BLUES.
=Cobalt Blues.=--Some of the compounds of cobalt with alumina,
phosphoric acid, silica, and tin, are remarkable for possessing a fine
blue colour of great permanency and indestructibility, and still find a
limited application. They are chiefly as follows:--
Cœruleum--a mixture of the oxides of cobalt and tin.
Cobalt blue--a mixture of the oxides of cobalt and alumina.
Smalts--a double silicate of cobalt and potash.
CŒRULEUM.--This is a light-blue slightly greenish colour, with
no purple tendency in artificial light. It is non-granular, covers
well, mixes with water or oil, and is a good artists’ colour for sky
effects. It is permanent in strong sunlight and impure atmosphere, and
resists acids and alkalies at normal temperatures. Hot hydrochloric
acid dissolves it, and addition of water to the pale blue solution
produces a violet red; evaporation to dryness restores the original
pigment. The green tint of a nitric acid solution is due to iron and
nickel impurites. Dilute sulphuric acid causes partial decomposition
of cœruleum, but it is proof against caustic potash, acetic acid, and
concentrated sulphuric acid. Its composition is given as
Cobalt oxide 18·66
Tin oxide 49·66
Silica and sulphate of lime 31·68
------
100·00
There are several methods of preparing cœruleum:--
(1) A solution of stannate of potash is added to one of cobalt. A blue
precipitate is thrown down, which, on washing, becomes first light-red
and then brown. When calcined at a white heat it assumes a blue colour.
(2) A solution of stannate of soda is mixed with a solution of nitrate
of cobalt, and the resulting precipitate calcined to bright redness
forms a blue pigment.
(3) Solutions of cobalt and tin are mixed and precipitated by soda, the
precipitate washed free from soda being calcined as in all the other
cases. The silicate of soda is the most satisfactory sodium salt for a
precipitant.
COBALT BLUE.--This rich pure blue pigment is not alone
permanent, but actually develops its full intensity only after exposure
to the air. With age, however, it acquires a greenish tendency,
and in artificial light it inclines to a violet tint. It is proof
against acids and alkalies, and mixes particularly well with water.
In combination with other pigments it is unaltered and has no effect
on them. Its tones are different from those of ultramarine. It slowly
decomposes when heated in strong sulphuric acid, yielding a violet
solution and a white precipitate, which latter dissolves and affords a
blue liquid on dilution with water. It consists principally of about 80
per cent. alumina, and 15 per cent. oxide of cobalt.
There are several ways of preparing it:--
(1) By Thénard’s original process, roasted cobalt, from Tunaberg,
Sweden, is dissolved under heat in an excess of nitric acid. The
solution, evaporated nearly to dryness, is boiled in water, and the
deposit of arseniate of iron is filtered out. Into the filtrate is
poured a solution of basic phosphate of soda, which throws down
a precipitate of basic phosphate of cobalt, varying in hue from
violet to pink. This precipitate is washed on a filter, and, while
still gelatinous, 1 lb. of it is intimately mixed with 8 lb. of
hydrated alumina, recently precipitated by ammonia from a solution of
potash-alum. The mass is first dried to brittleness and then calcined
in a covered clay vessel for half an hour at a cherry-red heat. The
resultant blue pigment is stored in glass receptacles.
The preparation of the gelatinous alumina is conducted as follows. The
potash-alum is dissolved in at least three times as much water as is
necessary, and is then precipitated by an abundant excess of ammonia,
with frequent stirring. When settled, the supernatant liquor is
siphoned off, and the precipitate is thoroughly washed several times on
a filter.
Thénard blue will vary in tint according to the proportions of
alumina used. A pure colour is obtained with 4-5 parts alumina to 1
of phosphate of cobalt; a greenish hue with equal parts of alumina
and cobalt salt; and almost any intermediate tone by varying the
proportions between these limits.
(2) To 16 parts of gelatinous alumina add 1 part of arseniate of
cobalt, obtained by precipitating the solution of cobalt by arseniate
of potash.
(3) A richer and more velvety blue is got by using oxide of cobalt and
substituting phosphate of lime for the alumina (Boullai-Marillac).
(4) Binder’s process is as follows:--Dissolve by boiling 6 lb. alum,
free from iron, in a leaden or earthenware vessel, and filter it into
a vat 5½ ft. high and 3 ft. across, one-third full of clean water.
Precipitate the alumina by solution of potash, fill up the vat with
water, settle, decant the clear liquor, and wash repeatedly till
barium chloride gives no precipitate. Dissolve ½ lb. sesquioxide
of cobalt in 1½ lb. hydrochloric acid at 22° B., and evaporate to
dryness. Dissolve the residue in 3 lb. hydrochloric acid, and pass
a stream of sulphuretted hydrogen through it, to throw down any
foreign metals. Filter clear, evaporate again to dryness, and dissolve
the residue in enough water to produce 4½-5 lb. of solution. Next
precipitate the cobalt solution (3 to 6 lb., according to depth of
tint required) by ammonia, avoiding excess. Wash the precipitate, and
add it to the water, holding the gelatinous alumina in suspension,
stirring thoroughly for ½ an hour. A reddish tint in the supernatant
liquor shows that some of the cobalt has been dissolved. Add a little
ammonia, and allow the precipitate to settle. Decant and add new waters
repeatedly. Finally collect on a fine filter cloth, drain, press, stove
dry, and calcine for 2-2½ hours at red heat in clay crucibles: then
cool, grind first in a mill and then on a slab, and sift.
SMALTS.--This pigment has not maintained its position in
competition with artificial ultramarine. Formerly it was very largely
used to correct the yellow tone of cottons, papers, and pottery. It has
a pale violet-blue tint, which, however, is not constant in artificial
light. Being a silicate it is very permanent, and proof against the
action of acids, alkalies, and sunlight, besides being inert when
mixed with other pigments. It can be used with either water or oil as
a medium, but is not a successful paint owing to its weak colouring
power. It is virtually a double silicate of cobalt and potash, or a
cobalt glass, containing a few impurities, of which the chief are
aluminium, iron, and lead oxides. The colour varies somewhat according
as these impurities fluctuate, and the finest ground sample is always
the palest. It is hardly ever adulterated, and the chief point to
secure is that it be ground to the finest possible degree.
Its manufacture is most extensively and successfully carried on in
Saxony. The raw materials used are cobalt speiss (an arsenide of cobalt
and iron), potash, and sand. The ore is broken up into convenient sized
pieces and roasted at red heat in a reverberatory furnace provided with
a tall shaft for discharging the sulphurous and arsenical fumes at a
high altitude. When the evolution of these fumes has ceased and the
mass begins to assume a pasty consistence, the roasted ore is removed
from the furnace, cooled, reduced to a fine pulverulent condition (then
known as “zaffre”) and passed through a silken sieve. Should it be
necessary,
[Illustration: Fig. 12.--FURNACE FOR ROASTING COBALT ORES.]
the cobalt ore is first spalled and hand-picked to remove the ores
of foreign metals which are associated with it; and then reduced to
a very fine state in an edge runner or mortar mill, and freed from
earthy impurities by washing. The concentrated ore is then dried and
dead-roasted in small charges at a time (about 4 cwt.) in a specially
designed reverberatory furnace such as shown in Fig. 12, of which _a_
is the hearth on which the ore is spread; _b_, the fireplace, the
products of combustion from which pass over the ore on the hearth, and
thence into the flues _c_, which repeatedly circle round the furnace
so as to provide abundant opportunity for the arsenious oxide derived
from the combustion (oxidation) of the arsenic in the ore to condense;
this highly poisonous arsenious oxide is collected in a solid form from
the flues at convenient intervals by means of the doors _d_. The ore
is charged and discharged at the door _e_. The roasting should not be
carried to such a point that the whole of the sulphur and arsenic are
removed when making smalts, as by leaving a portion of these substances
in the ore at this stage, the ultimate purification is better
accomplished.
