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

IV.
SUBSTANCES ADDED TO PRODUCE OPACITY.
The materials described in the previous section are those
forming the true glass substance. Other bodies have also
been proposed, as zinc oxide, by whose aid glasses suitable
for certain purposes can be prepared, though it is not advisable
to resort to such bodies in compounding enamel masses.
Enamels always contain certain infusible white substances
which render the glass opaque ;
and for this purpose, as already
mentioned, use is generally made of tin oxide or bone ash.
In view of the great influence exerted by these two materials
on the character of the enamel, a fuller description of them
must now be given.
TIN OXIDE (STANNIC OXIDE).
Tin oxide (SnO 2 ), when chemically pure, is a heavy powder
of a pure white colour, and by itself is totally infusible. When
suspended in a finely divided condition in, and enveloped by,
a fused colourless mass (glass) it renders the latter translucent
or altogether opaque, according to the proportion in which it
is used.
In commerce this oxide is met with as so-called " tin ash " ;
but as this is never quite pure and always contains lead oxide,
frequently associated with ferric oxide and small quantities
of cupric oxide, it is advisable to prepare one's own tin oxide
either by the wet or dry process.
In the wet process, fine tin is first granulated by melting
the metal and pouring it into cold water. This is then