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.


In the past forty years many clay additives or conditioners have been used in ceramic related industries. Additive A is just one in a series of non-traditional methods of increasing the plastic properties of moist clay. In the past potters used to “age” their moist clay in damp cellars or storage containers for years to achieve the effects that are now possible in a few minutes of clay mixing. Additive A was developed in the1950's to meet the country's needs of increased brick production for housing. At that time brick production throughout the United States was antiquated. Most bricks were fired in periodic kilns that we slow and labor intensive. Due to the increased demand for building bricks many plants automated their production processing and handling equipment for faster production. Today high-speed mechanized processes make bricks. Labor and production costs went down due to mechanization. However, brick loss rates increased due to the wet unfired bricks being formed and moved around the plant by machines. Many bricks were cracked or damaged by the high-speed machines. Due to high loss rates and the relative non-plastic characteristics of brick clays and low grade shale’s there was a need for a modifier to increase the green strength and plasticity of the bricks.

Ferro frit #3110 80  
EPK 20  
Bentonite (optional) 2% helps keep the glaze in suspension 
White Opaque Gloss    
Zicopax plus 15% causes opacity in the glaze
Blue Variation    
Mason stain #6271 Vivid Blue 10% develops color in glaze
Green Variation    
Mason stain #6271 Mint Green 10% develops color in glaze

Adjustments to the Raku Base Clear Gloss Crackle Glaze

Adjusting for Molten Viscosity

When using a glaze always calculate the glaze to a 100% batch with coloring oxides, stains, or suspension agents listed after the 100%. When adjusting a glaze by either adding or subtracting move materials by 5 part increments. When a modification is successful the revised glaze can easily be recalculated to the 100% batch weight.