BLUE MINERAL PIGMENTS—IRON-CYANOGEN PIGMENTS.
The pigments occurring in commerce under the names of Prussian,
Chinese, Berlin or Paris blue consist, when pure, of ferric
ferrocyanide, Fe₄[Fe(CN)₆]₃. This compound is obtained as a deep blue
precipitate when yellow prussiate (potassium ferrocyanide) is mixed
with the solution of a ferric salt.
The precipitate shows certain differences in composition according
to the exact method by which it is obtained. A different result is
obtained when the solution of the ferric salt is poured into that
of a ferrocyanide, to that produced by the reverse procedure, the
precipitate produced in the one case has properties which it does not
possess in the other. Similar differences often occur in precipitating
pigments.
If the solution of the ferrocyanide is poured into the iron solution,
the latter being in excess, an insoluble precipitate is formed; but if
the iron solution is poured into the ferrocyanide solution of which an
excess is present, the blue precipitate is formed, but it is soluble
in pure water, though not in water containing salts. Thus when this
precipitate is separated from the liquid and washed, the washings are
at first quite colourless and remain so whilst salts are present, but
when these have been completely washed away, a fine blue solution of
the ferric ferrocyanide is formed, from which the addition of a salt
solution again separates the dissolved Prussian blue.
Before proceeding to the manufacture of Prussian blue on the large
scale, it is necessary to consider the behaviour of ferrous salts
towards yellow prussiate, since these are commonly used instead of
ferric salts. When a solution of potassium ferrocyanide is mixed with
a solution of a ferrous salt, there is formed a pure white precipitate
of potassium ferrous ferrocyanide, K₂FeFe(CN)₆. In order to obtain a
completely white precipitate, it is necessary that the ferrous salt
should be completely free from ferric salts and that the solutions
should contain no dissolved oxygen; the solutions would therefore
have to be boiled before mixing. If one of the solutions contains the
smallest quantity of oxygen a bluish precipitate is formed. When the
white precipitate of potassium ferrous ferrocyanide is exposed to the
air it immediately acquires at the surface a dark blue colour, and is
completely changed to Prussian blue by sufficiently long contact with
air.
In accordance with this behaviour of ferrous and ferric salts different
methods may be used to obtain Prussian blue. Either the process is
commenced with a ferric salt from which a precipitate of Prussian blue
is at once obtained, or the solution of a ferrous salt is converted
into ferric salt by a powerful oxidising agent, such as chlorine or
nitric acid, or a solution of a ferrous salt is precipitated with
yellow prussiate and the white or bluish precipitate, which consists
chiefly of potassium ferrous ferrocyanide, converted into ferric
ferrocyanide or Prussian blue by treatment with nitric acid.
Prussian blue is also known, as we have said, as Chinese blue, Berlin
blue and Paris blue; Brunswick blue or mineral blue is another form.
Generally the product under the name of Chinese or Paris blue,
which has an intense dark blue colour, is quite pure, whilst under
the names of Prussian blue and Berlin blue mixtures with starch or
alumina are included, which consequently have a more or less pale
colour. The pigments known as mineral or Brunswick blue are the
least valuable, they have generally a less deep colour and are quite
without the metallic lustre which distinguishes Chinese blue and the
better varieties of Prussian blue. Mineral blue often contains very
large quantities of alumina, chalk, or even barytes; the addition of
the last is to be regarded as irrational, since it makes the pigment
conspicuously heavy.
=Chinese Blue.=—Pure Chinese blue appears in the form of deep blue
masses, characterised by a peculiar metallic lustre, which is
especially marked when the surface of a lump is rubbed with the finger
nail. This metallic lustre is accompanied by a copper-red shimmer
which is similar to that exhibited by fine indigo. In large pieces
pure Chinese blue appears very dark by artificial light; it possesses
enormous strength of colour. The following test serves for the
recognition of the complete purity of Chinese blue: A small quantity
is powdered and rubbed in a thin layer on white paper; if the metallic
lustre shows in undiminished strength the blue may be regarded as quite
pure, for Chinese blue quickly loses this property by additions of
other substances.
