THE MANUFACTURE OF ULTRAMARINE.
We have thought it necessary to an understanding of the manufacture
of ultramarine to discuss the production of this important pigment in
some detail, for it is only possible to properly conduct an operation
regulated by chemical laws when the processes which take place are
accurately known.
The mixture of the materials used in making ultramarine must be most
intimate in order that the constituents may act chemically upon one
another. In some works the mixing is performed in a very laborious
manner—upon a heap of the dried, levigated clay the remaining materials
are thrown, and the whole shovelled about till it is completely
homogeneous. Naturally, instead of this primitive and costly operation,
mechanical mixers may be used, but the mixture can also be effected
in a simpler manner, which almost dispenses with the necessity for
a mechanical process. Of the substances to be mixed only kaolin and
sulphur are insoluble in water; the others and also the by-products
of previous processes are easily soluble. The soluble and insoluble
constituents of the charge may be mixed in a very simple way by
bringing the levigated kaolin in the form of paste, without drying,
into a pan, adding the solutions of the salts and the powdered sulphur.
The pans are most conveniently heated by the fire gases from the
ultramarine furnaces. After mixing the solutions with the clay to
a thick paste the heating is commenced, the solid substances are
prevented from sinking to the bottom by continual stirring, the
mixture is slowly evaporated, and an extremely intimate and complete
incorporation of the constituents occurs. The heating is continued
until a dry mass is obtained, which may be used for the second
operation without powdering.
The mixture of raw materials is heated for a long time at a clear red
heat, or, in some works, at a white heat. The operation demands some
care; air must be excluded and the whole mass must acquire a uniform
temperature. If both these conditions are not obtained in heating it is
very difficult to obtain a product of uniform colour.
In different works different arrangements are used for the calcining
process. In the older methods crucibles or dishes of fire-clay were
used, of such a shape that the bottom of one crucible formed the lid
of the crucible beneath, just as is the case with the saggers used in
porcelain kilns. The mixture is closely pressed into the crucibles, of
which piles are built in the furnaces. These must be so arranged that
each crucible is surrounded by the fire. The furnaces are very similar
to porcelain kilns.
This method of calcining is obviously attended with many drawbacks; a
large number of costly crucibles is required, a certain proportion of
which is lost at each burning, either by breakage in the fire or in
filling. The pressing of the charge into the small crucibles requires
much labour, as also the placing of the crucibles in the furnace and
their removal; at the end of the nine to ten hours’ calcining, it is
necessary to wait some time to take the crucibles out of the furnace,
or considerable loss will be caused by breakage of the crucibles due to
rapid cooling.
For these reasons in most works crucibles have been abandoned in favour
of muffles, a large number of which are built in one furnace. The
charge is placed in the muffles; after completion of the calcining it
is quickly raked out, and the still hot muffle at once recharged, thus
the losses of time and heat are reduced to a minimum. The muffles are
generally about 1 metre long, 1 metre wide and 30 to 40 centimetres
high. They are made of fire-clay, and, when kept in uninterrupted use,
last a long time. The muffles have a small opening at the back which
communicates with the furnace, so that the gases evolved have a free
outlet without reaching the working place. The front of the muffle is
closed by an iron plate in which are small openings, through which the
contents can be examined without the entry of much air.
Three muffles are generally built in a furnace, but, by suitable
alterations in the construction, a considerably larger number may be
heated in one furnace. Whatever the number of muffles, the furnace must
be so arranged that the fire can be controlled at will. This is best
done by a good damper; all the muffles must be uniformly heated.
J. Curtius recommends cast-iron cylinders, lined with a thin layer of
fire-clay. For this purpose a covering of fire-proof cement is used to
protect the retorts. This covering, as it is gradually destroyed, may
be renewed by smearing over the damaged places with fire-proof cement,
mixed with water or some binding material. Aluminium silicate, graphite
and coke may also be used. The retorts, _a_ (Fig. 28), project through
the wall of the furnace at one end. They rest on fire-proof stone
bridges, and can be put into communication with the air by the pipe,
_m_, which can be closed air-tight. Short pipes, _k_ and _d_, which
can be closed, connect the interior of the retort with the cooling and
collecting chambers, _P_ and _g_, the latter of which is connected by
_r_ with the vessel, _h_, where the gaseous products are absorbed. The
porous plate, _f_, prevents the charge in the hinder part of the retort
from becoming open and falling to pieces during stoking. In order to
obtain a regular heating of the retorts the fire gases passing through
the space, _q_, are led away at two opposite places by flues into the
main flue, _F_. The intimate mixture of finely-powdered materials
is introduced into the hinder part of the retort, the porous plate,
_f_, placed in position, the cover, _t_, closed up air-tight, and the
connection with the chamber, _P_, closed. On heating, the volatile
products pass through the porous plate and the pipe, _d_, into the
chamber, _g_, the more volatile portions, without mixing with the
furnace gases, passing direct to the absorption vessel, _h_, or to a
lead chamber, whilst the sulphur which distils over remains in _g_.
