craze

A craze is a network of fine cracks that develops in the glaze layer of ceramic ware, typically appearing as a spiderweb pattern across the surface. Unlike a single fracture, crazing affects only the glaze itself, not the clay body underneath, though severe cases can eventually propagate deeper. The cracks are usually hairline thin and may be visible only in certain light, but they compromise the waterproofing of the piece and make it unsuitable for functional ware that will hold liquids.

Crazing occurs when the glaze and clay body experience differential cooling rates or expansion coefficients that cause the glaze to contract more than the underlying clay after firing. If the glaze shrinks faster or more than the body, tensile stress builds up in the glaze layer until it exceeds the material's strength. The stress releases suddenly in a burst of cracks radiating from weak points in the glaze. This is why rapid cooling or thermal shock, exposure to sudden temperature changes during or after firing, is a common trigger.

The root cause is a mismatch in the coefficient of linear thermal expansion (CTE) between glaze and clay body. A well-formulated glaze should have a CTE slightly lower than the clay body, so that upon cooling, the glaze is placed under slight compression rather than tension. Conversely, if the glaze has a much higher CTE than the body, it will pull away as it cools and craze. Potter control variables include glaze composition (alkali content and silica ratio), firing temperature, cooling rate, and clay body formulation. Industrial producers test for crazing resistance by firing test tiles and sometimes subjecting them to thermal cycling or boiling water exposure to accelerate craze formation.

Crazing is distinct from dunting, which is a crack through both glaze and body caused by sudden cooling, and from crawling, in which glaze beads up and leaves bare spots. In tableware production, crazing is a critical defect that triggers rejection of entire kilnloads if it appears in a high percentage of pieces. Earthenware is more prone to crazing than stoneware because of higher porosity and lower firing temperatures. Functional ware manufacturers often apply glaze twice or use high-temperature glazes to minimize the risk.

The term likely derives from the visual appearance of the craze pattern itself, resembling a surface crazed or strained to breaking point. In industrial ceramics, crazing is measured and reported as frequency (number of cracks per unit area) and sometimes photographically documented for quality control records. Understanding glaze fit, the relationship between glaze and body expansion, is fundamental to ceramic engineering and remains one of the most common sources of production loss in commercial pottery.