are any flaws in the surface quality of a Glaze, its physical structure, or its interaction with the claybody
Claybody/Glaze Interaction Problems
Many Glaze defects are the result of a problem in the way that the clay and glaze interact. The most common such problem is when the clay and the glaze do not physically fit each other.
Crazing is a condition when the glaze is compromized by a spiderweb-like network of tiny 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 harbor bacteria. Some glazes are intentionally formulated to cause this flaw for its aesthetic qualitys, especially Raku glazes. When glaze is intended to craze it is usually refered to as a crackle glaze.
Crazing is caused by uneven rates of shrinkage during cooling between the glaze and the clay. When the glaze shrinks more than the clay it is stressed by being streched over the surface of the clay, if the difference in shrinkage is significant enough then crazing will occur.
Shivering is a defect that results in tiny slivers of glaze seperating from the clay 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 seperate from the pot. The stresses that cause shivering are the opposite of those that cause crazing.
Dunting is an extreme form of clay/glaze mismatch that results in cracks that run not only through the glaze, but the clay as well; these cracks often completely destroy the pot. This defect is notorious for occuring days or even weeks after the pot is fired and sometimes it is only triggered by exposure to heat (this should not be confused with thermal shock
; in this case the temprature change is revealing an existing flaw and would not cause damage to a pot with a good glaze fit).
Dunting is usually caused by the clay shrinking significantly more than the glaze and therefore is an extreme form of shivering, but it could also be caused by the forces that cause crazing if they were severe enough. This kind of serious glaze fit problem is often caused by the addition of excessive amounts of sand to a clay body. an addition of more than 10% sand can effect the shrinkage rate of a claybody and increase the risk of dunting.