substances which were often coloured. It is possible that the
black varnish of Greek vases was obtained by such a method, and though
that point is not settled, we have many types of primitive pottery,
both modern and ancient, which are coated in this way. Such a coating
is only a substitute for glaze in the work of peoples who do not know
or have not mastered the technical secrets of true glazes. We can only
consider as glazes those definite superficial layers of molten
material which have been fired on the clay substance. Glazes are as
varied as the various kinds of pottery, and it must never be forgotten
that each kind of pottery is at its best with its appropriate glaze.
The earliest known glazes (Egyptian and Assyrian) were silicates of
soda and lime containing very little alumina and no lead. Such glazes
are very uncertain in use, and can only be applied to pottery
unusually rich in silica (i.e. deficient in clay). Consequently these
alkaline glazes cannot be used on ordinary clay wares, and when they
have been used successfully, the clay has always been coated with a
surface layer of highly siliceous substance (e.g. the so-called
Persian, Rhodian, Syrian and Egyptian pottery of the early middle
ages). The fact that glazes containing lead-oxide would adhere to
ordinary pottery when alkaline glazes would not was discovered at a
very early period; for lead glazes were extensively used in Egypt and
the nearer East in Ptolemaic times, and it is significant that, though
the Romans made singularly little use of glazes of any kind, the
pottery that succeeded theirs, either in western Europe or in the
Byzantine empire, was generally covered with glazes rich in lead.
Throughout Europe, and over the greater part of the world, leaded
glazes have been continuously used and improved for all ordinary
pottery, and it is only with certain special hard-fired types of ware
that they have yet been successfully replaced. Chinese porcelain and
all the European porcelains made by analogous methods are fired at so
high a temperature that a glaze by felspar softened by lime and silica
is found most suitable for them, and the hard-fired stonewares, rich
in silica, are often glazed with a salt glaze, or a melted earth rich
in oxide of iron.

Every kind of potter's clay (the mixture of clay, sand, flint, &c.,
from which the potter shapes his wares) has its own type of glaz