Glaze is a layer or coating of a vitreous substance which has been fired to fuse to a ceramic object to color, decorate, strengthen or waterproof it.
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 either raw, "greenware", or "bisque" fired (an initial firing of some articles before the glazing and re-firing). A wet glaze—usually transparent—is applied over the decoration. The pigment fuses with the glaze, and appears to be underneath a layer of clear glaze. An example of underglaze decoration is the well-known "Blue and white porcelain" porcelain famously produced in England, The Netherlands, China and Japan. The striking blue color 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 (i.e., gold leaf) over the glaze.
Overglaze colors are low-temperature glazes that give ceramics a more decorative, glassy look. A piece is fired first, overglaze is applied, and it is fired again. Once the piece is fired and comes out of the kiln, its texture becomes smoother because of the glaze.
During the Kofun period of Japan, high-fired, hard-bodied sue ware was decorated with greenish natural ash glazes. From 552 AD to 794 AD, differently colored glazes were introduced. The three colored glazes of the Tang Dynasty were frequently used for a period, but were gradually phased out; the precise colors and compositions of the glazes have not been recovered. Natural ash glaze, however, was commonly used throughout the country.
From between the eighth century, the use of glazed ceramics was prevalent in Islamic art and Islamic pottery, usually assuming the form of elaborate pottery. Tin-opacified glazing was one of the earliest new technologies developed by the Islamic potters. The first Islamic opaque glazes can be found as blue-painted ware in Basra, dating to around the 8th century. Another significant contribution was the development of stoneware, originating from 9th century Iraq. Other centers for innovative ceramic pottery in the Islamic world included Fustat (from 975 to 1075), Damascus (from 1100 to around 1600) and Tabriz (from 1470 to 1550).
In the thirteenth century, flower designs were painted with red, blue, green, yellow, and black overglazes. Overglazes became very popular because of the polished look they gave ceramics.