When the reaction is finished, by opening the cover, _t_, and the tube,
_m_, the crude green ultramarine in the retort can be rapidly oxidised
to blue by aspirating air at _m_, or after closing the opening, _d_,
and removing the plate, _f_, the green ultramarine can be raked into
the collecting chamber, _P_, and there oxidised. The retorts are then
immediately re-filled with the mixture and the temperature raised.
[Illustration: FIG. 28.]
The heating of a charge lasts nine to ten hours; the larger the
quantity of sodium sulphide in the mixture the shorter is the time
required. The mixture is spread out in the muffles in a layer 7 to
9 centimetres thick, and brought to a moderate red heat. When the
whole mass is uniformly hot it is more strongly heated; the change
of the constituents then becomes visible. The mixture takes at first
a brownish colour, and is somewhat similar in appearance to liver
of sulphur. The colour soon begins to incline to green, and finally
changes to a tolerably pure green, but always has a yellow tinge. Those
parts of the hot mass which come into contact with air are more blue,
and occasionally change to a pure blue. When crucibles are used the
upper layer of the contents is always bluer than the lower, and when
crucibles crack during the heating the portions near the cracks always
show distinctly a more or less blue colour.
When the calcination is finished the hot mass drawn out of the muffles
is at once brought into washing vessels, in which it is treated with
water so long as soluble materials are extracted. The compounds
dissolved in the water are chiefly sodium sulphide and sulphate and
a little alumina dissolved in caustic soda. The presence of a larger
quantity of caustic soda in the wash waters indicates too violent
heating. In this case the mass will not mix up easily in water; it
contains sintered lumps. The washed material is strained, spread out
on boards or linen stretched across frames, and completely dried by
artificial heat.
At this stage the product may be described as green ultramarine and
sold as such, or it may be converted into blue ultramarine. For the
latter purpose it is ground to a moderately fine meal, but green
ultramarine must be ground to a very fine powder; the finer the powder
the brighter the shade.
In order to obtain blue ultramarine from the washed and ground
substance, it is subjected for a short time to a moderate red heat
with unrestricted access of air. The heating by which the blue colour
is produced must be continued until the proper shade is obtained.
This operation is conducted in special muffles; in many works it is
customary to sprinkle powdered sulphur upon the heated mass during the
roasting. The sulphur burns, forming sulphur dioxide, which escapes
into the furnace through an opening at the back of the muffle. The
object of the addition of sulphur may be to keep the temperature from
rising beyond a certain point, or to prevent a possible reduction of
the sulphur compounds in the ultramarine.
The muffles used for finishing the ultramarine are similar to those
used in making green ultramarine; they are generally 50 centimetres
wide and 100 to 120 centimetres long. A muffle of this size will
contain 6 to 18 kilogrammes of green ultramarine. The temperature is
gradually raised to a gentle red heat, each muffle being provided in
front with an iron plate which prevents the cooling of the contents by
the external air. When the charge in the muffles is carefully observed,
it is noticed that the blue colour first appears at the surface and
the edges, _i.e._, at those places where the oxygen of the air has
unrestricted access. The mass is frequently turned over with an iron
rake, in order to expose all parts to the action of the air; samples
are frequently taken in order to observe the moment at which the
colour has reached its greatest intensity, when the heating is at once
stopped. By proper treatment about half an hour is required to convert
the charge of a muffle of the above dimensions into ultramarine of the
deepest blue. The hot mass is drawn out of the muffles and spread out
upon flags so that it may cool quickly. During the cooling the colour
is often observed to become considerably darker, which is a sign that
the ultramarine has not been heated for a sufficient length of time.
