In answer to my question on flint glass
this information here (
http://archive.org/stream/illustratedrecor00shaf/illustratedrecor00shaf_djvu.txt ) states:
[i
]'Mr. Apsley Pellatt, the talented reporter on the Glass Section, informs us, that the
masters of his craft divide glass into simple and compound. The former contains
only silica and flux, this flux being either soda, potash, lime, magnesia, alumina, or
mixtures of some of them : in which case, the glass is simply a silicate of an alkali. To
INTERNATIONAL EXHIBITION. 71
this "simple" glass belong plate, window, and bottle glass of every description.
" Compound glass," besides these simple elements of silica and alkali, contains also
the oxide of a metal ; and it is known among us as flint-glass, and on the continent
as crystal. It is employed for articles of luxury and domestic use. The oxide is
introduced to give more refractive power to the glass, by not allowing the rays to
pass through so freely as in simple glass, and the result is much greater brilliancy in
the metal. This flint-glass, wdiich is employed for achromatic purposes, and for
articles of luxury, requires the utmost attention of the manufacturer, as its quality is
of the highest importance. There is some difficulty in procuring the materials in a
state of perfect purity, and, perhaps, a greater difficulty in regulating the escape of
oxygen, while the elements are in a state of fusion. We are told, that " deoxidation
alone, supposing all the materials to be perfectly pure, will affect the colour of flint-
glass ;" and that "if oxygen be not supplied, the materials, when fused, will produce,
not a white, but a green tinted glass." It is for the purpose of retaining the requisite
amount of oxygen, that the black oxide of manganese is employed in the manufac-
ture of flint, as this substance has a strong affinity for oxygen, parting with it very
slowly, and not until it has escaped from the other ingredients of the metal.
A danger exists, on the other hand, of the glass being injured by the excess of
oxygen, in which case it receives a light-purple tint, and acquires a more frangible
character. '[/i]
my underlining
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