Why has my glaze peeled?

Peeling or "shivering" is the opposite of crazing and so is typically caused by overfiring of the clay. Unfortunately this fault cannot be reversed by refiring. Sometimes bare patches of glaze are mistakenly reported as peeling: In actual fact these are more correctly called "shelling" and are brought about by lack of adhesion between the unfired glaze layer and the underlying biscuit surface. This is sometimes brought about by dust or grease on the surface of the clay at the time of glaze application, or by dry powdery colour/oxide decoration, or by sprayed ware being too far away from the spray gun. It should be treated according to the reason for the fault, i.e. remove dust/grease, use a binder in the colour possibly also with a small amount of glaze mixed in with the colour, move the spray gun closer to the ware. Peeling may also be seen in slip cast ware particularly on rims and handles. Whilst the same reason might well apply to the fault here, it may also be caused by a build up of soluble salts in the casting slip. These salts migrate to the areas of the ware which dry at the fastest rate and become concentrated there. During the firing these salts modify the clay body slightly which causes a mis-match between the clay and glaze. Unfortunately affected ware may not always be salvageable since the affected areas will need to be ground away to remove the surface layer of clay and then re-glazed and refired. This is not always successful. To prevent further occurrences of the fault, the casting slip needs to be treated with barium carbonate, or disposed of and replaced with fresh material, the latter being the preferred option.