Mechanical arrangements are often used to continually turn over the
ultramarine in the muffles, which are then made in the form of
cylinders in which the ultramarine is continually turned by a stirrer
provided with wings. The whole arrangement is very similar to the
apparatus used for roasting coffee.
The ultramarine is then carefully ground to a very fine meal; it is
tolerably hard, and granite stones should be used, ordinary stones
would be too quickly worn down. It is not possible, by the most careful
grinding, to convert ultramarine into that condition of fine division
which is necessary if it is to be ground into paint; grinding must be
followed by levigation. The various qualities and shades brought into
the market differ only in regard to the fineness of their particles;
they do not vary in chemical nature, for the substance has always the
same composition.
It occasionally happens in ultramarine works that a charge does not
turn out a bright blue; the product is then sold as inferior quality
at a lower price, or is mixed with a larger quantity of good material.
In working by a settled process the different shades of ultramarine
vary in composition between narrow limits; the difference is caused by
variations in the raw materials.
It has been already remarked that there is no agreement as to the
composition of the colouring principle of ultramarine; some maintain
that it is an iron compound, whilst others regard the iron found in
ultramarine as an accidental impurity, which has no connection with
the colour. Recently this point appears to have been decided from the
results of many most accurate analyses of ultramarine. It has been
found that its chemical composition shows the greatest similarity with
the mineral nepheline.
Nepheline is a double silicate, its composition is expressed by the
following formula: Na₂O.SiO₂ + Al₂O₃.2SiO₂. By a comparison of the
analyses of green and blue ultramarine with that of nepheline, it is
seen that blue ultramarine may be regarded as nepheline combined with
sodium pentasulphide, whilst green ultramarine is nepheline combined
with sodium bisulphide. If this is the correct view, the two species of
ultramarine have the following formulæ:—
2{Na₂O.SiO₂ + Al₂O₃.2SiO₂} + Na₂S₅.
2{Na₂O.SiO₂ + Al₂O₃.2SiO₂} + Na₂S₂.
Ultramarine is one of the most permanent pigments. It is not altered by
the substances with which a pigment generally comes in contact. Being
a sulphur compound, it retains its colour completely in air containing
sulphuretted hydrogen. By acids, even weak organic acids such as malic,
citric or tartaric, it is rapidly decomposed, sulphuretted hydrogen
being evolved, and a greyish white residue left. It is sometimes found
that the sugar used in making lemonade produces a perceptible smell of
sulphuretted hydrogen. This is due to ultramarine, which has been added
to the sugar to hide its yellow colour.
From the preceding account of the manufacture of ultramarine it will be
understood that there exist only two species of ultramarine, green and
blue. In commerce a large number of different shades are found, which
are not to be regarded as pure ultramarine; they owe their particular
shade to additions. By mixing in a white pigment, such as barytes or
starch, the paler shades are obtained; by adding a small quantity of a
pure red pigment a colour with a violet tinge is produced. In a similar
manner any number of shades of ultramarine can be obtained.
The following account is taken from a new comprehensive work by J.
Wunder, on the different ultramarines.
=Preparation of Mixtures for Ultramarine.=—The following points are
to be observed: Chemically pure soda and ammonia soda react with
difficulty. The presence of caustic soda in the soda facilitates the
formation of sodium sulphide, and gives a finer product. It is also
of advantage to sprinkle the soda with a strong solution of sodium
sulphide. The more silica the mixture contains the more difficult is
the transformation into ultramarine, but the product is deeper and
better in shade, and has more resistance to alum and weak acids. The
sodium sulphide must react with silica and alumina. When oxygen enters
during the burning, silica is re-formed, which with soda and alumina
produces slags. In order to exclude oxygen many manufacturers burn
with restricted access of air; then the carbon bisulphide and sulphur
gases evolved are not burnt, but, together with tarry materials from
the coal, form an evil-smelling smoke, which renders the neighbourhood
objectionable. The coal consumption in burning is also greater. The
varieties rich in silica and stable towards alum are generally dark
reddish-blue. In order to obtain pure blue shades from mixtures rich in
silica they are burnt in closed crucibles to produce the green, which
is powdered and heated with restricted air supply, and the admission
of a little steam, to 160° to 180° C., when pure blue or greenish-blue
shades are obtained as required. In this way a mixture rich in silica
can be burnt to a good blue, containing 69·32 parts of silica to 30·67
parts of alumina, or 1 equivalent of Al₂O₃ to 3·84 equivalents of SiO₂;
whilst ultramarine rich in silica, made in the ordinary way, contains
at the most 66·7 parts of silica to 33·3 parts of alumina.
