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

74 ENAMELS AND ENAMELLING.
advantage as a decolorising agent. It must, however, beremarked
that many colours cannot be developed in plumbi-
ferous enamels ;
minium must not be employed.
and hence when such colours are in question
Manganese dioxide, as already seen, itself acts as a very
powerful pigment towards glass and enamel, provided no
reducing agents (carbon or ferrous oxide) are present, as these
destroy its colouring power by degrading it to a lower stage
of oxidation. Hence by adding to an enamel containing the
above bodies a sufficient amount of manganese dioxide to
convert them into carbon dioxide and ferric oxide respectively,
the enamel can be decolorised, or its colour reduced to a faint
yellowish tinge.
The greatest care is, however, necessary in the selection of
the manganese dioxide for this purpose, since many of the
commercial qualities contain as much as 30 and even 40 per
cent, of ferric oxide, and their use would result in the enamel
being turned brownish-yellow instead of decolorised. It is
essential therefore to take only the highest grades of com-
mercial manganese dioxide, i.e., such as are at least 90 to 95
per cent, pure ;
or better still the chemically pure oxide, since
the amount required is so small that price is a matter of no
importance.
No definite figures can be given as to the quantity of
decolorising materials to be used, as this depends of course on
how much of the substance (or substances)
to be oxidised is
present. The best, and, to the practical man, most convenient,
means of ascertaining the amount required in each case is by
fusion tests kor trial smeltings, small samples of the enamel
mass being fused and then compared with other samples of
the same, fused with known quantities of a decolorising agent.
In this way the object in view the perfect decoloration of the
enamel will be rapidly and assuredly attained.