Ceramic glaze
From Wikipedia, the free encyclopedia
- For other uses, see Glaze.
A glaze is a vitreous coating to a ceramic material whose primary purposes are decoration or protection. Glazes can be considered specialised forms of glass and therefore can be described as amorphous solids.
In pottery, glazing is the process of coating the piece with a thin layer of glassy material, called a glaze. After application, the pottery is fired, and the powdered coating melts into a hard, 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 variegation, and finished color. Glazes may also enhance an underlying design or texture which may be either the "natural" texture of the clay or an inscribed, carved or painted design.
Glaze may be applied by dry dusting a dry mixture over the surface of the clay body. Liquid glazes - suspensions of various powdered minerals, and metal oxides - can be applied by dipping pieces directly into the glaze, pouring the glaze over the piece, spraying it onto the piece with an airbrush or similar tool, with a brush, or with any tool that will achieve the desired effect.
To prevent the glazed article sticking to the kiln during firing either a small part of the item is left unglazed or special refractory supports, kiln spurs, are used as supports which are removed and discarded after the firing. Small marks left by these spurs can sometimes be visible on finished ware.
Decoration applied under the glaze on pottery, is generally referred to as underglaze. Underglazes are applied to the surface of the pottery, which can be in either raw, "greenware", or "bisque" fired, an initial firing of some articles before the glazing an re-firing. A wet glaze - usually transparent - 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 famously produced in England, The Netherlands, China and Japan. The striking blue colour is achieved by using the powerful colorant cobalt in the form of either cobalt oxide or cobalt carbonate, both of which are still commonly used in glaze formulation today.
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 (ie., gold leaf) over the glaze.
[edit] See also
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[edit] Reference
- Hamer, Frank and Janet. The Potter's Dictionary of Materials and Techniques. A & C Black Publishers, Limited, London, England, Third Edition 1991. ISBN 0-8122-3112-0.
