e, and
from the earliest time down to our own what the potter could produce
in form or glaze or colour has been largely decided for him by the
clay material at his command. With any good plastic clay which cannot
be fired at the highest temperature, lead glazes have always proved
the most practicable. A similar clay, to which large quantities of
sand are added, may be glazed by the vapours of common salt; and
mixtures rich in felspar, like Chinese or European porcelain, can be
glazed by melting felspathic materials upon them. Naturally those
species of pottery which are the hardest fired are the most
durable--the glazes of hard porcelain are more unchangeable than lead
glazes, and these in their turn than alkaline glazes.

The most important types of glaze are (1) alkaline glazes (e.g.
Egyptian, Syrian, Persian, &c.), the oldest and most uncertain; (2)
lead glazes, the most widespread in use and the best for all ordinary
purposes; (3) felspathic glazes, the glazes of hard-fired porcelains,
generally unsuited to any other material; (4) salt glaze, produced by
vapours of common salt, the special glaze of stonewares. Many
intermediate glazes have been devised to meet special needs, but these
remain the only important groups. Fuller details on this important
subject must be sought in the technical works.

_Colours._--The primitive potters of ancient and modern times have all
striven to decorate their wares with colour. The simplest, and
therefore the earliest, colour decoration was carried out in natural
earths and clays. The clays are so varied in composition that they
fire to every shade of colour from white to grey, cream, buff, red,
brown, or even to a bronze which is almost black. One clay daubed or
painted upon another formed the primitive palette of the potter,
especially before the invention of glaze. When glaze was used these
natural clays were changed in tint, and native earths, other than
clays, containing iron, manganese and cobalt, were gradually
discovered and used. It is also surprising to note that some of the
very earliest glazes were coloured glasses containing copper or iron
(the green, turquoise and yellow glazes of the ancient Egyptians and
Assyrians). Marvellous work was wrought in these few materials, but
the era of the finest pottery-colour dawns with the Persian, Syrian
and Egyptian work that preceded the Crusades. By this time