Glaze Defects


Crackle or fractures in the glaze.
The glaze is shrinking more than the clay. Glaze too thick. Glaze cooled too quick. Increase body expansion; fire hotter, soak body or increase silica in clay. Apply glaze thinner. Avoid fast cooling
Glaze completely separates from the clay.
Glaze too thick. Clay shrinkage is greater than the glaze. Glaze may have been applied to a dusty bisque surface. Thinner glaze application. Change or adjust clay body for a better glaze fit. Rinse bisque with water before glazing.
Small holes in the glaze surface.
Fired too rapidly. Air escaping from the bisque ware. Glaze is too viscous. Slower firing. Bisque fire one cone higher and dampen before glazing. Increase flux and decrease clay content 2-5%
Glaze actually runs down the sides of the ware and often onto the kiln shelves.
Over fired. Insufficient kaolin. Glaze too thick. Prevent over firing. Increase kaolin 2-5%. Thinner glaze application.
Bubbles appear in the glaze or on the surface.
Gases escaping from the clay during glaze firing Gases escaping from the glaze during firing. Trapped air in the clay body’s surface pores. Bisque fire a cone or two higher. Once maturing temperature is reached, soak kiln for 30 minutes to smooth surface. Moisten bisque before glazing to release air.
Glaze separates into clumps leaving bare areas of clay showing.
Glaze too thick. Bisque is dusty or greasy, Lack of adhesiveness of the glaze. Thinner glaze application. Rinse bisque with water to avoid contamination. Use gum in amounts of 1 teaspoon per pint of glaze.
Gas pockets trapped in the clay body causing blisters and swelling.
Over firing. Trapped air due to improper wedging or processing of clay. Carbon trap. Fire to a lower temperature. Improve wedging or clay mixing. Slower firing time.
The clay separates forming a crack. Can occur before or after firing.
Uneven or forced drying of ware. Firing damp ware. Fast cooling. Ware should be dried evenly and completely before firing. Slower cooling time. More uniform wall thickness.

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