For other uses, see Glaze.
Madonna with Child and Angels, ceramica glaze by Renaissance artist Andrea della Robbia.
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 [1] 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 [2]. 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.
Mug by Michelle Rhodes, with blue underglaze decoration on porcelain.
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.
Over glaze colors are low tempature glazes that give ceramics a more decorative, glassy look. They enrich the surface of ceramics. You should fire your piece first, then put overglaze on it, and then fire it again. Once a piece gets fired and it comes out of the kiln the texture has become smooth because of the glaze.
Earlist vessels date back to 10,000 BC. They were from northern Japanese people who made the vessels a pointed shape and marked with incised rope patterns and are similar to Siberian vessels. During the Tumulus Period, high fired, hard bodied sue ware were decorated with greenish natural ash glazes. From 552 AD to 794 AD different color glazes were introduced. The three colored glazes of the Tang were used a lot for a while but then died out. The actual colors used have not been figured out. Natural ash glaze, however, was commonly used through the country.
In the thirteenth century, flower designs begin being painted with red, blue, green, yellow, and black over glazes. Over glazes became very popular because of the polished look it gave ceramics . People would also use different colors mixed together to give it a more unique look. Another way to have a unique look is to paint most of the piece with under glaze then paint the main part with over glaze. Because the main part is what they want people to notice the most, the over glaze will give it the glossy look to make it stand out more, which under glaze does not give.
Click here for a collection of images showing a wide variety of glaze styles with information on each formulation and method.
