Crazing Description#2
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Crazing Description#2
Crazing is a very common flaw--the glaze will appear cracked. Most materials will expand when heated and contract when cooled. We're all familiar with the expansion joints on bridges which allow the road bed to expand and contract without buckling or cracking. A glaze and the clay body it covers will typically expand at different rates as they are heated. If the glaze has a much higher rate of expansion than the body, the glaze will craze. Picture a pot in the kiln at cone 10. The glaze is a very thick liquid (like melted taffy) spread over the surface of the clay. As the pot cools, the clay and glaze contract. But if the glaze wants to contract more than the clay, it builds up stress until it cracks. Usually a glaze will craze before it comes out of the kiln, but it can take a week or so to release the stress. Often we'll hear still warm pots pinging as they come out of the kiln--this is the glaze slowly crazing. Crazing can be used intentionally as a decorative technique. In this case we call i crackling. Realize that crazing is not so much a flaw of the glaze than it is of the combination of glaze and body. A glaze that doesn't craze on one clay body may very well craze on another. We usually try to remedy crazing by adding more silica to the glaze. Silica has a low coefficient of expansion, so adding it to the glaze makes it contract less.
| Guru Spotlight |
| Sherril Steele-Carlin |
