One of the most common defects is glaze crazing. While not technically a clay body defect it can be corrected through adjusting the clay body or glaze or both. Crazing is a fine network of lines in a fired glaze. When the clay body and glaze reach temperature in the kiln everything fits perfectly. At this point think of the glaze as a “honey” like viscosity on the rigid underlying clay body.

Upon cooling the glaze contracts more than the clay body. The glaze is now under tension. Glazes are most stable when under slight compressive loads. Some glazes can craze while cooling in the kiln or immediately upon opening the kiln. The primary craze lines can be joined by delayed crazing or secondary crazing days or months later. In some instances a glaze might look intact and then craze when in contact with moisture.

One of the many methods to correct crazing is the addition of low expansion material such as flint to the glaze. The addition of flint will bring the glaze fit into a slight compression. Additions of 5% to 10% flint to the glaze can correct crazing if the craze lines are further than ½”apart. Other corrections for the glaze depend on substituting low expansion oxides in the glaze formula for the relatively high expansion oxides that are presently in the crazed glaze. One question to ask is, do all or most of my glazes craze? If yes, the correction might be more readily found by adding 5% to 10% flint to the clay body. In this case the flint or silica remains as a crystalline solid within the clay body and not as a glass as when silica is added to the glaze, the end result being a compatible clay/glaze fit. Many glaze crazing defects can be traced to an immature clay body or a clay body that is over fired, both situations translate into the clay body and glaze not fitting correctly. An often-overlooked situation occurs when the potter fires the kiln to the correct temperature but the actual firing time to temperature is short. Ceramic materials need time to temperature as well as absolute end point temperatures to vitrify and mature.