PORCELAIN CLAY. 75 
steatite (124), or other substances; and the mixture is 
sifted several times through hair sieves. It is afterwards 
moistened with rain water, and, in the form of a paste, 
is put into covered casks. Here a fermentation soon 
takes place, which changes its smell, colour, and con- 
sistence. Its colour passes from white into dark grey: 
and the matter becomes both tougher and more soft 
than before. The peculiar mode of preparing this mix- 
ture, and the art of rightly managing it, are secrets in 
most porcelain manufactories. 
The next operation consists in giving to the paste 
thus formed the requisite shape of the vessels. This 
is done first by kneading it with the hands ; and then 
by taking up certain portions of it, and turning it on a 
lathe, in the manner of common pottery (115), but with 
more care. 
The third operation is the baking or firing. This is 
done in furnaces of a particular construction, and gene- 
rally lasts from thirty-six to forty-eight hours. The 
state of the baking is shown by proof pieces, as they 
are called, which are placed in convenient situations, 
and can be drawn out, from time to time, for examina- 
tion. The porcelain in this state, is named biscuit porce- 
lain ; and figures, and such other porcelain articles as 
are neither to be painted nor exposed to water are in 
the state of biscuit. 
A fourth operation is covering the surface of the bis- 
cuit with a varnish or enamel. This is composed of 
pure white quartz (76), white porcelain, and calcined 
crystals of gypsum (192) ; and sometimes principally 
of felspar (110). These substances are carefully ground, 
then diffused through water, and formed into a paste. 
When used, the paste is diluted in water, so as to give 
it considerable fluidity ; and the pieces of biscuit porce- 
lain are separately plunged into it, in such manner as 
to cover their whole surface. These are then exposed 
to a heat sufficient to melt the enamel or covering : and 
in this state they constitute white porcelain. 
If the porcelain is to be painted, it must again be 
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