The next stage is to fuse the roasted ore with potash and silica so
as to form a blue glass. The proportions in which the ingredients are
mixed depend upon the depth of colour in the zaffre operated upon and
the tint desired in the finished smalts; hence it is always determined
by a preliminary experiment, and is then most carefully adhered to,
each material being accurately weighed out. Only the best potash can
be used, as it must be quite free from soda, and iron or other metal;
the effect of soda is to render the blue greenish tinted. Quartz
affords the requisite silica, and is hand-picked to ensure freedom from
alumina, iron, and lime, which import dullness into the colour, and
then ground to a fine powder in an edge runner mill. The duly weighed
quantities of the several ingredients are intimately mixed in wooden or
cement lined vessels, so as to preclude the possibility of any metallic
iron finding its way in; and as a further protection against this risk
a little white arsenic is often added so that the iron may be carried
down in the regulus which is formed during the fusion in the crucible.
These crucibles are of refractory earthenware quite free from lime,
and measure about 18 inches across at top, gradually diminishing to 14
inches at bottom, so that an ordinary charge is about ¾ cwt. They are
placed in rows in a furnace which generally bears a close resemblance
to a glass furnace, the operation being very similar. The form of
furnace common in Saxony, where most of the smalts is made, is shown in
Fig. 13. By means of a series of openings a in the walls of the furnace
the pots _b_ are introduced on to the hearth of the furnace, whereupon
the openings _a_ are bricked up again and remain closed during the
operation. The ingredients are charged into the pots _b_
[Illustration: Fig. 13.--FURNACE FOR MAKING SMALTS.]
by means of long iron ladles which are introduced through the small
square apertures _c_, which can be temporarily closed by a half-brick
or other simple article. The fire is then lit in the fireplace _d_,
and the products of combustion circulate around the pots _b_, and
finally escape at the orifices _e_ at the top of the furnace into
flues leading to the chimney _f_. After about 8 hours’ firing fusion
commences in the pots, whereupon the contents are thoroughly stirred by
rods inserted through the working holes _c_. The temperature is then
increased till a white heat is attained, this being necessary for the
formation of a glass. The fused mass is repeatedly sampled, and when
it has become quite homogeneous, and the regulus or speiss containing
the iron, antimony, bismuth, arsenic, copper, nickel, sulphur and other
impurities has completely separated itself and collected at the bottom
of the pots, the blue glass is ladled out and dropped at once into cold
water, by which it is disintegrated and rendered very brittle ready for
the subsequent grinding. The regulus is then drawn off from the pots
through holes provided for the purpose, and removed by the orifices
_g_, after which the pots are ready for another charge. They ordinarily
remain serviceable for about six months.
The grinding needs to be done with great thoroughness, and is
accomplished partly by stamps and partly by edge-runner mills in the
presence of water. The particles as reduced are floated off by the
water to a series of settling tanks communicating one with another.
The portion which settles in the first of the series is too coarse for
use, and is returned to the edge-runner for further grinding; while the
portion in the last of the series possesses such a weak colour that it
is rejected, or put into the crucible to undergo a second fusion. The
selected portions are dried ready for the consumer.
=Copper Blues.=--These form an unimportant class, being unstable
and not endowed with great colouring power. Their tint is pale and
greenish, and though opaque in water, they are not particularly so when
mixed with oil. Exposed to the action of sulphur or its compounds,
whether present as sulphuretted hydrogen in the air, or in combination
with a metal, forming another pigment with which they may be mixed,
copper blues undergo an important chemical change, the carbonates and
oxides of copper being converted into the sulphide, which is black.
Under the influence of heat too the blue carbonate will lose its
carbonic acid, and be turned into the black oxide. Ammonia and the
acids dissolve them, but other alkalies are resisted until heat is
applied. The chief kinds of copper blue are Bremen blue, cæruleum, lime
blue, mountain blue, Péligot’s blue, and blue verditer.
BREMEN BLUE.--This is a more or less pure hydrated oxide
of copper, varying in its qualities according to the method of
preparation. When made by precipitating a neutral salt of copper from
solution, it forms a dense and compact mass; whereas a porous and
pulverulent pigment results when basic and insoluble copper salts are
treated with alkalies.
(1) The foundation of the manufacture of this colour was waste copper
scrap, such as ship’s sheathing, from which, in various ways, was
prepared a basic chloride or oxy-chloride. Some of the methods adopted
were:--(_a_) 100 lb. scrap copper, 99 lb. powdered sulphate of potash,
and 100 lb. salt, moistened with clean water; (_b_) 100 lb. copper
fragments, 60 lb. salt, and 30 lb. diluted sulphuric acid (3 volumes
of water to 1 of acid); (_c_) a solution of copper oxide (scales) in
pure hydrochloric acid poured over the scrap copper. The method (_a_)
produces a chloride of copper which, in contact with more metal,
becomes a sub-chloride; this, absorbing more oxygen from the air, is
converted into the basic green “oxide” of the factories. By the (_b_)
process, the hydrochloric acid set free, and the atmospheric oxygen
produce the same result. In the (_c_) process a similar effect is
obtained.
It is of primary importance that no trace of this sub-chloride of
copper shall be allowed to remain, as it undergoes decomposition
by caustic alkalies, and throws down an orange-yellow sub-oxide of
copper. Hence it has sometimes been the practice to prepare the basic
oxy-chloride twelve months in advance, and to stir it frequently before
use. Complete oxidation, however, can be as satisfactorily accomplished
by alternately wetting and completely drying the mass.
An interesting phenomenon takes place during the transformation of
this green magma into a hydrated oxide of copper. On this magma being
introduced by degrees into a caustic potash or soda lye of about 22°
B., the thoroughly washed and dried product is exceedingly fine, with
great covering power, and deepens on addition of a little water. When
the magma is diluted with an equal volume of water, and the mixture at
once poured into an excess of caustic lye, with constant agitation, a
few minutes’ rest will suffice for the mass to assume a most compact
consistence. The colour thus produced, when washed and dried, is much
lighter in colour, and has less body. A blue derived from any of these
products is unsatisfactory as regards freshness and intensity of
colour; whereas by adding a small quantity of concentrated solution of
sulphate of copper to the magma before treating it with the alkaline
lye, apparently a highly basic sulphate of copper is produced which
deepens the colour.
A pigment with good body can be made in the following manner. To 100
lb. of the thick magma of basic oxychloride add a concentrated solution
of 7 lb. sulphate of copper, and then 40 lb. of a concentrated caustic
lye (32°-36° B,), with vigorous and rapid stirring, finally adding
about 150 lb. of caustic lye at 20° B. When the decomposition is quite
complete, the precipitate is carefully washed, passed through a fine
hair sieve, and filtered. Drying is effected at a low temperature, to
ensure that the hydrated state of the oxide is not changed; and the air
of the drying chamber must be free from acid or sulphuretted vapours.
(2) If neutral nitrate of copper be decomposed by an insufficiency
of potash carbonate solution, the flocculent precipitate of copper
carbonate resulting is by degrees transformed into a sub-nitrate of
copper, which goes down as a heavy green powder. On treating this
sub-nitrate with a potassic solution of zinc oxide, a dark-blue
coloured pigment is formed, which is apparently a zincate of copper
mixed with a very small proportion of a highly basic nitrate of
copper. Though very light it has great covering power. In practice the
manufacture is conducted as follows.
Calcine copper scales in a reverberatory or muffle furnace till the
sub-oxide is entirely converted into protoxide, or until it dissolves
in nitric acid without evolving red nitrous fumes. Heat is applied to
the solution of nitrate of copper, which is decomposed by addition of a
clear solution of potash carbonate. After effervescence has subsided,
small doses of potash carbonate solution are added till but little
undecomposed copper is left. This residue is recovered by decanting the
clear liquor, and repeatedly washing the green precipitate with small
quantities of clean water, collecting all the washings, and finally
precipitating by potash solution. On introducing the green carbonate of
copper into a new solution of copper nitrate, it is transformed into a
basic salt. Crystals of nitrate of potash are obtained by evaporating
the previous liquors.
To obtain an economic solution of zinc oxide, clippings of metallic
zinc are treated with a solution of caustic potash or soda in a
cast-iron vessel. The immediate result is a disengagement of hydrogen,
and saturation of the alkali with zinc oxide, which behaves as an acid.
