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Devitrification, which is caused by
excessively slow cooling, occurs more frequently in the simple than the
compound Glasses, and destroys the crystalline properties of Glass.
Its appearance in crown or common bottle Glass is that of a multiplicity
of small petrified star-fish, or rays diverging from different centres,
in little groups resembling an opaque or partially opaque whitish clay,
or Reaumur's porcelain occasionally, surrounded by or encrusted within
the transparent Glass; when it exists in great masses, prior to its
becoming quite opaque, it has a beautifully variegated effect, like a
precious stone. It is the bane of the Glassmaker, and occurs when, from
a state of fusion, the Glass is allowed to cool too slowly. Sir James
Hall, in investigating the cause of devitrification, gave a piece of
green Glass this opaque character by very slow cooling; but the Glass,
in re-fusion, required a very intense heat to cause it to resume its
transparency. Whatever may be the cause, Glassmakers will know the
remedy; and when, from certain tendencies, this annoyance is likely to
occur, if there be thrown into the fluid Glass a quantity of common
clay or silicate of alumina, it will soon become fused in the mass,
and prevent the evil. Partial devitrification
materials, of sand, alkali, or lead, for what is termed
an overtaker,—that he needs
no additional manganese to neutralize the greenish tint. Dr. Faraday
observes upon heavy Glass, which in one of his experiments had Flint
Glass mixed with it—"The Glass, when finished and cold, was of
a deep purple colour; this was immediately referred to the manganese
in the Flint Glass, a supposition proved by repeating the experiment
with other Flint Glass, and then with Flint Glass of our own manufacture,
in which no manganese was used; the latter Glass gave no purple colour;
the former, a colour as deep as the first Flint Glass."
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