BLUE COPPER PIGMENTS.
=Bremen Blue and Green.=—The pigment known as Bremen blue or Bremen
green consists of copper hydroxide. It possesses all the disadvantages
of copper compounds; it is very sensitive towards sulphuretted hydrogen
and sulphur dioxide; in contact with air containing the former gas it
is quickly turned black. It has also other disadvantages; it cannot
be used in oil, for when ground in oil it is quickly discoloured, on
account of the formation of copper oleate. If a wall which has not
completely dried be covered with Bremen green, in a short time it will
be covered with spots. In spite of this small stability, Bremen green
is still largely used by artists on account of its low price, although
it could be replaced by other pigments almost as cheap and considerably
more durable.
Copper hydroxide comes into the market under many names. The pigments
known as blue verditer, lime blue and mountain blue contain essentially
copper hydroxide. The preparation of pure copper hydroxide on the
large scale will be first described. Copper sulphate is the usual raw
material. In order to obtain a colour of a pure blue shade it must be
free from iron, the presence of which would result in the production of
a discoloured precipitate.
A fairly concentrated solution of copper sulphate is warmed in a pan to
about 30° C.; weak caustic potash solution is added until the liquid
is slightly alkaline. A green precipitate separates, which consists of
copper sulphate and hydroxide. In order to remove the copper sulphate
from the precipitate, and impart to it the correct bluish-green colour,
it is left on a strainer until it has acquired a pasty consistency,
when it is brought into a tub and mixed with weak caustic potash
solution. Care must be taken that this potash solution is not too
concentrated, or it will withdraw water from the copper hydroxide and
produce black copper oxide. In order to prevent this result, which
would render the whole mass useless, a portion of the precipitate
is tested with the caustic potash solution. If it is blackened, the
solution must be diluted with the necessary quantity of water before it
is used. Instead of diluting with water, potassium carbonate solution
is often added. The addition of the caustic liquor is continued until
the colour of a test portion of the precipitate is not deepened by
further additions. When this point is reached the colour must be at
once withdrawn from the action of the alkali. It is brought into a
large tub filled with water, in which it is most carefully washed with
many waters.
Basic copper chloride may also be used as the raw material for this
pigment. It is obtained by treating scrap copper with 60 per cent. of
common salt and 30 per cent. of sulphuric acid. The pale green basic
chloride is then treated with caustic potash solution. This method
is now rarely used; copper sulphate is considerably lower in price
than formerly and copper hydroxide is prepared from it with the least
trouble.
=Neuberg Blue= is a mixture of copper blue with Chinese blue. The
larger the quantity of the latter component the deeper the colour.
Neuberg blue can be more easily ground with oil than pure copper blue.
It should not be used as a distemper colour, for the Chinese blue
is decomposed by the action of lime, and the colour will quickly be
spoiled by the separated ferric oxide.
=Lime Blue= is ordinary copper blue, mixed with varying quantities
of gypsum. It is paler than pure copper blue; different shades are
obtained by increasing the quantity of gypsum.
This pigment is made by dissolving 15 parts of copper sulphate in 1,300
parts of water, adding a strong solution of 121 parts of ammonium
chloride, and then milk of lime prepared from 30 parts of quicklime.
The precipitate is well washed, strained, ground, and finally dried.
Lime blue is brought into the market in several forms; in cubes
obtained by cutting up the stiff paste and then completely drying,
and in irregular lumps or a coarse powder. The pigments containing
copper hydroxide as their colouring principle are used less as artists’
colours than for distempering.
=Payen’s Mountain Blue= is a mixture of copper hydroxide with varying
quantities of calcium carbonate. It is prepared by adding calcium
chloride to a solution of copper sulphate and stirring in dilute milk
of lime. A green precipitate of basic copper chloride is formed;
potassium carbonate solution mixed with milk of lime is then added. The
latter mixture contains caustic potash, which decomposes the copper
chloride, copper hydroxide and calcium carbonate being simultaneously
precipitated. Different shades may be obtained; by increasing the
quantity of lime the product is paler. This pigment commonly comes
into the market in the paste form. When it is dried the operation must
be conducted at a low temperature; copper hydroxide is converted into
copper oxide at relatively low temperatures.
=Oil Blue.=—The pigment known under this name consists of copper
sulphide (CuS). It can be made in several ways. The simplest process,
which also gives a good product, is here described.
Sulphur is boiled in a glass flask with a long neck. When the heavy
reddish brown vapours begin to fill the neck, copper turnings or wire
free from oxide are thrown in. The copper burns in the sulphur vapour
with a red flame to cupric sulphide. When it has once commenced to
burn, the flask requires little heating, so much heat is produced
by the combustion of the copper that the sulphur continues to boil.
The introduction of copper is continued until only a small excess of
sulphur is left, the flask is then closed and allowed to cool. When the
contents have reached the temperature of the air the flask is broken,
and the mass boiled with caustic potash solution in order to remove
excess of sulphur.
The product obtained by this process has a handsome violet blue colour,
which must be shown by the whole mass. If portions taken out of the
flask are black, an excess of copper has been used. In order to improve
the colour of a faulty batch, it is mixed with sulphur and quickly
heated to the boiling point of sulphur, air being excluded. The excess
of sulphur is then removed by boiling with caustic potash solution.
Oil blue is a handsome but not very durable pigment. When used in
varnish, which protects it from the action of air, it is fairly
permanent.
=Copper Hydroxide.=—By precipitating carefully a solution of a copper
salt with caustic soda, a blue precipitate of copper hydroxide is
obtained, which, though not distinguished by any particular beauty, is
adapted for use as a distemper colour and for similar purposes. The
operation is conducted at ordinary temperatures; on account of its
voluminous nature the pale blue precipitate requires a long time for
washing. Hot solutions must not be used; at a temperature near the
boiling point of water copper hydroxide is completely decomposed into
black copper oxide.