COMPOUNDED GREEN PIGMENTS.
Mixtures of a yellow and a blue pigment produce a green; according as
one or the other predominates colours are obtained inclining to yellow
or blue.
In some cases the mixing can be accomplished in the actual production
of the pigments, so that a green precipitate is directly produced.
This is, however, rarely the case; generally the compound pigments
are obtained by simply mixing the two colours. The mixing can be done
either in the dry or the wet way; colours in the wet state are more
easily mixed than when dry, so that even when dry colours are employed
the mixing is done with the addition of water. Although the mixture
then requires a second drying, this method is still to be recommended,
in the first place, because the mixing can be more quickly done in
consequence of the greater mobility of the mass, and in the second,
because the formation of poisonous dust is completely avoided.
The mixing is generally accomplished by mechanical arrangements. If
dry colours are to be mixed, rotating cylinders may be used; these are
filled with the materials to be mixed, well closed and rotated about
the axis as long as may be necessary. It should be observed that when
colours which have a very different specific gravity, such as chrome
yellow and Prussian blue, are to be mixed, the cylinders must be
rotated for a much longer time than when colours of approximately equal
specific gravity are to be united.
In working in the wet way, which is to be preferred, sufficient water
is added to the colours to make a paste thin enough to be mixed by
spatulas; the mixture is thoroughly stirred and the pulp ground in
ordinary mills until the mixture is quite uniform. Already whilst on
the mills the pulp becomes thicker in consequence of evaporation; when
the grinding is finished it is spread out in thin layers so that it may
dry as quickly as possible. This is important in the case of mixtures
whose constituents have very different specific gravities, otherwise
the heavier pigment may sink to the bottom of the paste and the mass
thus lose its uniformity.
The compound green pigments come into the market under most varied
names, which are often entirely without connection with their chemical
composition. Such names are: mineral green, English green, oil green,
green vermilion; the most common are chrome green and Brunswick green.
It should be noted that these names are more strictly applied to simple
colours previously described.
=Chrome Green= is generally made by mixing deep chrome yellow and
Prussian blue; it can naturally be obtained in all possible shades.
The brightness of this already handsome pigment can be considerably
increased by the addition of a small quantity of indigo carmine. In
this case the mixing is best done by adding a solution of the indigo
carmine to the stiff paste and again sending the mixture through the
mill.
By additions of white substances paler shades of chrome green are
obtained; levigated terra alba or white pipe clay can be advantageously
employed.
=Elsner’s Chrome Green= is obtained by preparing a solution of yellow
prussiate of potash and potassium chromate and another solution of lead
acetate and ferric chloride. The two liquids are mixed with vigorous
stirring; according to the proportions of the materials a blue or a
yellow shade of green is produced.
=Silk Green.=—Forty-one parts of lead nitrate are dissolved in 20 to 30
times the weight of water, the solution is boiled in a copper pan, and,
according to the shade required, 10 to 30 parts of fine Chinese blue
are added. After well stirring, a solution of 10 parts of potassium
bichromate and 1 part of nitric acid is poured into the boiling liquid,
the mixture is again well stirred, the precipitate allowed to settle,
washed and dried. The green pigment obtained in this way has a peculiar
silky lustre, hence the name ”silk green”.
=Natural Green= is a mixture of Guignet’s green with picric acid; it is
used in making artificial flowers instead of emerald green.
=Non-arsenical Green= was proposed as a substitute for emerald green,
which, however, it does not equal in brilliance. It is obtained by
mixing copper blue (basic copper carbonate) with chrome yellow, chalk
and ferric oxide, in somewhat variable proportions. It usually contains
80 to 82 per cent. of the copper compound and 13 to 15 per cent. of
chrome yellow.
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In making mixtures of pigments it is to be remembered that chemical
actions may occur in the mixture; colours which act upon one another
should not be mixed: the mixture would contain the cause of its own
destruction. For example, a lead pigment should not be mixed with one
containing sulphur in the form of a sulphide or sulphate. By following
this important rule the number of pigments which may be mixed together
is considerably diminished; the advantage is that durable colours are
produced, which do not change in a short time to an unrecognisable
shade.