From what has been said above concerning the different behaviour of
ferrous and ferric salts, the simplest method of making Chinese blue
would be to precipitate the solution of a ferric salt with yellow
prussiate, keeping the ferric salt in excess, to wash and dry the
precipitate. Ferric salts are, however, dearer than ferrous salts, so
that the latter are generally used.
In making Chinese blue from a ferrous salt ferrous sulphate or green
vitriol is generally taken. This salt is dissolved in water; it is
advisable to add a small quantity of sulphuric acid, since the water
will contain carbonates, which produce ferrous carbonate. Air oxidises
the latter to ferric hydroxide, which is brown, and if present even
in small quantity will spoil the shade of the blue. This addition of
sulphuric acid is of special importance when the white precipitate
produced by the prussiate solution is to be oxidised by the air.
A solution of yellow prussiate is added to the iron solution in such
quantity that a very small excess of iron salt is left; there results
a white or rather, since the ferrous sulphate always contains small
quantities of ferric oxide, a pale blue precipitate. This is allowed
to settle, the liquid drawn off as completely as possible from the
precipitate, and nitric acid then added, or a solution of bleaching
powder, followed by the amount of sulphuric acid required to decompose
it. The nitric acid or the chlorine liberated from the bleaching powder
speedily effects the oxidation, changing the pale blue colour to the
deep blue of Chinese blue. The precipitate should be left for several
days in contact with the liquid in order to complete the oxidation;
it is then well washed and dried. Pieces of definite shape and size
bearing the trade mark of the firm are often pressed from the blue when
it has acquired a stiff consistency.
There are many formulæ for the preparation of Chinese blue, differing
in regard to the quantities of green vitriol, yellow prussiate, and
nitric acid or bleaching powder to be used. It is, however, clear
that only _one_ formula can be correct, that in which the quantities
of materials are equivalent, since the reactions take place between
equivalent quantities. In practice the equivalent quantities would not
be weighed to the tenth of a gramme—there would be no object in such a
proceeding, since in works chemically pure substances are not used; but
manufacturers should work according to equivalent quantities in order
to use up their materials as completely as possible. No increase in
labour is involved, the same labour weighs one or another quantity.
The proportion of materials in which the least loss is involved is here
given. Nine kilogrammes of green vitriol are dissolved in 100 litres of
water, 15 kilogrammes of strong sulphuric acid are added, and then a
solution of 15 kilogrammes of yellow prussiate in 100 litres of water.
The solutions are kept in constant motion during the mixing. As soon
as the decomposition is finished, without waiting for the precipitate
to settle, steam is led in and 20 kilogrammes of nitric acid of 1·3298
specific gravity added in small portions. The heating by direct steam
is continued until red vapours are no longer evolved from the liquid,
which is a sign that the oxidation is finished.
Although the use of direct steam is very convenient for heating, since
wooden vessels can be used and the heating quickly accomplished, it
may be dispensed with, and the solutions heated in pans or even not at
all. In this case the oxidation lasts considerably longer than when the
liquid is heated. In the process recommended by Gentele 109 parts of
yellow prussiate are used to 20 parts of green vitriol, each dissolved
in much water. The precipitate is heated for a short time with 51 parts
of nitric acid of 1·2285 specific gravity and 16 parts of sulphuric
acid. The mixture is allowed to stand for several days to complete the
oxidation before the precipitate is separated from the liquid.
In Hochstätter’s method, solutions of 6 parts of yellow prussiate in
15 parts of water and of 6 parts of green vitriol in 5 parts of water
are mixed. To the precipitate 24 parts of strong hydrochloric acid and
1 part of sulphuric acid are added, and then a solution of bleaching
powder in 80 parts of water until the liquid smells distinctly of
chlorine. Apart from the fact that this method does not employ
equivalent quantities of iron salt and yellow prussiate, it is also
to be regarded as unsuitable, since the addition of sulphuric acid
produces calcium sulphate, which, being soluble with great difficulty,
will be precipitated along with the blue, making the shade lighter. The
addition of sulphuric acid should be omitted and only hydrochloric acid
used, the calcium chloride formed by the decomposition of the bleaching
powder being very soluble in water. Many formulæ given for the
manufacture of Chinese blue are said to produce a pigment of special
beauty. This, however, is incorrect. Chinese blue is a compound of
fixed composition, which is always formed when equivalent quantities of
the salts are used and the precipitate converted into the blue compound
by one of the oxidising agents given. It is most simple to use nitric
acid as the oxidising agent; for this purpose the presence of chlorine
in the acid is harmless. When sufficient space and time are allowed, a
special oxidising agent may be omitted and the oxidation of the white
precipitate accomplished by atmospheric air.
