436 A POPULAR TREATISE ON GEMS.
maintained
for any length of time, by which process the alkali contained in the
soluble glass is set free, the silica combines with the lime, and more
particularly with the fluor-spar (fluoride of calcium), so durable a
cement is formed thereby, that it will not admit of the smallest
absorption of moisture, and consequently is absolutely un-attackable by
frost. By applying the chloride of calcium in solution to the cement,
the supposed objection that the salts of soda, or alkali, are
efflorescing by degrees, is hereby obviated, for the chloride of
calcium at once absorbs the alkali.
Soluble
glass may be colored by various metallic oxides, so as to produce, when
heated, very sharp colors, similar to enamels, and may also be employed
for a coating over other paints, such as fresco, &c.
As
a cement for joining together heterogeneous and homogeneous
substances, it is unsurpassed, and when applied, renders the substances
so coated both water and fire proof.
If soluble glass is intended for a varnish, the proper specific gravity is 1-1C5, but for a paint it may be reduced to that of water.
In
France, soluble glass is much used in coating common building-stones,
for the purpose of rendering them damp-proof. Marble buildings and damp
cellars may be made impervious to dampness by varnishing the surface
with soluble glass; although the proper mode is to exhaust the air from
the stone or brick, and then impregnate it with soluble glass by
pressure. A patent was lately taken out in England, for preserving
building, pier, and wharf stones, by first coating them with a wash of
chloride of calcium, and afterwards by the application of the
concentrated solution of soluble glass,—repeating the operation several
times. Soluble glass was introduced into the United States, by the
author of this work, in the year 1831, under
