UNDERSTANDING CRAZING

What is crazing?
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 spiral upward. These are filled in horizontally with finer cracks. Crazing is caused by the glaze being under too much tension. This tension occurs when the glaze contracts more than the body during cooling. Because glazes are a very thin coating, most will pull apart ar craze under very little tension. Crazing can make foodsafe glazes unsafe and ruin the look of a piece. There are two types of crazing, each with a different cause: 1) immediate crazing appears when the piece is removed from the kiln or shortly thereafter and is caused by glaze body fit(glaze fits too tightly to body) and 2) delayed crazing, which shows up weeks or months later and is caused by moisture getting into the ware.

Immediate crazing:
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 may occur. It is important for ware and glaze expansion and shrinkage to match or crazing can occur.

Glazes during firing:

1. During firing, glaze undergoes physical and chemical changes.
2. As heating progresses, glaze melts.
3. With further heating, more liquid forms until it is viscous or thick fluid.
4. At this point, viscous (thick flowing) glaze still conforms to the size of the bisque.
5. Any gas evolving from the body will form blisters which can heal if the glaze is still fluid.
6. When the kiln shuts off, the glaze and the body cool together.
7. During cooling, both the body and the glaze shrink.
8. Eventually, the glaze becomes a hard glass that will no longer flow.

Solution to glaze and body fit:

1. Test samples for a good fit.
2. Bisque to 1-2 cone numbers hotter than glaze to insure that the body is mature.
3. Use self-supporting witness cones to verify the heatwork.
4. Recognize that bodies and glazes will have different fits for different heatwork. A glaze mit fit bisque fired to 03, but craze on 07 bisque.

Delayed crazing:
This type of crazing shows up weeks or months later and is practically always caused by underfiring. If ware is underfired (does not reach maturity), it can, in time, expand when moisture fills the pores causing the body to expand. Sudden changes in temperature can cause crazing if the body and glaze do not expand or contract uniformly. Either the body expanding or the glaze shrinking can cause fine hairline cracking (crazing) to occur. Refiring to the proper cone will sometimes solve the problem. Firing to the proper cone number is critical to help eliminate crazing problems. Witness cones must be used to verify the heatwork the ware receives. If the kiln-sitter turns the kiln off and a witness cone is not properly deformed, then the ware is not fired to maturity.

Underfiring can occur because of:

1. variations in kiln heating uniformity
2. kiln-sitter out of adjustment and shutting off kiln early
3. controller thermocouple inaccurate
4. differences in heatwork between kiln shelf and kiln-sitter location.

Lead free glazes
Lead-free glaze formulations today have less of a firing range. They develop their fired properties more quickly and this makes proper firing more critical.

Printed with permission of Orton Firing Institute.

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