Glaze: Encyclopedia II - Glaze - Pottery glazes

Glaze - Pottery glazes

In pottery, glazing is the process of coating the piece with a thin layer of a glassy material. After applying a glaze, the pottery is fired, and the powdered coating melts into a glass-like coating.

Glazing is functionally important for earthenware vessels, which would otherwise be unsuitable for holding liquids due to porosity. Glaze is also used on functional and decorative ware made of stoneware and porcelain. In addition to the functional aspect of glazes, aesthetic concerns include a smooth pleasing surface, the degree of gloss and finished color. Glazes may also enhance an underlying inscribed, carved or painted design.

Glazes are most often a mix of dolomite, frit, silica/flint, feldspar, sodium borate, clay and whiting plus metal oxides or carbonates. Although a dry glaze mix can be useful, the chemicals are usually mixed with water, with the addition of a material such as bentonite to keep the mixture in suspension. Glaze recipes are carefully formulated to melt at appropriate temperatures and produced a surface with desired characteristics.

Glaze may be applied by dusting a dry mixture over the clay, or by dipping the piece in the slurry of glaze and water. Liquid glaze can also be applied by splashing or with a brush. Brushing tends not to give an even covering, but can be effective with a second coating of a coloured glaze as a decorative technique.

With all glazed items, a small part of the item (usually on the base of the piece) must be left unglazed, else it will stick to the kiln during firing. To prevent glazed and stacked pieces from sticking together, kiln spurs were invented. Early pottery, such as European faïences until the early 18th century, often has clearly visible marks from such spurs.

Decoration applied under the glaze on pottery, is generally referred to as underglaze. Pigments are applied to the surface of the pottery piece, either in a raw or green state or having been bisque fired. The wet glaze is applied over the decoration. The pigment fuses with the glaze, and appears to be under a layer of clear glaze. An example of underglaze decoration is the well known "blue and white" porcelain produced in China and Japan. The striking blue colour is achieved by using a cobalt pigment, fired at the correct temperature.

Decoration applied on top of a layer of glaze, is referred to as overglaze. Overglaze methods include applying one or more layers or coats of glaze on a piece of pottery or by applying a non-glaze substance such as enamel or metals over a glaze.

Adapted from the Wikipedia article "Pottery glazes", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki