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.

Gerstley Borate has been a valuable glaze constituent for many years and is a key part of many interesting and successful glazes over a range of firing conditions.As the supply has dwindled due to closing of the supply source, the Hammill & Gillepsie technical staff went to the lab and began experimenting. Beginning with x-ray diffraction to determine the mineral constitution of Gerstley Borate and following up with other physical and chemical testing, we fully characterized the material and identified three key characteristics:  extended particle size distribution including a colloid fraction, a specific and unique distribution of oxides, and a fired property that promotes a variegated mottled texture in many glaze compositions.

As with crazing glaze shivering is also a clay body glaze non-fit event upon cooling. Shivering is most likely to be observed on the edges or ridges of ceramic forms. If one or two pieces in a kiln load show this defect it is always a good practice to hit lightly with a metal object (screwdriver) the remaining pieces to see if shivering can be induced. Some ceramic pieces might look stable but shiver at a later date.