GREEN MINERAL PIGMENTS—GREEN COPPER PIGMENTS.
=Copper Carbonate.=—In nature two salts occur containing copper,
carbonic acid, and water in different proportions—one, the mineral
malachite, has a fine emerald green colour; the other, azurite, is sky
blue. Both minerals are ground and levigated, and brought into commerce
as mountain green and mountain blue.
Compounds of copper with carbonic acid, made artificially, are
used under different names as pigments. The terms mountain blue
and mineral blue are applied indiscriminately to various copper
compounds; as mountain blue, or Brunswick green, pigments come into
commerce containing varying quantities of copper carbonate (made by
precipitating a copper salt with soda) mixed with barytes, zinc white,
or another white pigment, in order to obtain paler and deeper shades.
The poisonous Scheele’s green is also given the same name as that under
which copper carbonate is sold, _viz._, mineral green, so that the
composition of this colour can only be decided by analysis.
A green pigment, similar in composition to malachite, is obtained
by precipitating copper sulphate with soda in the cold and washing
quickly. If the precipitate is left in contact with the liquid,
its colour changes and it is converted into a blue mass similar in
properties to ground malachite. A green precipitate is obtained with
more certainty by precipitating a boiling solution of copper sulphate
with soda solution.
Both green and blue copper carbonate are pigments of little depth;
as a rule ground and levigated malachite and azurite have a greater
intensity of colour than the artificial pigments.
=Copper Arsenite.=—The compounds of arsenious acid with copper are
among the most handsome and bright colours which exist. It is thus
to be regretted that these pigments are disappearing more and more,
and indeed with every reason, from the pigments permitted by law.
In the end they must fall into complete disuse. It is a matter of
common report that these pigments, and particularly emerald green,
have frequently caused arsenical poisoning. Numbers of people have
been repeatedly made ill by the dust of emerald green fixed on light
fabrics, such as are used for ball dresses, simply by albumin or
dextrine, and brought into the air by the movements of the dancers.
It has been further shown that the air of rooms papered with hangings
coloured by emerald green contains small quantities of the most
poisonous of all arsenic compounds, arseniuretted hydrogen.
Although it is in the interest of colour makers to produce the
brightest possible colours, yet, on account of their dangerous
properties, pigments containing arsenic should be quite excluded.
In several countries their use is forbidden by law. Since there are
several pigments which can compete in brightness with the arsenic
colours, it is to be hoped that the latter will shortly disappear from
commerce.
=Scheele’s Green, Swedish Green.=—A solution of 32 parts of potash is
boiled with 11 parts of white arsenic (arsenious acid) until the latter
is completely dissolved. This solution of potassium arsenite is mixed
with a hot solution of 32 parts of copper sulphate free from iron. The
beautiful green precipitate is washed with hot water and dried. These
directions for the preparation of copper arsenite are due to Scheele,
the discoverer of the pigment. It can be prepared by several other
methods.
When white arsenic and copper sulphate are dissolved together, and
caustic soda added to the boiling solution, Scheele’s green is
precipitated. When made by this method the pigment must be very
carefully washed; if the excess of caustic soda is not completely
removed it is very hard when dry, and is difficult to powder. In order
to obtain darker greens the quantity of copper sulphate is increased.
A particularly bright green is obtained by dissolving copper sulphate
in water, with the addition of 12 to 15 per cent. of white arsenic, and
precipitating by a solution of potassium zincate, which is obtained
by boiling caustic potash solution with zinc filings, hydrogen being
evolved.
The shade of Scheele’s green depends on the relative quantities of
arsenious acid and copper sulphate employed. If the arsenious acid
predominates, a pale product is obtained; when excess of copper
sulphate is used, the colour is pure green. The pigment made according
to the foregoing directions possesses only small covering power. A
product better in this respect is obtained by precipitating a solution
of 100 parts of copper sulphate by a solution of 90 parts of potassium
carbonate, in which 66 parts of arsenious acid have been dissolved.
Scheele’s green is a bright green pigment, which resists the action
of the atmosphere tolerably well. It is easily decomposed by dilute
acids; when strongly heated arsenious acid is set free, and copper
arsenate and arsenide are produced. Copper arsenite is the essential
constituent of the pigments sold under the name of mineral green, which
is generally a mixture of copper arsenite with varying quantities of
mountain green.
=Brunswick Green, Green Verditer=, is a mixture of copper arsenite,
copper hydroxide and gypsum. It is prepared by dissolving 100 parts
of copper sulphate in water, adding a solution of ¼ part of arsenious
acid and 10 parts of anhydrous potassium carbonate, and finally
precipitating with milk of lime made from 20 parts of quicklime. The
pale to bluish-green precipitate is washed and pressed into flat cakes,
which are brought into the market and used as lime colours. This
pigment is not adapted for use in oil; when applied in that medium it
darkens considerably.
=Neuwied Green= is made in almost exactly the same manner as Brunswick
green, the chief difference being that a larger quantity of arsenic
is used, and thus a product obtained inclining more to green than to
blue. As a rule 2 to 2½ parts of arsenious acid are used to 100 parts
of copper sulphate. The colours found in commerce under this name very
often consist of mixtures of emerald green and finely ground barytes.
=Copper Oxychloride= was at one time largely used. At present it is
scarcely employed; it has been replaced by handsomer and cheaper
pigments. Copper oxychloride is made by exposing a wet mixture of
copper, copper sulphate and common salt to the action of the air.
To 111 parts of copper sulphate 112·5 parts of copper and the same
quantity of common salt are employed. The mixture is made into heaps,
which are wetted and from time to time shovelled about, so that the
air may come in contact with all portions of the mass, and the copper
chloride be converted into basic chloride. It is advisable to allow the
heaps to dry frequently after they have been turned over, so that the
air may more readily penetrate into the interior. Copper oxychloride
has a pale green colour of little brilliance. On this account it is
hardly ever used now; it forms the raw material for the preparation of
Bremen green.