The cleared liquor serves for decomposing the basic nitrate of copper.
The pigment produced is a handsome blue, and the potash liquor can be
evaporated down till it yields crystals of saltpetre. The economy of
this method lies in producing nitric acid cheaply from soda nitrate and
obtaining saltpetre as a bye-product.
(3) An inferior and cheaper pigment is made in the following manner.
To a solution of copper sulphate add one of barium or calcium chloride
till a white precipitate ceases to go down, and from the cleared blue
liquor all the copper is precipitated by addition of fresh milk of
lime. Usually the weight of quicklime required is 20 per cent. of the
copper sulphate. The settled, washed, and dried precipitate is the
pigment desired. The cleared barium or calcium chloride solution may be
used anew as a precipitant for the next batch.
CÆRULEUM.--This name has been given to the beautiful blue
pigment used in Egyptian and Pompeian mural paintings, and exhibiting
the same bright blue after 1000 years’ exposure to the weather as when
first used. Its composition has been given by Fouqué as approximately
63½ per cent. silica, 21 per cent. copper oxide, and 14 per cent.
calcium oxide, and he regards it as a double silicate of copper and
calcium. It is supposed to have been produced by fusing together
copper ore, sand and lime, but experiments have not yet resulted in a
successful imitation of the pigment, a difficulty being encountered
in the fact that if too high a temperature be permitted, destruction
of the blue colour ensues, and a green glass results instead. This is
unfortunate, as its remarkably bright and stable hue would make it very
popular if it could be manufactured at a moderate cost. It withstands
sulphuretted hydrogen, and even prolonged boiling with any of the acids
or alkalies.
LIME BLUE.--This pigment is essentially a mixture of hydrated
oxide of copper and calcium sulphate. It resists the action of alkalies
in the cold, but turns black when boiled in caustic soda, and is
completely soluble in hydrochloric acid. Ultramarine has largely, if
not entirely usurped its place. There are several ways of making lime
blue:--
1. Any soluble copper salt the acid of which will make a soluble salt
with lime is suitable, the only precaution necessary being that if in
the decomposition of the copper and lime salts, the combination of
the whole of the sulphuric acid with the lime is not attained, there
should be an excess of copper sulphate in the liquor rather than of
the lime salt. The resulting copper solution, containing very little
lime sulphate, is settled in a cool place for 24-36 hours, filtered,
and diluted with clean water down to about 18° B. Meantime a milk of
lime is prepared with very white and well-burned lime, slaked and mixed
with abundance of pure water, and kept stirred for a long time in a
lead-lined vat. After a short rest to permit sand, &c., to precipitate
itself, the milk is drawn off, and left to settle in lead-lined or
copper pans. The deposit is collected, ground in a mill where contact
with iron is impossible, and passed through a very fine sieve.
The mixture of lime and copper solution is made in the proportion
of 100 lb. dry lime with 175 dry copper salt, if the most intense
coloration is desired, but the proportion of lime may be much increased
without detriment to the pigment beyond lessening its intensity of
colour. After complete settlement of the precipitate, the clear liquor
is decanted; the pigment is carefully washed with clean soft water,
and drained on filter cloths till it is of a convenient consistence
forming a green paste. A definite weight of this paste calculated on
the dry pigment is taken for further incorporation, consequently it is
first necessary to ascertain how much water is in the paste. Usually
it amounts to 75 per cent., and on this basis 5 lb. of the paste are
stirred up with 1 gal. clean water in a lead-lined vat, with addition
of ½ lb. wet lime under constant agitation. Subsequently ¼ pint of
clear solution of best potash at 15° B. is well stirred in, and the
mixture is immediately taken to the mill and most thoroughly ground.
Further, for each 10 gal. of green paste is prepared a clear solution
of 1 lb. pure salammoniac in 2 gal. water and another solution of 2 lb.
copper sulphate in 2 gal. water. The liquid paste is drawn off from the
mill into a stoneware vessel, and the two solutions of salammoniac and
copper sulphate are immediately added. After complete agitation and
combination, the mixture is left for 4 or 5 days to settle, and turned
into a lead-lined vat, where it is repeatedly washed with clean waters
until turmeric paper is not discoloured.
(2) Precipitation of copper sulphate by excess of thin milk of lime in
the cold, followed by washing and drying, will give a lime blue which
will dry without turning black. Or 100 lb. of the copper sulphate may
be treated with a milk of lime prepared from 30 lb. quicklime and
addition of 12½ lb. salammoniac. When the liquor has become colourless,
the pigment is prepared from the precipitate; but the lime should be
ground after slaking, and the milk of lime left to stand for some days,
before use. The salammoniac seems to be essential to the production of
a pure full blue. Milk of lime poured drop by drop into the ammoniacal
copper solution gives a precipitate which redissolves on agitation,
and remains long in solution under heat, but finally throws down a
permanent precipitate, while the liquor on standing gives beautiful
blue crystals. From experiments it appears that out of seven atoms of
copper sulphate in the liquor, five are precipitated by milk of lime
and the last two are decomposed by ammonia. A greater proportion of
lime will produce a precipitate holding a certain quantity of less
valuable pigment. A smaller proportion of lime yields a finer coloured
and more crystalline pigment, because it crystallises partly in the
excess of solution, so that by incomplete decomposition a smaller
yield of superior pigment is obtained. The proportions necessary for
formation of the colour are 7 equivalents of copper sulphate, 5 of
lime, and 2 of ammonia, and if the 2 equivalents of ammonia be replaced
by 2 of lime and 2 of salammoniac, the proportions furnishing the
best colour will be 100 lb. copper sulphate, 24 lb. lime, and 22½ lb.
salammoniac.
Both caustic soda and caustic potash produce a fine blue precipitate
in a solution of ammoniacal copper sulphate with excess of ammonia,
but the liquor decolorises only on evaporation of the ammonia. The
precipitate becomes lighter-hued the more it is washed, and consists
of hydrated oxide of copper with a little carbonic acid; it does not
turn brown even when heated in presence of excess of potash or soda.
Moreover the presence of ammonia renders the hydrated oxide of copper
much more permanent. The composition of this pigment is given by
Gentele as 33½ per cent. copper oxide, 23½ sulphuric acid, 16 lime, and
the remainder water, &c.
MOUNTAIN BLUE OR AZURITE.--This natural blue pigment consists
essentially of a basic carbonate of copper, and is found in quartz
rocks in England, France, Bohemia, Hesse, Saxony, the Tyrol, and
Siberia. It affords a rich sky-blue paint of a permanent character,
but being comparatively costly is not largely employed. Its composition
is about 69 per cent. copper oxide, 25½ carbonic acid, and 5 water. The
only preparation needed is exceedingly fine grinding.
PÉLIGOT BLUE.--(1) Whereas the hydrated oxide of copper
precipitated from a solution of a salt of copper by excess of potash
or soda rapidly blackens even though washed with cold water, Péligot
obtains a blue hydrated oxide which resists boiling and heating at 212°
F. He uses any soluble copper salt, but preferably the sulphate. A
very dilute solution of the copper sulphate is treated with ammonia in
excess (aqua ammoniæ or an ammoniacal salt) and precipitated by soda or
potash.
(2) On adding water in excess to a slightly ammoniacal solution of
copper nitrate, the same pigment is obtained.
(3) A mixture of 73 parts silica, 16 oxide of copper, 8 lime, and 3
soda, is fused together at a temperature not much exceeding 800° F. At
higher temperatures there is risk of the pigment turning black.
VERDITER.--This sky-blue and not very durable pigment, used
in water-colour painting, closely resembles Bremen blue (see p. 34) in
composition and manufacture. It consists chiefly of copper carbonate,
mixed with a lesser proportion of hydroxide, sulphate, or oxide,
and occasionally a small quantity of sulphate of lime; and is most
satisfactorily prepared from copper chloride or nitrate, though almost
any salt of copper may be used. The mode of fabrication varies.
(1) To a solution of the nitrate or sulphate is added one of potash
or soda carbonate so long as any precipitate is formed, and this
precipitate, when filtered and washed, is treated with a weak caustic
soda solution.
