Glaze Defects

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Glaze Defects are any flaws in the surface quality of a glaze, its physical structure, or its interaction with the clay body.

[edit] Clay body/Glaze Interaction Problems

Many Glaze defects are the result of a problem in the way that the clay and glaze interact. The most common problem is when the clay body and the glaze do not physically fit each other.

[edit] Crazing

Crazing appears in the glazed surface of fired ware as a network of fine hairline cracks.. This can be seen as an aesthetic flaw, and in the case of functional pottery it is a sanitary concern because these cracks, although microscopic in width, can potentially harbour bacteria. Some glazes are intentionally formulated to cause this flaw for its aesthetic qualities, these are known as crackle glazes. Such effects are popular for glazes on Raku.

Crazing is caused by the glaze being under too much tension. The tension occurs when the glaze contracts more than the body during cooling. The different thermal expansion or shrinkage properties of the clay body and the glaze will determine if the glaze crazes.

[edit] Shivering

Shivering is a defect that results in tiny slivers of glaze separating from the body. Often this is only noticed in a pot several days after it has been fired when it has been sitting in one place long enough for a noticeable amount of glaze fragments to accumulate on the surface beneath it.

Shivering is caused by the claybody shrinking more during cooling than the glaze. If this stress is great enough it will cause pieces of glaze to crack and separate from the pot. The stresses that cause shivering are the opposite of those that cause crazing.

Faults arising from a mismatch of the respective thermal expansions of the body and glaze may be evident immediately from the kiln or otherwise may only occur days or even weeks after firing. In extreme case the fault may run not only through the glaze, but the body as well; these cracks often completely destroy the pot. Such a condition is occasionally referred to as dunting, however this would be incorrect as the term is specific to a crack that results from thermal shock