GREEN COBALT PIGMENTS.
From mixtures of chromium oxide, cobalt carbonate and alumina in
different proportions a series of pigments can be obtained varying
in shade between pale blue and bluish-green. On account of the great
stability of these colours at the highest temperatures they are of
great importance in porcelain painting, for which they are principally
used. It is necessary that the porcelain should be free from iron,
since ferric oxide forms a black compound with chromium oxide, very
small quantities of which would be sufficient to considerably damage
the fineness of the colour.
=Cobalt Green=, which is also known as Rinmann’s green or zinc green,
is a compound of the oxides of cobalt and zinc. It is always produced
when a cobalt compound is ignited with zinc oxide. Rinmann’s green
has not so deep a colour as the poisonous emerald green, but it is
distinguished by extraordinary durability, and deserves to be more
largely used than it is at present. This pigment is most simply made by
precipitating the mixed solutions of a zinc salt and a cobalt salt. It
is also formed by moistening pure zinc oxide with a cobalt solution and
igniting.
In precipitating the mixed solutions of zinc and cobalt salts products
of different shades are obtained, according to the proportions in
which the salts are used. If equivalent quantities are used, an almost
black product is obtained, quite useless as an artists’ colour. The
best result is obtained by mixing intimately pure precipitated cobalt
carbonate with zinc oxide and igniting the mixture; a mixture of 9 to
10 parts of zinc oxide with 1 to 1·5 part of cobalt carbonate gives
colours between pale green and dark green.
Especially fine Rinmann’s green is obtained by igniting cobalt arsenate
with zinc oxide and arsenious acid. The addition of the last-named
substance can only have the object of preventing the temperature from
rising too high, which might injure the beauty of the colour; arsenious
acid is volatile at a fairly low temperature.
Cobalt green is also obtained by evaporating a solution of cobalt
nitrate and zinc nitrate and igniting the residue, or by igniting
a mixture of the sulphates. In the latter case a tolerably high
temperature is necessary in order that the sulphates may be decomposed.
The author has found that a particularly handsome product is obtained
by mixing pure zinc oxide with a dilute solution of cobalt chloride,
drying and then slowly heating to redness in a crucible with a
well-fitting lid. Towards the end of the operation the heat is
considerably increased and maintained for a short time, after which the
mass is rapidly cooled.