Glaze

• Glaze is used on pottery and porcelain to give it a waterproof finish.

• Glaze may be transparent, opaque or coloured. 

• The main glazes were lead, tin and salt.

• Glaze is a form of glass, consisting basically of glass-forming minerals (silica or boron) combined with stiffeners (such as clay and fluxes) and melting agents (such as lead or soda). 

• In raw form, glaze can be applied either to the unfired pot or after an initial unglazed, or biscuit, firing. The pot is then glaze fired; the glaze ingredients must melt and become glasslike at a temperature that is compatible to that required for the clay. 

For glazing, two main materials were used:

Lead: in the form of Lead Sulphide, Galena, was found in Derbyshire and may have been brought from Cumberland and North Wales as well, but Derbyshire was the main source. 
Lead glazes are transparent, with traditional types made of sand fused with sulphide or lead oxide. They were used on earthenware by Roman, Chinese, and medieval European potters and are still employed on European earthenware. 

Salt: from Cheshire, where at Marbury, near Northwich, salt in the form of brine was mined. Salt glazing was practiced for about 100 years in Staffordshire, so this close source of supply was an important factor. 

Tin glazes, opaque and white, were introduced by medieval Islamic potters and were used for Spanish lustreware, Italian maiolica, and European faience and delftware. Eventually the Chinese and Japanese made such glazes for the European market.

Metal oxides give colour to glazes (see also the decorative over glaze technique of enameling). Copper will make a lead glaze turn green and an alkaline glaze turquoise; a reduction kiln will cause the copper to turn red. Iron can produce yellow, brown, grey-green, blue, or, with certain minerals, red. 

Feldspars (natural rocks of aluminosilicates) are used in stoneware and porcelain glazes because they fuse only at high temperatures. The effects of specific glazes on certain clay bodies depend both on the composition of each and on the potter's control of the glaze kiln.

 Porcelain fired without a glaze, called biscuit porcelain, was introduced in Europe in the 18^th century. It was generally used for figures. In the 19^th century biscuit porcelain was called Parian ware. Some soft-paste porcelains, which remain somewhat porous, require a glaze. After the body has been fired, the glaze, usually containing lead, was added and fired to vitrify it. Unlike feldspathic glaze, it adheres as a relatively thick coating.

Painted decoration on porcelain is usually executed over the fired glaze. Because painting under the glaze-that is, on a fired, unglazed body-must be fired at the same high temperature as body and glaze, many colours would "fire away." Thus underglaze painting on porcelain is largely limited to the extremely stable and reliable cobalt blue found on Chinese blue-and-white wares. Most porcelain colours - called overglaze, enamel, or low-temperature colours - are painted over the fired glaze and fired at a much lower temperature.