Why is my fired glaze peeling off the pot?
When a glaze shivers it is under extreme compression as it cools, peeling off the underlying clay body. This defect is most prevalent on pottery lips and handles, which are the areas of highest surface tension as the glaze hardens upon cooling.
Shivering glaze chips can range from 1/16” to 2” (.15 cm to 5 cm)in size often looking like a paint chip coming off the fired clay body. Importantly, if an individual glaze has shivered, the entire kiln load of the same clay body and glaze can be in question even though there are no apparent indications of defect in the other pots that might fail in the future. Many times hitting the edges of the pot with a metal tool can induce shivering in such highly stressed compromised glazes. Glaze shivering can also occur when a high iron content clay body is over reduced in the kiln firing. The result being a layer of carbon interfering with the covering glaze that flakes off when cooled.
If only one glaze formula has shivered among several, the corrections should take place with the defective glaze. If more than one glaze formula has shivered, an adjustment to the clay body is required.
In some instances, as little as a 2% to 4% increase of feldspar or other high expansion materials in the glaze formula will bring the glaze into a stable configuration without altering the surface texture, color and opacity of the glaze.