BLUE COBALT PIGMENTS.
=Cobalt Blue, Thénard’s Blue, Cobalt Ultramarine, King’s Blue, Leyden
Blue.=—The pigment known under these names is a compound of cobalt
oxide with alumina, and is prepared in a similar manner to Rinmann’s
green. Of all the blue pigments used in painting, cobalt blue is the
most handsome and most durable. It is completely unaltered by the
atmosphere. The most powerful chemical reagents have but a slight
action upon it, and their action is less the higher the temperature
used in the manufacture. Cobalt blue is made by precipitating mixed
solutions of a cobalt salt and alum, washing, drying, and igniting the
precipitate. A product of pure blue colour is only obtained when alum
absolutely free from iron is used. If but very small quantities of iron
are present, the red ferric oxide has a very considerable influence on
the shade of the product.
The depth of colour varies according to the proportion of cobalt salt
used. The variation is not so considerable as in the case of cobalt
green; as a rule, 50 to 100 parts of cobalt sulphate or nitrate are
used to 100 parts of alum. The temperature at which the precipitate is
ignited must be much higher than that used for cobalt green; it should
be raised to a white heat. The crucible should be provided with a
well-fitting lid—the fire gases damage the shade of the colour.
When cobalt sulphate is ignited with ammonia alum, cobalt blue is
obtained; but this method is not to be recommended. The ignition must
be of long duration to completely drive off the sulphuric acid. The
author has made a cobalt blue of remarkable beauty in the following
manner: Alumina obtained by precipitating electrolytically an alum
solution (page 49) was mixed with a solution of pure cobalt chloride
to a thin pulp, which was quickly dried in a shallow porcelain dish.
The mixture, whilst still hot, was pressed into a crucible placed in
a larger one filled with magnesia; the crucible was heated to a good
white heat, at which it was maintained for an hour, when it was allowed
to cool. The cobalt blue made by this method has the purest blue
colour, being without that admixture of red which is not rarely to be
perceived in this pigment. To avoid the appearance of this red tint,
certain additions are made; a small quantity of a zinc salt is most
frequently employed, according to the quantity of which more or less
pale shades are produced.
=Cæruleum= is a pale blue pigment consisting of a compound of the
oxides of cobalt and tin. It is made by converting tin by the action
of nitric acid into white stannic oxide, adding a solution of cobalt
nitrate and evaporating to dryness in a porcelain dish whilst stirring;
whilst still warm the powder is strongly ignited. In cæruleum the
chemical combination is less firm than in the compounds of cobalt oxide
with zinc oxide or alumina, strong hydrochloric and dilute sulphuric
acids decompose it, continued treatment with nitric acid extracts all
the cobalt. Cæruleum resists completely all atmospheric influences.
The circumstance that all the cobalt in this compound may be removed by
treatment with nitric acid leads to the conclusion that the pigments
composed of cobalt oxide and another oxide are not chemical compounds
of the two oxides, but that the alumina, zinc oxide and tin oxide
serve simply as bases for the blue modification of cobalt oxide. That
a chemical compound cannot be present follows from the fact that the
quantities of the oxides uniting together may be very considerably
altered and a coloured product still obtained. The author has shown in
1868 that the blue modification of cobalt oxide is obtained by heating
the red. The change of the red colour of cobalt chloride into the blue
is not caused, as was formerly thought, by loss of water, but by the
transformation of the cobalt oxide contained in the compound from the
red into the blue modification. Strong bases are able to retain the
cobalt oxide in the blue form, and strongly basic oxides are therefore
used in the manufacture of this series of pigments.
=Cobalt Zinc Phosphate.=—This pigment is made by precipitating a
solution of zinc sulphate, free from iron, by sodium phosphate, adding
a solution of cobalt sulphate and again sodium phosphate so long as a
precipitate is produced. Excess of sodium phosphate is to be avoided,
otherwise a considerable quantity of cobalt remains unprecipitated. In
the directions given by Gentele it is stated that the sodium phosphate
must be in excess, and that the liquid above the precipitate must
possess a reddish colour. The author’s experiments have shown that a
pigment in every respect equal to that produced by Gentele’s process is
obtained when all the cobalt is precipitated. The precipitate, which
consists of variable quantities of zinc and cobalt phosphates, is dried
and ignited, when it acquires a very deep blue colour.