Glaze defects

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

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.

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 in cases where the glaze material has a higher coefficient of thermal expansion than the body.The different thermal expansion or shrinkage properties of the clay body and the glaze will determine if the glaze crazes.

hivering

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

Glaze Surface Defects

Pinholes

Pinholes are pinhead sized areas where the glaze has receded and left bare clay exposed. When these form they will generally occur in large numbers all over the glazed surface of a pot but they can be present in smaller numbers and isolated areas. The most common cause of pinholes is the release of gases from the claybody during firing. This outgasing causes small bubbles to form in the glaze that will then leave a bare area when they burst if the surface tension of the glaze is sufficient.

This defect occurs more commonly in low fired ceramics than stoneware. For this reason it is a common practice, when a pot will be glazed with low fire glaze, to fire the pot one or two cones higher when bisqueing than it will be fired with glaze. For instance: a pot intended to be glazed with cone 06 glaze might be bisque fired to cone 04. By burning off all of the gasses during the bisque this technique prevents them from disturbing the glaze surface during the glaze firing. This technique is impractical for stoneware however, because bisque firing to a stoneware temperature would vitrify the clay and make it quite difficult to apply glaze.

Crawling

Crawling is a glaze defect characterized by glaze beading up and leaving large, irregular areas of bare clay on the surface of a pot. This defect is caused by an error in glaze formulation that gives the glaze high enough surface tension to pull away from the surface of the clay.

Crawling is also thought to be caused when a dirty pot is glazed without being cleaned first. If a pot has built up a significant level of dust (which is a common occurrence in many potter's studios) and glaze is applied over that layer of dust it can cause problems during the firing. When the dust combusts during the firing, gas can build up under a viscous glaze and cause it to crawl away from the surface of the clay.

It should be noted that crawling is not always considered a defect. There are certain glazes, such as Shino, that are intentionally formulated to crawl in a more predictable and regular fashion as a form of surface decoration.

External links

* [http://sankey.ws/glazeexpansion.html A study of Glaze expansion]