(2) A hot solution of chloride of lime is added to a hot solution
of sulphate of copper at 62½° Tw. till the precipitate ceases to go
down. The solution of chloride of copper which constitutes the liquor
is filtered off, diluted with water to about 31½° Tw., and treated
with repeated small doses of slaked lime ground exceedingly fine in
water till no more copper is precipitated. The resulting green paste
is drained, filtered, washed, and put into wooden vats; here 8 lb. of
lime paste and 5 pints of potash carbonate solution at 25½° Tw. are
added for every 70 lb. of _dry_ colour contained in the green paste,
the whole mass being thoroughly agitated, then allowed to rest till the
development of the required shade is accomplished, when it is filtered,
washed, and dried.
(3) In some German works the final green paste as prepared in (2) is
put into air-tight vessels, and a solution of 3 lb. ammonium chloride
and 4 lb. sulphate of copper in 7 gal. of water is introduced for each
70 lb. of _dry_ colour in the green paste. After complete admixture of
all the ingredients, the receptacles are fastened up for several days
so that the reactions may proceed out of contact with the air, and
finally the pigment is removed, washed, and dried for use.
=Indigo.=--The well-known blue colouring matter termed indigo is
produced by a great number and variety of plants, distributed
throughout all the tropical countries of the globe. Commercially, it is
obtained chiefly from species of _Indigofera_, as _I. tinctoria_, the
cultivated species of India, furnishing the chief article of commerce,
found also in Madagascar, St. Domingo, &c.; and _I. Anil_, in the
Punjab, W. Indies, and on the Gambia river. Some is also obtained from
_I. argentea_, in Africa and America: _I. Caroliniana_; _I. disperma_,
the cultivated plant of Spain, America, and some of the E. Indies;
_I. cærulea_, the “black indigo” of India; _I. glauca_, in Egypt and
Arabia; _I. pseudo-tinctoria_, cultivated in some parts of the E.
Indies, and said to yield the best dye; _I. cinerea_, _I. erecta_, _I.
hirsuta_, and _I. glabra_, in Guinea. Considerable local supplies are
obtained from the following plants:--_Isatis tinctoria_, in Europe and
China (see Woad); _I. indigotica_, cultivated in some parts of China;
_Amorpha fruticosa_, in Carolina; _Baptisia tinctoria_, wild, in the
United States; _Gymnemia_ (_Asclepias_) _tingens_, in Burmah; _Polygala
tinctoria_, in Arabia; _Polygonum Chinense_, _P. tinctorium_, _P.
perfoliatum_, _P. barbatum_, _P. aviculare_, in China and Japan, and
introduced into Belgium; _Ruellia indigotica_, largely cultivated in
Assam, as well as in India, and at Che-king, in China; _Tephrosia
tinctoria_, and _T. apollinea_, in India and Egypt; _Wrightia
tinctoria_ (_Nerium tinctorium_), the Palas indigo of the Carnatic.
The cultivation of indigo (chiefly _Indigofera tinctoria_) is very
extensively carried on in India, especially in the district included
between 20° and 30° N. lat. The soil best suited for the culture is a
rich loam, with a subsoil which is neither too sandy nor too stiff;
alluvial soils give the best returns, but good crops are sometimes
raised on higher grounds. The land is ploughed in October-November,
after the rains; the seed, about 12 lb. to the acre, is sown in
February-April. Too rapid growth diminishes the yield of dye. In
July-September, the plants are in full blossom, and the harvest takes
place. The preparation of the dyestuff may be performed in either
of two ways, which are distinguished as the “dry-leaf,” and the
“green-leaf” process. The latter is considered the better, and is
the more general; it is conducted as follows:--The flowering plants
are cut down at about 6 in. from the ground, and immediately taken
to the steeping vats, within which they are spread out and pressed
down by beams fitted to the side posts of the tanks. Enough water is
then admitted to cover the plants; if this be delayed, fermentation
may set in and spoil the product. The duration of the steeping is
liable to considerable modification, and needs much judgment and
experience; with a temperature of 96° F. in the shade, 11-12 hours may
suffice; in cooler weather, 15-16 hours may be necessary. Moreover,
very ripe plants require less time than young and unripe ones. The
following general conditions indicate the time for suspending the
maceration:--(1) The sinking of the water in the vat; (2) the immediate
bursting of the bubbles that arise; (3) an orange tint mingled with
the green, when the surface water is disturbed; (4) the emission
of a sweetish, pungent odour, quite distinct from the raw odour of
the unripe liquor. At this point, men enter the vat, and stir up its
contents either by hand or by a wooden paddle. The agitation is at
first gentle, but increases as the fecula begins to separate; this
is known by the disappearance of the froth, and by the colour of the
liquor changing from green to blue. The “beating,” as it is called, is
continued for 1¾-3 hours, the following conditions being a guide as
to its sufficiency:--(1) The ready precipitation of the fecula from a
sample of the liquor, and the madeira-wine colour of the latter; (2)
a brownish colour observed on dipping a cloth into the liquor, and
wringing it out; (3) the appearance of a glassy surface on the liquor,
and the subsidence of the froth with sparkling and effervescence.
Next a little pure cold water, or weak lime-water, is sprinkled over
the surface of the liquor, to hasten the settlement of the fecula,
which occupies 3-4 hours. After this, the water is drained away
from the top, by means of plug-holes in the side of the vat. The
precipitated fecula is then removed to a boiler. Here it is made to
boil as promptly as possible, and is kept boiling for 5-6 hours; it is
constantly stirred, and skimmed with a perforated ladle. After boiling,
it is run off to a straining table, where it stays for 12-15 hours to
drain; next it is pressed for about 12 hours, and then cut, stamped,
and placed to dry. The ordinary dimensions of a steeping-vat are 16 ft.
by 14 ft. by 4½ ft. deep; this will contain about 100 _maunds_ (8200
lb.) of plants, which may yield from 40 lb. downwards of indigo. The
beating-vat is less deep.
Such are the methods of cultivation and manufacture most generally in
use throughout India. In limited districts, however, some modifications
are in vogue. On land subject to inundation, the plants last only one
year. South of the Ganges, the seed is sown at the beginning of the
rains, and the plants remain on the ground for two years, thus giving
a double crop, the second of which is the larger and better. In very
strong land, a third crop is sometimes secured. Occasionally, sesame
is sown on the same ground, and harvested before the indigo is cut.
Small quantities of indigo are grown on poppy lands, and irrigated. The
seed is sown in March-April, and the crop is gathered at the end of
the rains, in time for an opium crop to be taken off the land. Indigo
is sometimes manufactured by collecting the fecula, and dropping it in
cakes to harden in the sun; this is “gaud” indigo, of very inferior
quality. The fecula is improved by boiling it in coppers and pressing
it into boxes. The production of the indigo blue is the result of the
decomposition of the colouring principle of the plant, which exists as
a glucoside. Plants grown on poor soils, and in dry climates, yield
almost the whole of this glucoside to the ordinary process of steeping
and beating described above; but plants raised on rich alluvial soil,
and in damp heat, contain an amount of glucoside which cannot be
utilised by the ordinary process. In order to prevent this waste, which
causes the richest plants to give the least return, it is necessary
either to prolong the fermentation, and raise the heat to 95°-100° F.,
or to add a solution of sugar or glucose to the vat-liquor. Olphert
adopts the use of steam, to raise the temperature of the vat to 111°
F., and thus obtains 25 per cent. more colouring matter.
Japan possesses several large factories for preparing indigo from the
native _Polygonum tinctorium_. The plants, 2-3 ft. high, are cut into
three parts, the uppermost being the most valuable. The best dye is
made from the leaves alone, which, after a few hours’ exposure to air
and sun, are placed in straw bags. They are afterwards removed from
the bags and moistened with water, which must be proportioned with
the greatest exactitude. They are then spread upon, and covered by,
mats, for a few days, after which the sprinkling is repeated. The
process continues for about 80 days, the moistening being renewed
about 25 times for the best leaves, and 9 for the inferior. After
this fermentation, the leaves are pounded in wooden mortars for two
consecutive days, by which they are reduced to a pulp; this is then
formed into balls of dark-blue colour.
