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
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
Porcelain fired without a glaze, called biscuit
porcelain, was introduced in Europe in the 18th century. It was
generally used for figures. In the 19th 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.