Crazing

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Last Updated: Thursday, 19 June 2014 19:43

Ever notice a pattern of fine spider web-like series of lines across your glazed ware? Unless you have used a crackle glaze where the intent is to create this look, you are experiencing a phenomenon called “crazing”.

Crazing can occur immediately upon removal from the kiln or can develop over time (delayed crazing). Common causes of crazing are:

• A mismatch of the co-efficient of thermal expansion (CTE) of the clay body and the glaze. When using a new glaze or new clay body, we suggest that you perform a test fire to ensure compatibility.
• Immature bisque. If your ware has not been fired to maturity before applying glaze, the bisque will continue to expand/contract during the glaze firing, causing a misfit between the body and the glaze. We recommend a two shelf cone difference between your bisque fire and your glaze fire. If you are using glazes that mature at cone 05-06, then we would suggest that you bisque fire to 03/04.
• Opening the kiln too soon, resulting in thermal shock. Hot ware meets cool air, causing the glaze to cool faster than the bisque. We recommend that you do not open the kiln until 150 degrees or less or cool to the touch.
• Dry-footing (no glaze applied to the bottom of ware) a low fire clay body will cause the ware to absorb moisture. Moisture causes the ware to expand therefore putting tension on the glaze and crazing it. If producing a functional piece of low fire clay body the entire piece must be glazed to help prevent crazing and assure it to be food safe.
• A glaze applied excessively heavy may also cause a glaze to craze.

If you experience crazing on dinnerware, we would recommend that you discontinue use. While you may have used non-toxic glazes, which are food safe, the hairline cracks could be difficult to clean, harboring bacteria.

If you have crazing or other glaze defect issues, please contact Kathy McCourt of Mayo’s Technical Service for assistance at 614-675-2031 or This email address is being protected from spambots. You need JavaScript enabled to view it.