The central provinces of Java yield large quantities of indigo, which
are exported to Holland, and thence widely distributed. The indigo
prepared by the natives is of an indifferent quality, in a semi-fluid
state, and contains much quicklime; but that prepared by Europeans
is of a very superior quality. An inferior variety, having smaller
seeds, and being of quicker growth, is usually planted as a second
crop on land where one rice crop has been raised. In these situations,
the plant rises to a height of about 3½ ft. It is then cut, and the
cuttings are repeated three, or even four times, till the ground is
again required for the annual rice crop. But the superior plant, when
cultivated on a naturally rich soil, not impoverished by a previous
heavy crop, attains a height of 5 ft., and grows with the greatest
luxuriance. The plants intended for seed are raised in favourite spots,
on the ridges of rice-fields in the neighbourhood of the villages,
and the seed of one district is frequently exchanged for that of
another. That of the rich mountainous districts, being esteemed of best
quality, is occasionally introduced into the lowlands, and is thought
necessary to prevent that degeneration which would be the consequence
of cultivating for a long time the same plant upon the same soil. The
climate, soil, and state of society of Java seem to offer peculiar
advantages for the extensive cultivation of this plant. The periodical
droughts and inundations of the Bengal provinces are unknown in Java,
where the plant, in favoured situations, may be cultivated nearly
throughout the whole year, and where it would be secure of a prolonged
period of that kind of weather necessary for the cutting. The dye is
prepared in a liquid state by the natives, by infusing the leaves
with a quantity of lime; in this state, it forms by far the principal
dye-stuff of the country. The indigos prepared in Java by Sayers’s
process are of unusually high and constant quality. They contain an
average of 70½ per cent. of indigotine, and a minimum of 65-66 per
cent.; and an average of 2·77 per cent. of ash. Ordinary commercial
indigos seldom attain 65-66 per cent. of indigotine; and their ash
averages about 16½ per cent.
The Philippines produce considerable quantities of indigo, the best
coming from Luzon. The plants suffer from locusts and storms, but the
cultivation is very profitable. The yield of indigotine is large, but
the preparation is conducted in such a primitive manner that the value
of the product is much deteriorated.
In many parts of Africa, as Sierra Leone, Liberia, Abeokuta, the Niger
valley, Natal, Cape Colony, Tunis, and the Soudan, species of indigo
plants are found in a wild state, and from them the natives prepare an
inferior dye-stuff.
In some of the S. States of America, notably S. Carolina, indigo
culture has been attended with more or less success. The method of
preparation pursued here varies but very slightly from the ordinary
Indian process, almost the only important modification being the
addition of a little oil to the liquor in the beating-vat, when the
fermentation becomes too violent. The precipitated fecula is placed
in coarse linen bags, and hung up to drain. The drying is finished by
turning it out of the bags upon a floor of porous timber, and working
it up. It is frequently exposed to the sun for short periods at morning
and evening, and is then placed in boxes or frames, to cure till it is
fit for the market.
Several of the Central American States have figured conspicuously as
indigo producers. The dye is precipitated in the beating-vat by the sap
contained in the bark of Tihuilate (_Yonidium_), Platanillo (_Myrosma
Indica_), or Cuaja tinta. The fecula is left during the night; and,
on the following day, is boiled, filtered, pressed, and sun-dried.
In most districts, the cultivation is declining, partly owing to the
carelessness exhibited in the preparation of the dye.
Indigo is judged commercially by its lightness, by a copper gloss on
the surface, and by exhibiting no foreign ingredients when broken.
There are several ways of testing it chemically, to ascertain
the exact proportion of indigotine present; one method is as
follows:--Finely pulverised indigo, 1 part; green copperas, 2 parts;
and water containing 10 per cent. of caustic soda, 200 parts; are
well boiled in a flask, and left to cool. The clear liquor is exposed
in shallow vessels to the air, when the soluble indigo is oxidised,
and precipitated as pure indigotine. The residue in the flask is thus
treated three times; the whole indigotine is then collected on a
filter, dried, and weighed. The consumption of indigo is still very
large. Artificial indigo has not, as yet, been manufactured on a
commercial scale, nor at a commercial price; but it has been produced,
in the laboratory, from coal-tar derivatives, and further experiment
may reveal a process for preparing the article at a sufficiently low
price to compete with the natural colour.
Several preparations of indigo are in use:--(1) Sulphopurpuric acid,
phenicine, or indigo-purple, is made by mixing 1 part of indigo with 4
parts of sulphuric acid (sp. gr. 1·845), and heating for ½-1 hour; the
acid mass is thrown into 40-50 parts of water, when the purple falls
down; it is collected on a filter, and washed with dilute hydrochloric
acid. (2) Sulphindigotic acid is prepared by mixing indigotine, 1
part, with sulphuric acid (sp. gr. 1·845) 6 parts; the operation must
be performed in a leaden vessel, cooled outside, and the indigo must
be added by degrees, to avoid heating; the mixture is then left for 8
days, when the conversion will be complete. Fuming or anhydrous acid
may be used, in less proportion, but the reaction is more difficult
to manage. Weaker acid will require a longer period, say a month for
“brown acid” (145° Tw.). (3) Sulphindigotic acids are transformed into
neutral paste, or “carmine,” by neutralising with carbonate of soda,
and washing the paste, on a woollen filter, with a solution of chloride
of sodium (common salt).
=Manganese Blue.=--(1) Kuhlmann found a blue mass of manganate of lime
in furnaces used for making calcium chloride by calcining a mixture
of chalk and residues from chlorine making. The formation of this
beautiful coloured manganate he attributes to the decomposition of the
calcium chloride by steam, and to a certain solubility of the lime
in undecomposed calcium chloride. Unsuccessful attempts to reproduce
this result were apparently due to the lime not being under such
favourable conditions for acting upon the manganese oxide as when it
is in solution in the calcium chloride. As accidentally produced in
reverberatory furnaces, the manganate of lime is of an ultramarine
tint, and is insoluble in water though not permanent under its
influence; it is acted upon by the weakest acids.
(2) Bong has proposed several formulæ for making manganese blues,
the ingredients in each case being heated to redness in an oxidising
atmosphere, taking special care to avoid iron. The following are his
recipes:--(_a_) 6 parts soda ash, 5 of calcium carbonate, 3 of silica,
and 3 of manganese oxide; (_b_) 8 of barium nitrate, 2 of kaolin, and
3 of manganese oxide; (_c_) 8 of barium nitrate, 3 of silica, and 3
of manganese oxide. The tint can be varied from violet to green by
altering the proportions.
=Prussian Blue.=--This blue owes its colour to a combination of iron
and ferrocyanogen. The commercial products vary very much in tint,
depth of colour, covering power, and solubility. They are used for a
variety of purposes, nearly all of which require the blue to have some
property different from what it should have for other uses. For some
purposes a green shade blue is wanted, for others a violet shade blue;
some users want the blue to be soluble in water, others for it to be
soluble in oxalic acid, others require it to be insoluble. Ordinary
Prussian blue is insoluble in water, acids, and alkaline salts;
bleaching powder has no action on it, and therefore it is largely used
for tinting paper. It is capable of resisting acids; but alkalies,
such as caustic soda, caustic potash, the carbonates of the same
metals, lime and ammonia, decompose it, oxide of iron and a solution
of a ferrocyanide of the alkali being formed, the decomposition being
shown by a change of colour from blue to a reddish brown. On this
account Prussian blues cannot be used for colouring soaps and alkaline
products, or used as a pigment in distemper painting along with lime.
The change of colour, from blue to brown, by the action of alkalies,
distinguishes this blue from other blues.
Prussian blues require to be tested for their solubility in oxalic
acid, by taking about 20 gr. of the blue, mixing with 1 oz. of water
and 20 gr. of oxalic acid, in which a good blue ought to dissolve
completely. Some brands are soluble in strong hydrochloric acid while
others are not. It is decomposed by boiling sulphuric acid, and turned
green by boiling nitric acid. For all the ordinary uses of a pigment
Prussian blue is quite durable, and possesses a depth of colour and a
definite tint which is proof against the destructive agencies of light
and air; and though its covering powers are not great, it is one of the
most important blue pigments in use.