In working in this manner the precipitate is washed several times
with water, and spread in a thin layer upon boards which are placed
in rows one above the other. In a short time the mass turns blue on
the surface, but the pale colour remains underneath much longer, since
the access of oxygen to the interior is difficult. The boards must be
left until the mass has a uniform colour throughout. In order that the
paste shall not dry too thoroughly whilst on the boards it is desirable
to make it very fluid. If the precipitate has been completely washed
before the oxidation the product is finished after drying, but if a
portion when completely dried shows a crystalline incrustation, salts
are still present, which must be removed by careful washing.
=Prussian Blue.=—As we have already stated, Prussian blue is identical
in chemical nature with Chinese blue, from which it differs only by
certain additions which are made in order to reduce the price. Prussian
blue should therefore be regarded as a more or less impure Chinese
blue; less pure materials are used in its preparation than for the
fine Chinese blue. Instead of pure potassium ferrocyanide, the crude
salt, which is sold by the prussiate makers at a lower price, is used
for Prussian blue; in addition to the ferrocyanide this crude salt
contains a considerable proportion of other salts, including a quantity
of potassium carbonate.
The ferrous sulphate used for Prussian blue is mixed with alum, which
neutralises the potassium carbonate, precipitating alumina, by which
the precipitate formed from the yellow prussiate and ferric salt is
made paler. If the pale blue precipitate were treated with acids they
would first dissolve out the alumina; this should be avoided, since the
alum has been added with the intention of increasing the quantity of
precipitate and making the pigment paler. The precipitate can therefore
only be changed into the blue pigment by spontaneous oxidation; this
is accomplished by the oxygen of the air in the manner previously
described. The oxidation is more rapid according to the quantity of
ferric salt contained in the green vitriol at the commencement; it
is convenient to prepare a large quantity of green vitriol solution
and expose it in shallow vessels to the air so that the oxidation may
proceed. But if the solution of green vitriol were exposed alone to
the action of the air a rust-coloured precipitate of basic salt would
be deposited at the bottom of the vessels. To prevent the formation of
this basic salt, sulphuric acid is added in small quantity, and, as the
oxidation proceeds, further small quantities of acid are added. In this
way a solution of ferric sulphate is slowly formed, which gives with
yellow prussiate a blue precipitate requiring little further oxidation.
=Mineral Blue, Brunswick Blue.=—This pigment, the most reduced of the
colours of this class, is obtained by mixing white pigments, barytes,
pipe clay, etc., with the already impure Prussian blue.
=Soluble Prussian Blue.=—The modification of ferric ferrocyanide easily
soluble in water is obtained by adding a solution of a ferric salt
to a solution of yellow prussiate, the latter being in considerable
excess. Brücke gives the following method: a solution is prepared of
217 grammes of yellow prussiate in 11 kilogrammes of water. Another
solution contains 100 grammes of ferric chloride in 1 litre, 1 litre
of which is mixed with 2 litres of a saturated solution of Glauber’s
salt and the mixture added to the prussiate solution so long as a blue
precipitate forms. The Glauber’s salt plays no part in the formation
of the colour; it is added to prevent the solution of the precipitate,
which is insoluble in salt solutions. The precipitate is brought on to
a filter and washed with water until the washings begin to be blue,
which is a sign that the salts have been almost completely removed.
The precipitate cannot be further washed without the loss of a large
quantity which would be dissolved by the water; without further washing
it is slowly dried in the air.
