Humans have made objects and vessels from clay for at least 10,000 years and have been applying a form of glaze for at least 7,000 years, beginning in Egypt. Potter's wheels have been around for 6,000 years or more. About 2,700 years ago, the Chinese had created true high fire glaze. Crazing occurs when the glaze on a pot cracks. Most crazing is a pattern of very fine cracks spread evenly across the piece.
In a finished piece of pottery, the glaze is the coating of the pot. It's made of impermeable glass formed by minerals and metals in the final firing, where temperatures can reach 1260 degrees Celsius. The first glazes were discovered by accident, when chemicals found in wood ash melted onto and stuck to the clay body during firing. Glaze makes pottery waterproof. It strengthens and decorates the clay with many different colours and effects.
The Cause of Crazing
Because the clay and glaze of a piece have different chemical compositions, they respond differently when heated in the fire of a kiln. Both clay and glaze will expand under heat, but they do so at different rates. They also contract when cooled, again, at different rates. If the rates of contraction are too different, and the glaze contracts more than the clay, the glaze will crack under the tension.
Most crazing occurs after the final firing of a piece, when the piece has cooled. Some crazing occurs at other times, during thermal stress or if the pottery is left exposed to extremely cold weather.
Crazing is not the effect the potter originally wanted. Unless otherwise intended, glaze should be perfectly smooth, showing the texture of the clay underneath. Crazing spoils this effect. Also, while the cracks are often microscopic, they can harbour bacteria, making it impossible to sanitise the surface. If the pieces are used to handle food, they must be destroyed. In antiques, crazing is an indication that the piece has not been well-handled, and it can lower the value of the piece. There is no way to fix crazing once it has occurred.
To prevent crazing in ceramics, the expansion coefficient of the clay and the glaze must be as close as possible. For chemists, this means a glaze composed of 20 per cent silica, in the form of diatomite, is essential. For the rest of this, it means checking the glaze label against the type of clay and ensuring they are both intended for the same type of firing.
There is one type of ceramics where crazing is pursued, and that is raku. Raku was originally invented in Japan for the cups used in the Zen Buddhist tea ceremony. Originally called ima-yaki (now-ware), it later took the name raku from the family for which one of the original artists made tea bowls. Raku emphasises simplicity in form and interest in texture, hence the acceptance of crazing.