GREEN MANGANESE PIGMENTS.
=Manganese Green, Rosenstiehl’s Green.=—This pigment, which is handsome
but difficult to produce, consists of barium manganate. It can be
obtained by several methods, all of which produce a good green, but
with very unequal properties. Manganese green made from barium nitrate
has little durability; when made from barium hydrate it is more costly,
but very durable.
Barium manganate is most easily obtained by precipitating a boiling
solution of potassium manganate by barium chloride. The almost blue
precipitate becomes nearly white when washed and dried, but when it is
gradually heated on a porcelain plate to a dark red heat it acquires
a fine green colour. The heating must be carefully conducted; if the
temperature rises too high, the colour changes to a dirty greyish
brown, owing to the reduction of the manganic acid.
This pigment is also made by heating 14 parts of manganic oxide, 80
parts of barium nitrate and 6 parts of barytes with access of air until
the desired shade appears. The mass is then ground in a continuous
current of water until it is changed into a very fine powder and
nothing more is dissolved by the water. The best result is obtained
by Rosenstiehl’s process: 4 parts of barium hydrate, 2 parts of
finely-powdered barium nitrate and 0·5 part of artificial manganese
dioxide are rapidly mixed; the mixture is moistened and heated to a
dark red heat. The fused mass is boiled out with water and the residue
dried under a bell jar, under which are dishes of sulphuric acid and
caustic potash, the former of which absorbs the water, whilst the
latter keeps the air free from carbonic acid, which would injure the
shade of the moist substance.
Manganese green is amongst the pigments more recently discovered.
On account of its high price it has found but limited use up to the
present. It may be used in any vehicle, and is distinguished by great
permanence.
In regard to the preparation of this colour by Rosenstiehl’s method,
which of all gives the best results, it should be observed that the
shade largely depends on the quantity of barium hydrate used; the
greater this is the more the shade inclines to blue. If it is desired
to produce a colour of a greener tone than is produced by the fusion,
this can be accomplished by boiling for a very long time with very
weak hydrochloric acid, which extracts a portion of the base from the
compound and produces a deeper shade.
=Böttger’s Barium Green.=—A beautiful green is obtained by the process
given by Böttger, as the author has found. It consists of barium
manganate. The process is, however, somewhat costly, so that the
pigment would only be available for artists’ purposes.
A solution of potassium manganate is first made by gradually adding 2
parts of very pure pyrolusite, finely powdered, to a fused mixture of
2 parts of caustic potash and 1 part of potassium chlorate, bringing
the mass to a low red heat after the introduction of the whole of the
pyrolusite, and finally extracting with water, in which the potassium
manganate dissolves to a fine emerald green solution. This process
is unattended by difficulty if the pyrolusite contains a sufficient
quantity of manganese peroxide and is in a sufficiently fine powder,
which is most important. Potassium manganate is a very unstable
substance. Cold water must be used to dissolve it, and the solution
should not long stand exposed to the air, but should at once be used to
obtain barium manganate.
When the solution of potassium manganate is mixed with a solution of a
barium salt, a handsome violet precipitate immediately forms. This is
washed with water and quickly ground with ¾ to 1 part of its weight of
barium hydrate. The mixture is heated in a copper dish with constant
stirring to a low red heat; the colour changes gradually to a very fine
green. When the mass has reached the proper shade it is treated with
cold water to remove excess of barium hydrate, until the washings show
no trace of alkaline reaction.
=Manganous Oxide= is occasionally used as a green pigment, especially
for painting metal work; it is obtained in the following manner:
Manganese sulphate solution is precipitated by soda solution, and
the manganous carbonate obtained strongly ignited in a crucible. In
order to avoid oxidation of the manganous oxide, which would readily
occur in the process, it is necessary to prevent air from reaching
the substance. With this object the crucible in which the heating is
conducted is covered by another from which the bottom has been removed
and which is filled with coal. The air entering the crucible on cooling
must pass through the layer of glowing coals, by which it is deprived
of oxygen.
=Manganese Blue= is obtained, according to G. Bong, by igniting a
mixture of 3 parts of quartz, 6 parts of soda ash, 5 parts of limestone
and 3 parts of manganese oxide, or of 3 parts of quartz, 8 parts of
barium nitrate and 3 parts of manganese oxide, air being admitted and
reducing gases excluded. All materials must be free from iron. The
quantity of manganese oxide regulates the depth of the colour, but not
its shade; by increasing the soda a greener, by increasing the quartz a
more violet, shade is obtained.