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

8 ENAMELS AND ENAMELLING.
4. Opaque glasses : soda
or tin oxide.
-lime glass containing bone ash
5. Opaque coloured glasses : soda-lime glass (frequently
also mixed with lead glass) containing bone ash or tin oxide,
and coloured with metallic oxides.
Classes 4 and 5 are the kinds with which we have most
concern, those of class 4 forming the white enamels, whilst
all the coloured enamels are comprised in class 5.
The most prominent of the general properties of glass
is its indifference towards chemicals, and it is precisely this
quality which is of such importance in the industrial employment
of enamels.
With the exception of hydrofluoric acid and the caustic
alkalies, no chemical reagent has any apparent influence on
properly constituted glass unless the exposure be prolonged
for years: consequently, it may rightly be said that glass is
insusceptible to chemical influences, apart
from the bodies
just referred to. This property of indifference is, however,
only met with in perfection in the potash-lime glasses, those
containing lead having an inferior capacity of resistance ;
and
enamels are again inferior to the latter, because usually less
attention is bestowed on their possession of this quality than
on their ready fusibility.
In connection with enamels, however, one should bear in
mind the purposes for which they are intended. Thus, when
it is merely a question of embellishing an artistic or decorative
article, then durability becomes a relatively unimportant
matter, since such articles are never exposed to the full
influence of the weather ; consequently the composition of
the enamel need not be specially designed with a view to
ensuring durability or resisting power.
On the other hand, the conditions are different for enamels
destined to protect the surface of metals from the action of
chemicals, beauty of colour being in such event a minor con-