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

70 ENAMELS AND ENAMELLING.
cobaltous oxide is one of the most valuable pigments available
for the purposes of the enamel industry.
VIOLET PIGMENTS.
Only one single metallic oxide is known to be capable of
producing violet ; but this is one of such common occurrence
in nature as to be available for the manufacture of cheap
enamels .The substance in question is :
Manganese Dioxide.
It is found native in many places, in the form of pyrolusite,
and is won by mining, being in large demand for many
industrial purposes (chlorine manufacture, cloth printing,
dyeing, glass-making, the iron industry, etc.). The natural
oxide (commercially termed "
manganese ") frequently contains
a very high proportion of extraneous substances ferric oxide
in particular and for this reason cannot be used in an un-
prepared state for the production of pure shades of colour.
When this latter condition is essential, as it is in the case
of enamels, chemically pure manganese dioxide must be used.
To obtain this compound use may be made of the solution of
manganese perchloride obtained as a by-product in the manufacture
of chlorine gas, the liquor being boiled in a glass or
.porcelain vessel so long as chlorine gas continues to come off.
This done, the solution contains only rnanganous chloride.
A clear solution of calcium hypochlorite (bleaching powder)
is then added until the deep brown precipitate of pure manganic
hydrate ceases to form, which hydrate, when dried at a gentle
heat, may be used direct as a pigment.
For ordinary purposes the finely powdered native oxide,
which is a very powerful pigment, may be used, and, provided
it be employed in merely small quantities, will produce very
handsome shades of violet, the actual amount of iron present
being
then so minute that it cannot affect the colour.
Whichever material is taken, care is necessary, in fusing