When a salt of the higher oxide of iron is added to a solution of
yellow prussiate of potash (or ferrocyanide of potassium) a blue
compound is formed which is called Prussian blue. But this is not the
method adopted for its commercial manufacture. In that case a ferrous
salt, the proto-sulphate of iron, is added to a potassium ferrocyanide
solution, the result of which is that a dirty bluish white precipitate
is thrown down. On adding to this a little solution of bichromate of
potash and sulphuric acid, the full deep blue is obtained. This is the
industrial method of manufacturing Prussian blue.
_Yellow Prussiate._--The first step is the preparation of the yellow
prussiate of potash. The manufacture of this substance, although
an industry of considerable importance, is comparatively little
understood, either from a scientific or a practical point of view. At
all events, many prussiate makers seem completely at sea with regard
to the most favourable conditions for carrying on the manufacture, and
there can be no doubt that in many cases great waste occurs, through
ignorance of the various reactions which take place during the process.
The raw materials usually consist of carbonate of potash, iron filings
or turnings, and organic matters containing carbon and nitrogen--such
as dried blood, woollen rags, horn, hair, leather scraps, &c. The
most suitable substances for use are, of course, those containing the
largest proportion of nitrogen. The following are the percentages of
nitrogen in various kinds of animal matter:--
Horn 15 to 17
Dried blood 15 to 17
Woollen rags 10 to 16
Sheep shearings 16 to 17
Calves’ hair 15 to 17
Bristles 9 to 10
Feathers 16 to 17
Hide clippings 4 to 5
Old shoes 6 to 7
Horn charcoal 2 to 7
Rag charcoal 2 to 12
Animal matters always contain more carbon than is necessary for the
formation of cyanogen by combining with the nitrogen also present.
Consequently, when such substances are heated with pearlash, the
excess of carbon reduces a portion of the carbonate to the metallic
state, and this potassium combines with the cyanogen to produce
potassium cyanide. The manufacture of yellow prussiate of potash may
be conveniently divided into three stages: (1) The production of the
molten mass technically known as “metal”; (2) the lixiviation; and (3)
the crystallisation.
(1) The “metal” is made by fusing animal matters with pearlash, almost
invariably with the addition of iron scrap. The animal substances are
sometimes used in their original condition, whilst sometimes they are
previously charred. Generally speaking, however, a judicious mixture
of the fresh and charred materials has been found to give the best
results. The charcoal which is left on carbonising animal matters
contains a certain amount of nitrogen, decreasing in proportion as the
temperature rises; but a smaller quantity of charcoal is also thereby
produced. For example: 100 parts of rags carbonised at a certain
temperature left 75 parts charcoal containing 12 per cent. of nitrogen,
while the same rag carbonised at a higher temperature yielded 25 parts
of charcoal, which contained only 2 per cent. of nitrogen. The animal
matters employed should not leave much ash on ignition, as this would
both thicken the mass and decompose a portion of the potash. In this
respect sand is specially objectionable, for on ignition 1 part will
decompose 2 of pearlash, owing to formation of silicate of potash. It
is not necessary that the pearlash should be quite pure; in fact, a
certain proportion of sulphate is stated to be useful, as it is changed
into sulphide by ignition with the carbonaceous materials.
The theory of the formation of yellow prussiate of potash may be
briefly stated as follows: The carbonate and sulphate of potash
react with the carbon, nitrogen, and iron, forming in the first
instance sulphide of potassium, which afterwards converts the iron
into sulphide, whilst potassium cyanide is simultaneously produced.
It should be here explained that ferrocyanide of potassium (yellow
prussiate) is not formed during the ignition of the above mentioned
materials, but results from the lixiviation of the fused mass with
water, when the cyanide of potassium and iron sulphide decompose
each other, producing ferrocyanide and sulphide of potassium. It is
quite obvious that even if any ferrocyanide were produced during the
process of fusion, it would almost immediately be decomposed, at the
intense heat to which the mass is subjected, into potassium cyanide,
iron carbide, and nitrogen gas. If any doubt were felt on this point,
the experiments of Liebig conclusively prove that the formation of
ferrocyanide takes place on dissolving the ignited mass in water, but
not previously. Liebig found that if the fused mixture be allowed
to cool, and then treated with moderately strong alcohol, potassium
cyanide alone is extracted, and the residue when dissolved in water
no longer yields ferrocyanide. As ferrocyanide is not formed during
the process of fusion, the presence of iron in the preliminary stages
may appear superfluous; but such is not the case. The presence of iron
is necessary for two reasons, firstly, because the sulphate of potash
which is generally present is converted into sulphide and bisulphide,
and these, in the absence of iron, would decompose some of the cyanide
of potash into sulphocyanate, thereby causing a loss of cyanogen so
far as yellow prussiate is concerned; and secondly, because potassium
bisulphide has a very corrosive action on the iron pot in which the
fusion takes place. When iron is present it readily decomposes any
alkaline sulphides, thereby preventing formation of sulphocyanate, and
being itself converted into iron sulphide, which is again changed into
prussiate by the action of the aqueous cyanide.
Pear-shaped iron pots were formerly used for fusing the raw materials.
The arrangement now generally adopted in large English works consists
of a series of iron pots almost hemispherical in shape, set in
brickwork, and each heated by a separate fire and circular flue. These
vessels are closed by iron lids, with apertures for the admittance of
animal matters, the aperture being at once closed by a slide after each
addition. Through every lid there passes a vertical spindle, carrying a
set of blades for mixing the materials, and set in motion by a suitable
shaft worked by steam power. Instead of the ordinary iron pots,
reverberatory furnaces are often employed, especially in Germany. The
reason for this preference is, that ordinary iron vessels are worn out
in a comparatively short time, the destructive action being greatest on
the under surface of the muffle. A much larger quantity of raw material
can also be operated upon at one time if a reverberatory furnace be
used. The mode of procedure depends to some extent upon the condition
of the organic materials employed. If fresh, the muffle or furnace must
be left open, so as to permit the mixture to be well and frequently
stirred, and additions to be made at intervals until eventually
ammonia ceases to be evolved. The furnaces are arranged in such a
manner that when the carbonate of potash has once become fused the
doors of the fire-place may be shut, and no fresh firing is required
during the introduction of the animal matters. The molten mass is kept
well stirred by means of a thick iron bar, suspended by a chain, and
fixed in an aperture in the side of the furnace. By the use of this
arrangement the stirring is much more easily and thoroughly effected
than is the case with the old fashioned pots. Ordinary reverberatory
furnaces cannot be used for the fusion, because the silica in the
hearth would combine with the potash to form silicate of potash.
Gas generators with air blast are now sometimes employed instead of
ordinary fuel in the manufacture of yellow prussiate of potash. Several
advantages are gained by operating in this manner, especially that of
permitting the regulation of temperature and the admission of oxygen,
so as to obtain an ordinary, a neutral, or a reducing flame, according
to requirements. In the preparation of the “metal,” for every 100
parts of pearlash from 100 to 125 parts of fresh animal substances are
required, together with 6 or 8 parts of iron in some form or other. The
pearlash, or a mixture of 1 part of pearlash with 2 to 4 parts “blue
salt” or “blue potash” (this substance will be referred to later on),
is melted in the furnace and heated to bright redness, so that the
temperature of the mass may not be reduced too much by the addition
of the animal matters. These, in their original condition, or an
equivalent quantity of carbonised materials, together with the proper
proportion of iron, are then introduced--first pretty frequently,
afterwards at longer intervals. Each addition of animal matter causes a
somewhat violent frothing and escape of combustible gases, along with
water and carbonic acid, and the whole becomes thick--not so much owing
to the introduction of solid substances as by the fall of temperature,
resulting from the production of such large quantities of gas. In
order to hasten the decomposition, vigorous stirring must be applied.