Soluble Prussian blue has but a limited application; anatomists use its
solution to inject specimens, and it was formerly used for blue writing
inks, for which purpose aniline dyes are now preferred. It may also be
remarked that ordinary Prussian blue readily dissolves in a solution of
oxalic acid; the solution was also used as a writing ink, but since it
attacks steel pens, it has now been replaced by solutions of dyes which
have no action on steel.
=Special Processes for the Manufacture of Chinese Blue.=—In place of
chlorine or nitric acid manganic chloride can be used to oxidise the
white precipitate obtained from yellow prussiate and green vitriol.
This process is based upon the property of manganic chloride to readily
give up part of its chlorine and be changed into manganous chloride
(the ordinary chloride of manganese). The conversion of the white
precipitate into the blue is thus also in this case indirectly effected
by chlorine.
Many varieties of pyrolusite contain considerable quantities of
manganic oxide which are of little use in the manufacture of chlorine,
for which pyrolusite is chiefly employed. If such pyrolusite be
digested with crude hydrochloric acid for several days, the manganic
oxide is dissolved with the production of manganic chloride.
The oxidation of the white precipitate is performed in the usual way;
after running off the liquid an excess of manganic chloride solution is
added, as soon as two tests taken at an interval of a few minutes show
no further increase in the depth of the colour, the reaction is ended
and the solution of manganous chloride is drawn off.
Any powerful oxidising agent may be used to oxidise the white
precipitate; chromic acid has been proposed for this purpose, but up to
the present the chromates are too dear to be used in place of the cheap
oxidising agents previously mentioned.
In regard to the present endeavours to utilise by-products of the
chemical industries, a method for preparing this pigment may be
mentioned which would certainly be very profitable on the large scale.
Coal gas before purification contains some quantity of ammonium
cyanide, which is decomposed by ferric oxide with the production of
Prussian blue. This pigment might be made by passing coal gas over
very finely divided ferric oxide, until it was completely converted
into Prussian blue. The process is, however, not practicable on the
large scale: the ferric oxide would have to be exposed in an extremely
thin layer to be completely converted into Prussian blue, and the
transformation would require a long time. A better process is to make
yellow prussiate instead of Prussian blue. A mixture of ferric oxide
with coarse sawdust is spread out in thin layers, over which the impure
coal gas passes. When the mass has acquired a blue colour owing to the
Prussian blue produced, it is treated with a mixture of quicklime and
potassium sulphate. Potassium ferrocyanide is formed; it is obtained
in crystals by the evaporation of the solution and can be used in the
preparation of Prussian blue.
The “Laming’s mass,” now generally used in gas works, contains, after
use, a considerable quantity of cyanogen compounds, and it would only
require a short trial to decide whether it was more advantageous
to make this mixture by exposure to air again suitable for the
purification of coal gas, or to use it to obtain yellow prussiate.
=Turnbull’s Blue.=—When a solution of a ferrous salt is precipitated
by potassium ferrocyanide, a fine blue precipitate is formed,
identical in physical properties with Chinese blue, but differing in
chemical constitution. It is ferrous ferrocyanide, Fe₃[Fe(CN)₆]₂.
The manufacture of Turnbull’s blue is expensive; the pigment has no
specially advantageous properties and is therefore seldom made.
In preparing this pigment, a solution of yellow prussiate is
transformed into red prussiate by passing chlorine through it so long
as it is absorbed by the liquid; the mother liquors from which red
prussiate has been crystallised may be used with advantage. When to the
solution of red prussiate green vitriol solution is added so that the
former remains in excess, a soluble Turnbull’s blue is obtained, but
when excess of green vitriol is used the insoluble form is produced.
Chinese blue and Turnbull’s blue are used both as oil and water
colours, and these handsome but not particularly durable pigments are
also employed in colouring wall papers.
=Antwerp Blue= is a mixture of Prussian blue with zinc cyanogen
compounds. It is obtained by precipitating a solution of 2 parts of
zinc sulphate and 1 to 2 parts of green vitriol (according as pale or
deep blue is required) by a dilute solution of yellow prussiate.