as a mass of fused Flint Glass: it will detect the presence
of metallic colouring matter, especially iron, although the most
carefully conducted analysis may fail in discovering the slightest
trace of it.
Mr. Josiah Wedgwood found that
part of gold would give a rose-coloured tint to
Flint Glass. With the best possible recipe, and the purest materials,
good results depend upon an intense and continuous fusion: too little
caloric will fail to refine it, and drive off air-bubbles, and the
colouring matter of the manganese; and too long continuance of intense
heat will destroy the manganese, cause the Glass to attack the pot,
and become striated, gelatinous, and greenish. Extra time, at a lower
rate of temperature, will not make up for want of continuous intensity.
most intense heat can scarcely be considered too great; but the moment
the metal is fully fused, and refined by continuous rapid fusion, the
high temperature of the furnace should be reduced from its maximum
heat to a working temperature: this period being considered the crisis.
Achromatic plate should be made while the metal is in this quiescent
state: it is then most free from striæ; but, should intense heat be
subsequently renewed, it tends to reproduce them. There is, therefore,
a period of comparative perfection and purity, at which, if Flint Glass
be not worked for optic use, the opportunity is for ever lost.
When Flint Glass is kept in fusion beyond
the crisis, it not only assumes a greenish tint, by acting upon
the iron of the Stourbridge clay pot, but takes up a small portion of its
alumina; which, by its inferior density, rises to the surface, frequently
with detached portions of the pot, causing striæ, and other
impurities, which render it entirely unfit for optical purposes.