When the reaction is at an end, the semi-fluid mass is transferred to
cast-iron dishes, and the furnace is again filled with carbonate of
potash and heated. In this way four or five charges may be accomplished
every day, and the process carried on continuously. The most favourable
conditions for effecting the melting part of the process are attained
when the heat approaches whiteness, and a bright, clear flame is
produced as soon as the raw materials are introduced. According to one
authority, woollen rags and good pearlash, with a small proportion
of waste iron, have produced the largest yield of yellow prussiate,
although even in this case two-thirds of the total nitrogen present was
lost in the form of ammonia.
(2) Lixiviation.--The fused mass, if properly prepared, should yield
about 16 per cent. of prussiate on dissolving in water. In this part
of the process, the “metal” when cold is broken into lumps and placed
in cold water mixed with the weak lyes from former operations. Heat
is then applied until the temperature rises to about 180°-190° F.,
and the liquid is stirred vigorously so as to promote rapid solution,
because some of the potassium cyanide is apt to be decomposed during
lixiviation. When the solution attains a density of 30°-40° Tw. it
is left to clarify, the heat being withdrawn. The clear solution is
decanted, and evaporated in pans, which are generally heated by the
waste heat of the furnaces. When it has a density of 54° Tw. it is run
off into the crystallisers, where it deposits the crude salt.
(3) Crystallisation.--This is a very important stage of the
manufacture, as it is the final process by which the crude prussiate
is rendered sufficiently pure to be placed on the market. The impure
substance is dissolved in warm water until the solution stands at 54°
Tw.; after all insoluble matter has deposited, the clear liquor is
placed in the crystallising vessels. These are occasionally made of
wood; but when such vessels are used, the crystallised salt generally
possesses a green colour, which is believed to be due to the tannin
present in the wood. On this account cast-iron crystallisers are more
frequently employed. The crystallisation proceeds slowly--often going
on for several weeks in large vessels. The mother liquor is then drawn
off, and if not too impure is used for dissolving fresh quantities
of the crude prussiate. The ferrocyanide is deposited in crusts in
the crystallisers; but by hanging lumps of the solid salt in the
solution, long clusters of crystals may be obtained, and by suspending
these in fresh prussiate lyes immense crystals are produced. From 100
parts crude prussiate about 90 parts pure potassium ferrocyanide are
obtained, or sometimes in the case of purer materials 97 parts.
Sulphate of potash is often present in commercial yellow prussiate.
The separation of this impurity is best effected on the large scale by
evaporating the prussiate solution to a density of 62° Tw., at which
point most of the sulphate will crystallise out. If the clear liquor be
then drawn off, diluted to 52° Tw., and allowed to cool, almost pure
potassium ferrocyanide will gradually deposit. This may be rendered
absolutely pure by gently fusing the crystals, dissolving in water, and
treating with a small quantity of acetic acid, which will decompose
any carbonates and cyanides. On adding sufficient strong alcohol, the
ferrocyanide is precipitated, and when crystallised once or twice more
from water it may be regarded as chemically pure.
Blue salt.--This substance, to which we have previously referred, is a
residue obtained in the manufacture of prussiate of potash. The last
mother-liquor contains a large quantity of carbonate of potash, along
with smaller amounts of hydrate, silicate, chloride, and sulphocyanate.
It is concentrated until the liquid has a density of 90° Tw., when most
of the chloride, silicate, &c., separates out, and the strong liquor
containing the greater proportion of the carbonate is evaporated to
dryness, and calcined in a reverberatory furnace. The dry residue
constitutes the “blue salt” or “blue potash,” and contains from 70
to 80 per cent. carbonate of potash. It may be employed instead of
pearlash, or mixed with it, for the next batch of yellow prussiate. The
composition and amount of the insoluble residue left on lixiviation of
the “metal” vary according to the proportions and character of the raw
materials used. Other conditions being equal, horn gives the lowest
percentage of insoluble matter on lixiviation.
The large proportions of potash and phosphates contained in the
insoluble residues render them well suited for use in the manufacture
of artificial manures. As already mentioned, when regarded from a
scientific or economical point of view, the yellow prussiate industry
is carried on under very imperfect conditions. In addition to the
amount of potash, there is a very considerable waste of nitrogen,
firstly, because the larger proportion of that element present in the
animal substances is not converted into cyanogen at all, but passes
off chiefly in the form of ammonia salts; and, secondly, because
part of the potassium cyanide which is actually produced is lost by
decomposition, and another portion is left in the mother liquor. It
has been calculated that out of every 100 parts of ferrocyanide which
should theoretically be obtained, 4 parts are lost when fairly pure
materials have been employed, and 14 in the case of impure ingredients.
The following analyses indicate the percentage composition of two
samples of insoluble residue:--
No. 1. No. 2.
Sulphate of potash, &c. 9·06 3·21
Phosphates of lime, magnesia and iron 13·74 6·24
Oxide of iron 13·34 19·58
Lime and magnesia 5·08 7·23
Sand and silica 23·97 29·24
Charcoal and moisture 34·81 34·50
------ ------
100·00 100·00
According to Karmrodt, the following proportions of the nitrogen
contained in various animal substances are actually converted into
cyanogen during the manufacture of yellow prussiate of potash:--
Per cent.
Woollen rags 16
Horn 20
Leather cuttings 33
Cow hair 14
Dried blood 16
Horn charcoal 56
Rag charcoal 33
As is well known, human excreta contain a considerable proportion of
nitrogen, and there seems no reason why this should not be employed
in the manufacture of yellow prussiate. It is quite possible that
municipal bodies might find this a convenient and profitable plan
of disposing of a portion of the sewage with which they have to
deal. It is obvious to all persons who have given this subject much
consideration, that the nitrogen required in the manufacture of yellow
prussiate of potash might be obtained with comparative ease from the
surrounding atmosphere. Indeed, from a theoretical point of view this
seems a charming process. About fifty years ago the Society of Arts
awarded Lewis Thompson a medal in connection with this very process.
Thompson ignited a mixture of 2 parts pearlash, 2 parts coke, and 1
part iron turnings in an open crucible for a considerable time at a
full red heat. The resulting black mass was found to contain a large
quantity of ferrocyanide, together with excess of carbonate of potash,
&c. This process, or a similar one, in which a current of air was
passed over a mixture of charcoal and iron saturated with carbonate of
potash, was tried on a large scale for two years at Bramwell’s works
at Newcastle. About 1 ton of yellow prussiate was made daily by this
process; but it was not found to work profitably, and was eventually
abandoned, chiefly, it is said, owing to the large amount of fuel
required, and because the cylinders, whether of iron or fireclay, were
not able to stand for any length of time the intense heat to which they
were subjected.
The annexed illustrations, Figs. 14 to 17, show the arrangement of a
prussiate of potash furnace at Sir E. Buckley’s works, at Clayton,
Manchester, which are well designed to prevent nuisance: A, iron pot;
B, fire-place; _a_, cover of pot; _b_, stirrer; _c_, hinged pipe
conveying vapours to the flues; _d_, flues surrounding the pot, and
leading to the chimney-shaft; _e_, chain to lift up cast-iron vapour
hood.
Brunquell, a German manufacturer, has criticised the present method
of conducting operations, and proposes that it is necessary as far
as practicable to aid the secondary formation of cyanogen by ammonia
and incandescent charcoal, and to avoid loss of potash by using pure
animal substances, and preventing contact with the solid products of
combustion from the furnace. With this view he adopts a horizontal
reverberatory furnace, the hearth of which is a cast-iron tray about
4½ ft. long, 4 ft. wide, and 3½ in. deep. The crown of the furnace is
built as flat as possible, the working space is limited, and the charge
is kept from contamination by the fire. Such a furnace, despite certain
drawbacks, presents important advantages. Fuel is economised; the
process is hastened so that seven or eight charges can be dealt with
in a day, instead of only four; and the furnaces cost less and endure
longer. The charge consists of 220 lb. potash, of which two-thirds is
from evaporated mother-liquors, and one-third fresh; 44 lb. animal
charcoal from the carbonisation of substances poor in nitrogen; 140-150
lb. of pure animal matters as dry as practicable; and 17½ lb. iron. The
firing is urged and the charge is stirred till all the potash is fused,
when the ash-pit is closed, and the damper turned on for charging half
the animal charcoal. The firing and stirring are again pushed on till
the proper consistency is attained, and potassium vapour begins to burn
off. In this state the mass is ready to receive the animal
[Illustration: Figs. 14, 15, 16, 17.--YELLOW PRUSSIATE
FURNACE.]
substances, those rich in nitrogen being first added in small portions
at a time. Their effect is to render the mass hard, dry, and difficult
of fusion, whereupon the remainder of the animal charcoal should be
promptly introduced. After thorough agitation, the working door is
closed for a short time, and the contents of the furnace are rapidly
discharged into a covered iron pan.
