This clay is not fusible by the highest
heat of our furnaces, though the felspar, from the decomposition of
which it is derived, forms a spongy milk-white glass, or enamel, at
a low white heat. But felspar, when decomposed by the percolation of
water, while it forms a constituent of granite, loses the potash,
which is one of its ingredients to the amount of about 15 per cent,
and with it the fusibility that this latter substance imparts.
The siliceous ingredient is calcined flint; and in some of the
porcelain works, (particularly, I believe, those at Worcester,) the
soapstone from the Lizard-point, in Cornwall, is employed. These are
all the avowed materials; but there is little doubt that the alkalies,
or alkaline earths, either pure or in combination, are also used,
in order to dispose the other ingredients to assume that state of
semi-fusion characteristic of porcelain.
(The principal processes are) the grinding and due mixture of the
ingredients, in order to obtain a mass sufficiently plastic; the
forming this mass on the wheel; the subsequent drying of the ware;
the first firing, by which it is brought to the state of biscuit; the
application of the firmer colours occasionally on the surface of the
biscuit; the dipping the biscuit in the glaze; the second firing, by
which the glaze is vitrified; the pencilling in of the more tender
colours on the surface of the glaze; and the third and last firing
that is given to the porcelain.
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