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

BAW MATERIALS FOR THE MANUFACTURE OF ENAMELS. 15
Consequently, the materials used for making glass must
contain one or other of the above-mentioned compounds.
The matrix of enamel must also contain a very finely
divided infusible white substance, generally tin oxide or bone
ash, to render the glass opaque :
and, as one or other of these
is an essential constituent of all enamels, they
must also be
reckoned among the fundamental materials. As the success
of the resulting product depends on the suitability of these
materials, a more precise account of their properties and
method of preparation will now be given.
Silica.
This substance is one of the most extensively distributed
bodies met with in our planet, being not only a main con-
stituent of the rocks (e.g., granite and gneiss) forming the
highest and largest mountains in the world, but also occur-
ring in a pure state in numerous deposits.
The minerals, rock crystal, quartz and quartz sand,
as well as flint, consist of almost perfectly pure silica ; as
do also the minerals known under the names of amethyst
(violet), citrin (yellow), smoky topaz and morion (smoke brown
to black), the only admixture being the minute proportion of
extraneous matter producing the particular colour in each
case.
From the chemical point of view, silica is composed of silicon
{Si) and oxygen (O 2 ) = SiO 2 . There are two allotropic modifica-
tions of this body : the crystalline (found almost pure, as rock
-crystal and quartz), and the amorphous form -e.g., flint, hyalite
and opal.
Silica belongs to the class of bodies that are infusible when
heated alone. It is inert towards other substances, and is
unattacked by the strongest acids ;
in fact there are only two
bodies capable of acting upon it at all, viz., hydrofluoric acid
and the alkalies. On boiling quartz in a solution of caustic