BROWN MINERAL PIGMENTS.
=Lead Brown.=—When red lead is treated with nitric acid, lead monoxide
is dissolved, whilst deep brown lead peroxide remains. When the action
of the nitric acid is finished the residue is well washed and dried.
Lead peroxide is now extensively used for lucifer matches; it readily
gives up oxygen on heating, and thus accelerates the ignition of the
composition. Brown match-heads owe their colour to lead peroxide.
=Manganese Brown= consists of manganic oxide; it occurs in nature, but
seldom in a state of sufficient purity to be used as a pigment; it is,
therefore, generally made artificially. The process is very simple.
A solution of manganese sulphate is precipitated by caustic soda
solution, the manganous hydroxide produced is quickly changed by the
action of the air into manganic hydroxide. The colour is most rapidly
developed when the precipitate is spread out in thin layers; it is well
washed after it has changed to brown.
=Pyrolusite Brown= is manganese peroxide in a state of fine division.
It can be made from the residual liquors of the preparation of chlorine
by adding sodium hypochlorite. A brown precipitate results, which is
kept in contact with the liquid until it no longer changes in colour.
It is then washed with water slightly acidified with sulphuric acid,
and finally with pure water. When finely powdered, manganese peroxide
has a very fine brown colour. It is quite unaltered by the air and
thus, in consideration of its cheapness, deserves a much greater use
than it has yet found.
=Prussian Brown.=—When Prussian blue is heated in air it soon begins to
glow, and is converted into a brown mass, the shade of which depends
on the amount of impurity in the Prussian blue. It is desirable to use
Prussian blue quite free from impurities, so that a brown of the same
shade is always obtained. The brown obtained can be shaded by additions
of indifferent substances.
=Iron Brown.=—By igniting a mixture of 100 parts of finely levigated
natural yellow ochre with 5 parts of common salt a brown product is
obtained, which is pale or deep according to the temperature used. It
is a cheap and durable colour.
=Copper Brown.=—According to Cartheuser solutions of 2 parts of copper
sulphate and 1 part of Epsom salts are mixed, and a strong solution of
potassium carbonate gradually added so long as a precipitate forms. The
precipitate is washed, dried and ignited, when copper brown is obtained
as a brown powder of great permanence in air.
According to Schreger solutions of 2 parts of copper sulphate, 2 parts
of alum and 0·5 to 1 part of ferrous sulphate are mixed, potassium
carbonate solution added, the precipitate dried and ignited. The
redness of the shade is proportional to the quantity of ferrous
sulphate.
=Hatchett Brown= is copper potassium ferrocyanide. It is obtained by
precipitating a soluble copper salt with potassium ferrocyanide. Copper
salts behave towards potassium ferrocyanide in the same manner as iron
salts; different compounds result according as the ferrocyanide or the
copper salt is in excess. Hatchett brown is somewhat largely used for
painting wood.
=Chrome Brown.=—When solutions of potassium chromate and a copper
salt are mixed a precipitate of the composition CuCrO₄.2CuO.2H₂O is
obtained. On drying it acquires a fine brown colour; it forms a very
stable pigment.
Chrome brown can also be made by dissolving 10 parts of potassium
bichromate in 20 parts of water, heating to boiling, adding 13·5 parts
of solid copper chloride and then gradually a boiling solution of 10
parts of soda in 20 parts of water, until effervescence no longer takes
place. On cooling, chrome brown separates as a soft brown precipitate.
=Cobalt Brown= is a very durable compound pigment of a pleasing shade.
It is obtained in various hues by adding ferric oxide to the mixture
of alumina and a cobalt salt used for making cobalt blue. The process
may also be carried out by igniting ammonia alum with cobalt sulphate
and ferrous sulphate. The temperature required in this case to obtain a
bright colour is very high, and must be maintained for a long time to
decompose the whole of the ferrous sulphate.
The pigment can be obtained at a far lower temperature when ferric
chloride is used in place of ferrous sulphate. The mixture is made by
grinding together 5 parts of cobalt hydroxide and 25 parts of ammonia
alum, then adding a solution of ferric chloride, rapidly drying the
whole to a powder, and whilst still hot filling it into the crucible
in which the ignition is performed. When small quantities of ferric
chloride are used chocolate brown shades inclining to violet are
produced; the larger the quantity of the iron compound the more pure is
the brown.
It has been stated that it is important, in the preparation of cobalt
pigments, to prevent the entry of fire gases to the heated mixture;
the reducing action of the gases would materially damage the beauty
of the colour. It has been proposed to place a small quantity of
mercuric oxide at the bottom of the crucible. This would decompose
on heating, and create an atmosphere of pure oxygen in the crucible.
Apart from the cost of the mercuric oxide, which is considerable, it
would only be effective at the commencement of the operation, since it
is completely decomposed at a low red heat. The author has found an
addition of pyrolusite considerably more effective. This may be applied
by spreading a small quantity, about 5 per cent. of the mixture to be
heated, on the bottom of the crucible and covering it with powdered
glass, upon which the colour mixture is placed. An alternative course
is to place the crucible in a second, filling the space between the
crucibles with powdered pyrolusite. At a strong red heat oxygen is
slowly evolved from the pyrolusite; the entry of fire gases into the
crucible is thus prevented. In using the first method the crucible
is frequently spoilt, whilst in the second method the surrounding
pyrolusite protects it, so that it can be used again.