Jeff Zamek Ceramics Consulting Services

Glaze Defects - Causes and Corrections

As potters we have all been faced with a clay body defect either in the forming, drying, or firing stages when working with clay.  Sometimes it is difficult to determine if the clay body or the glaze caused the defect. While there are some types of defects that are unquestionably clay body related and some types of defects that are attributable to glazes, sometimes the defect is caused by the interaction between kiln firing/clay body and glaze. Clay and glaze defects can also be caused by any number of other miscalculations which can range from, choosing the wrong clay body or glaze formula for the firing temperature, inferior clay forming techniques and a general lack of knowledge on how ceramic materials react in their construction and firing stages. There are many possible clay body defects along with compound defects, which result in more than one type of defect on a pot or ceramic sculpture.

Traditionally, potters operating at stoneware temperatures fired pottery to cone 9 (2300 0 F.)  resulting in a dense hard vitrified clay body and glaze. One characteristic of such high temperature firings is the well developed interface where the clay body ends and the glaze layer begins. The clay and glaze surfaces fuse together in a seamless transition looking and feeling like one entity. Such integration of ceramic materials is often not the case with earthenware or low temperature firing of clay and glazes. Often the glaze layer looks “painted” on the underlying clay body surface.

Pottery tools have been around since the first potter picked up clay and decided a smooth rock or wooden twig would be a useful implement in the forming or finishing process. One tool’s function has remained largely unchanged since it was first used to form a pot. The potter’s rib, when brought against the moist clay, can impart its straight or curved profile and transform the pliable clay. Any potter would recognize rib tools brought up from an archeological dig.