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

18 k
2 ENAMELS AND ENAMELLING.
antimony
oxide 1
part, alum 1, sal ammoniac (ammonium
chloride) 1, and lead oxide 1 to 3 parts. These ingredients are
finely powdered, intimately mixed by grinding in small
quantities in a mortar, and then slowly heated ; the sal ammoniac
is gradually volatilised in the form of dense white
fumes, and when these have all disappeared the mass is ready
for use and will furnish a yellow colour. It is fired with a
flux of silica, borax and lead oxide, in quantity equal to, or at
most not exceeding double its own weight. All pigments con-
taining antimony or lead oxide require only a small quantity
of frit.
The most handsome violet is obtained from manganese
dioxide, provided care be taken that no organic matter is
present in the mass when fired, and that no reducing gases
obtain access ; otherwise the dioxide will be easily reduced
and give only a brownish colour in place of violet.
When, as frequently happens in enamelling jewellery,
small quantities of enamel have to be melted before the blow-
pipe, the various properties of the enamel masses and the
contained pigments must be sufficiently borne in mind. As
already described, the inner blowpipe flame has a powerful
reducing action, only the outer flame being an oxidising one ;
consequently the latter alone must be used in enamelling,
except in a few rare instances, such, for example, as when a
green (ferrous oxide) enamel or a cuprous
oxide red mass is to
be fired. In such cases the oxidising flame would convert the
ferrous or cuprous oxide into the ferric or cupric state, and
would change the green of the former to brown red and the
red of the cuprous oxide to green.
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