Enamels and enamelling; an introduction to the preparation and ...

RAW MATERIALS FOR THE MANUFACTURE OF ENAMELS. 17
Quartz sand resulting from the decomposition of granite
or gneiss, mostly contains small quantities of the minerals
associated with quartz in these rocks. These minerals,
however, are felspar and mica, the former in the state of
(generally white) crystals, whilst the latter is generally in the
condition of very tine crystalline flakes, green or black in
colour. The presence of these impurities in quartz sand
greatly reduces the value of the latter for our purpose ; since,
on the one hand, they considerably diminish the fusibility of
the glass, and, on the other hand, mica [imparts a strong
coloration to the product. As it is a difficult, and on a large
scale impracticable, operation to purify the quartz sand from
these admixtures, such sands must be discarded when the
production of fine glass is in question, though they may be
advantageously employed for making commoner qualities.
In many places fine-grained sandstones are met with,
consisting almost entirely of pure quartz sand, the individual
grains of which are cemented together by a relatively small
amount of lime. When treated in a suitable manner to get
rid of this lime, sandstones of this class form a very useful
source of silica for glass- or enamel-making.
An excellent material for enamels and tine glass is afforded
by the so-called infusorial or diatomaceous earth (Kieselguhr)
which occurs as a fine, nearly pure white mass, so soft as to
be easily reduced to an almost impalpable powder by rubbing^
between the fingers. Microscopic examination shows this
wrongly named " infusorial earth " to consist of the siliceous
integument of diatoms, polythalamia and other lower or-
ganisms, the organic constituents of which have entirely dis-
appeared, leaving behind only the outer shells of nearly pure
silica.
For the purposes of the glass-maker, however, diatomaceous
earth is a somewhat troublesome material to deal with in
working on a large scale, being so bulky as to give rise to
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