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

28 ENAMELS AND ENAMELLING.
alabaster). The carbonate is the usual variety employed in
making glass and enamel.
Limestone.
Ordinary limestone is composed of calcium carbonate,
and almost invariably contains foreign admixtures capable
of exerting an injurious influence. Most limestones contain
small quantities of organic matter, ferrous carbonate, mag-
nesium carbonate and occasionally clay as well.
Organic matter, unless in excessive amount,
is not harm-
ful, the carbon formed therefrom in the process of smelting
being capable of removal by the addition of small quantities
of oxidising subs^nces.
If, however, the proportion of organic
matter is so large as to render the limestone dark grey
or black, the latter is hardly fit for use as a constituent of
enamel, unless previously treated, for which purpose the
limestone may be advantageously burnt in kilns, an operation
resulting in the elimination of both the carbon dioxide and the
organic matter.
More ominous is the presence of ferrous carbonate as
a constituent of limestone, since even small quantities of
this salt will impart a noticeable bottle-green coloration to
enamel. Ferric oxide has a less powerful influence on the
colour than ferrous oxide ; consequently, in testing the
suitability of a limestone for the manufacture of enamel, it is
advisable to make a small trial smelting, with an addition of
some oxidising agent, to convert the ferrous oxide into ferric
oxide, which latter will tinge the glass yellow, but less
powerfully than the ferrous oxide would colour it green.
Furthermore, the suitability of a limestone for our pur-
pose can be gauged by its appearance after prolonged exposure
to the air, a reddish-yellow coloration of the lumps, or the
formation of a number of red veins therein, indicating the
o
presence of iron in large proportion; such a limestone may
be rejected as unsuitable, without any further test.