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

II.
THE COMPOSITION AND PROPERTIES OF GLASS.
In point of chemical composition every glass must be
regarded as a mixture of two or more salts, there being,
really, only one exception to this rule, viz., the so-called
"water glass" which consists of a single salt. Formerly
silica was the only acid body employed to combine with the
basic substances present in glass ; but at the present time
boric acid is also used in some kinds of glass, a matter of some
importance in connection with the object now in view, since
this last-named acid has considerable influence on certain of
the properties with which glass is endowed. One result
of the use of boric acid is that, whereas formerly glass could
be considered as a mixture of silicates alone, this point of
view now requires modification, according to circumstances,
and glasses containing boric acid must be characterised as
borosilicates.
At one time the bases employed in glass-making were few
in number, and consisted solely of potash, soda and lime, so
that the product could be entitled either a silicate of potash
and lime, or a silicate of soda and lime. Nowadays, how-
ever, white glass contains, in addition, zinc oxide and lead
oxide, which are used to impart certain qualities to the product.
The bases just named share with magnesia the property
of forming with silica colourless compounds which can be
melted or fused by exposure to a suitable decree of heat.
Other bodies, such as alumina, are, it is true, also capable of