The character of the animal matters employed varies so much that it is
impossible to lay down hard and fast rules for the proportions of the
several ingredients, or the duration of the roasting. Nor is the value
of a raw material always in proportion to its richness in nitrogen,
because the poorer material may waste less potash, consume less fuel,
and require less labour. The addition of iron filings or turnings is
useful only in prolonging the life of the cast-iron crucibles.
_Combination of the Cyanide and Iron Solutions._--A great number of
recipes are in vogue for combining the two solutions of ferrocyanide
and an iron salt, both with reference to their proportions, and to the
addition of foreign matters of various kinds. These variations in the
formulæ give rise to distinct names for some kinds of Prussian blue,
which will be referred to below. The ordinary common Prussian blue has
a greenish tendency, and is chiefly made according to one or the other
of the following directions:--
(1) Mix a solution of 100 lb. yellow prussiate with a solution of 100
lb. green copperas (ferrous sulphate) and 18 lb. alum, to which 9 lb.
sulphuric acid has been added, and let the mixture stand for 2-3 hours,
or until the solid portion has completely settled out. Decant the clear
supernatant liquor, and well wash the precipitate with clean waters.
Finally throw it on a filter and subject it to repeated disturbance,
so as to ensure the admission of air to every particle, in order that
the requisite oxidation may take place. The proportion of alum used
is subject to very great variation according to individual fancy; it
renders the subsequent grinding of the pigment a very much easier
matter, but it causes the shade of blue to be paler than it otherwise
would be.
(2) The simple solutions of green copperas and yellow prussiate in
equal proportions are mixed together without any other ingredient being
added, and the precipitate produced is washed, filtered, and aërated as
in (1). It is, however, inferior by reason of the oxide of iron formed
in the pigment spoiling the purity of the colour, and necessitating the
treatment of the wet mass with hydrochloric acid, at some expense, for
removal of the iron oxide.
ANTWERP BLUE.--This pale variety of Prussian blue has but
little importance now. It is prepared by adding a solution of 4 lb.
yellow prussiate in 5-6 gallons of water to one of 2 lb. sulphate
of iron, and 1 lb. each of alum and sulphate of zinc in an equal
quantity of water. The resulting pigment consists of a mixture of the
ferrocyanides of iron, alumina, and zinc; it is washed, filtered,
aërated, and dried as other forms of Prussian blue.
BONG’S BLUE.--When cyanide of potassium is added to an
acid solution of a copper salt, a red colour is produced, which has
already been mentioned by different observers. The substance formed
is very changeable, at least in the liquid where it is formed. It
is decomposed by acids, alkalies, cyanide of potassium, and even
decomposes spontaneously, the colour changing to yellow. It is
precipitated by insoluble cyanides; hence when a dilute acid is added
to the red solution, the dye is at once thrown down along with the
cyanide of copper. If the precipitate thus obtained is treated with
sulphuretted hydrogen, it is decomposed and the substance is set free.
This substance can combine with iron, like cyanogen, so as to conceal
the properties of the iron. This compound is very permanent, and has
lately been studied by Bong, who gives the following directions for its
preparation:--
Cyanide of potassium is added in excess to an acid solution of a
copper salt until the red colour at first formed has disappeared,
when a ferric salt is at once added. On the addition of the iron salt,
of course, a copious precipitation of Prussian blue takes place,
and the liquid again turns to a dark purple-red. To separate the
colouring substance from the alkaline salts in the liquid, a dilute
acid is added, which precipitates it and the cyanide of copper. This
precipitate is combined with the Prussian blue, which also contains
a considerable quantity of the colouring substance, and then treated
with a boiling solution of carbonate of ammonia, in which it dissolves.
As the cyanide of copper also goes into solution, it is separated by
again precipitating it with an acid, and treating the precipitate with
sulphuretted hydrogen. The colouring substance thus liberated now
contains a certain amount of hydroferrocyanic acid, which is removed
after neutralisation by acetate of lead. It is now filtered, and the
purification is completed by precipitating with a silver salt and
treating the precipitate with sulphuretted hydrogen.
This purple-coloured compound crystallises very indistinctly. To
determine its composition, Bong precipitated it with acetate of copper.
When dried at 212° F., the rose-coloured precipitate had the following
composition: Carbon 24·31, nitrogen 28·04, hydrogen 1·88, iron 13·66,
copper 17·67, oxygen 14·44. Total 100·00. These numbers correspond to
the formula Cu, Fe Cy_{4} (HO)_{4}.
This substance is likewise precipitated by salts of zinc, mercury, and
silver. All these precipitates are pink or purple, very beautiful, and
of remarkable brilliancy. They are soluble in alkalies. Iron salts
yield no precipitate, nor do lead salts, except in the presence of
ammonia, when a blue-violet precipitate is formed. When treated with
sulphuretted hydrogen, these precipitates yield purple-red and acid
liquids, which undergo change in the air, especially if warm, forming
Prussian blue. When these liquids are neutralised with alkali, purple
compounds are formed, which are permanent in the air, soluble in water,
slightly so in alcohol, and insoluble in ether. Their colouring power
is exceptionally great. These pigments will unite with ferrocyanides,
and in its preparation such a compound is produced in considerable
quantity; it is likewise of a purple colour, and gives a rose-coloured
precipitate with acetate of lead. Both alone and in this compound it is
very permanent; it resists the action of sulphurous acid, concentrated
and boiling alkalies, and dilute acids, but is rapidly destroyed by
chlorine and nitric acid. If this pigment could be prepared cheaply
enough, it would probably be used with advantage in the arts, on
account of its resistance to chemical reagents and light, the variety
of its shades, and its brilliancy. It does not colour fibres directly,
but can readily be fixed on them from slightly acid solutions, if they
are previously mordanted with metallic oxides.
BRUNSWICK BLUE.--This pigment is made in pale, medium, and
deep shades, and is an extremely useful colour, being very fine,
requiring no grinding, thoroughly permanent in light and air, hardly
acted upon by acids, but turned brown by alkalies, and liable on
standing to separate into two portions--a white and a blue--the latter
coming to the surface while the former sinks, and necessitating a
thorough stirring of the paint before use.
It generally consists simply of barytes, or gypsum, or china clay,
coloured by a small percentage of Prussian blue, with or without the
addition of a lesser proportion of ultramarine. The barytes or other
base is very thoroughly agitated in water, while a solution of green
copperas and a solution of yellow prussiate are gradually added without
ceasing the agitation. When the incorporation of the ingredients has
been completely accomplished, the precipitate is settled, washed,
filtered, and dried. Following are a few recipes:--
Pale. (1) 1 cwt. barytes, 1 lb. green copperas, 1 lb. yellow
prussiate.
Pale. (2) 1 cwt. china clay, 2 lb. green copperas, 2 lb. yellow
prussiate.
Pale. (3) 1 cwt. gypsum, 1½ lb. green copperas, 1½ lb. yellow
prussiate.
Medium. (1) 1 cwt. barytes, 3 lb. green copperas, 3 lb. yellow
prussiate.
Medium. (2) 1 cwt. china clay, 6 lb. green copperas, 6 lb. yellow
prussiate.
Medium. (3) 1 cwt. gypsum, 4½ lb. green copperas, 4½ lb. yellow
prussiate.
Deep. (1) 1 cwt. barytes, 5 lb. green copperas, 5 lb. yellow
prussiate.