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Crazing is one of the most common problems related to glaze defects. It appears in the glazed surface of fired ware as a network of fine hairline cracks. The initial cracks are thicker, and filled in with finer cracks.
Crazing is caused by the glaze being under too much tension. This tension occurs when the glaze contracts more than the clay body during cooling. Because glazes are a very thin coating, most will pull apart or craze under very little tension.
Crazing can make a foodsafe glaze unsafe and ruin the look of the piece. There are two types of crazing, each with a different cause.
Immediate Crazing
1) Appears when piece removed from kiln or shortly thereafter.
2) Caused by glaze body fit-glaze fits too tightly to clay body.
All ceramic bodies change in size during heating (firing) and cooling. What is desired is for the glaze to shrink a little more than the body during cooling. If it doesn't, then glaze problems can occur. It is important for ware and glaze expansion and shrinkage to match.
Delayed Crazing
1) Shows up weeks/months later.
2) Caused by moisture getting into the ware.
This type of crazing shows up weeks or months later and may be caused by underfiring. When moisture is introduced to the clay body through stilt holes, expansion and contraction of the clay body can also cause crazing. Therefore handwashing pieces as opposed to constant dishwasher use may the preferred method with functional low fire ceramic ware. This would not be the case with stoneware when it has been fired to its proper maturity because the clay body will not be as porous as low fire peices.
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