize glaze cracking before firing:

  • Apply glaze in thin, even coats to avoid excessive thickness.
  • Test glaze application and firing on small test pieces before applying to larger works.
  • Adjust glaze recipes and firing schedules to match the specific clay body used.


Understanding the factors that contribute to glaze cracking before firing and taking appropriate measures can help you achieve successful glaze application and firing results.

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