=Ultramarine Violet= was first introduced into commerce about the
year 1859 by the Nuremberg Ultramarine Works, under the direction of
Leykauf. By the reaction of moist calcium chloride on ultramarine
blue in the warm chambers above the furnaces the blue was changed to
violet. By the action of air a portion of the moist calcium chloride
is decomposed into lime and hydrochloric acid, the latter of which,
together with air, reacts on the blue. In 1872 Wunder, intending to
apply to the estimation of sulphur in ultramarine blue a method which
gives good results with metallic sulphides, passed chlorine over heated
ultramarine. The pigment was converted into a brownish-red substance,
which on washing combined with water and turned violet. The red
substance is not produced at every temperature; 300° C. is the most
favourable. On washing sodium chloride is dissolved; the washed violet
is free from chlorine, but has taken up water, which is expelled on
heating, when the violet turns to blue. A similar brownish-red body
is formed when sulphur trioxide is led over heated ultramarine blue,
and when sulphur chloride acts on warm ultramarine blue. When the
brownish-red chlorine compound is introduced into absolute alcohol, a
reddish-violet compound is produced, containing organic matter. Ethyl
chloride is formed at the same time. When ammonia gas is passed over
the heated chlorine compound it is absorbed; a violet ultramarine
is produced, from which the ammonia is not removed by washing with
water, but only by heating nearly to redness, or, better, by fusing
with caustic alkalis. Aniline combines in a similar manner. In order
to obtain a good violet the brownish-red chlorine compound must not
be formed; the violet is produced by leading chlorine and steam over
ultramarine blue at 160° to 200° C. Blue rich in silica is most
suitable for transformation into violet. The violet so obtained is not
decomposed by lime.
C. Mahla produced the chlorine in the blue itself by the reaction:
NH₄Cl + 2NH₄NO₃ = 6H₂O + 5N + Cl. By heating a mixture of ultramarine
blue with ammonium chloride and nitrate in crucibles at 200° C. a fine
violet is produced. It contains ammonia, which cannot be removed by
washing, but only by strongly heating or heating with concentrated
alkalis. This violet is decomposed by the prolonged action of lime,
in three days it is changed to a grey green. In the course of the
manufacture it was noticed that less nitrate was required in the
mixture according to the length of time it was exposed in porous
crucibles to the action of air and heat; by heating for a sufficient
length of time with access of air, a good violet is obtained with
ammonium chloride alone. In the manufacture of violet by means of moist
chlorine hydrochloric acid is formed, which is also the active material
in its formation by means of ammonium chloride. It was therefore to
be expected that chlorine gas might be replaced by hydrochloric acid,
if it were accompanied by sufficient air. This has been found to be
the case. The temperature must be maintained between 180° and 230° C.
Below 150° C. the blue is decomposed by moist hydrochloric acid gas; at
temperatures above 230° C. it is unchanged. Hydrochloric acid gas and
air, without steam, give with ultramarine blue on heating a brownish
red substance similar to that produced by dry chlorine; it also is
changed to violet by washing with water. The violet obtained by means
of moist chlorine differs from the blue from which it was formed, in
that the latter has lost one-sixth of its sodium and combined with
water and much oxygen. The following is the approximate formula for a
blue:—
Na₆Al₄Si₆S₄O₂₁,
and for the violet obtained from it
Na₅HAl₄Si₆S₄O₂₄ + H₂O.
The violet contains much thiosulphate. If it is decomposed by nitric
acid and silver nitrate added to the filtered solution a precipitate
is obtained which changes in colour from white to yellow, orange and
brown, just as the precipitate given by silver nitrate in solutions of
thiosulphates containing nitric acid.
=Chlorine and Steam Process.=—Chlorine does not attack iron at 150° to
250° C. The reaction is carried out in heated iron boxes 1 metre wide,
2 metres long and 65 centimetres high. The blue is spread out in a
layer 2 centimetres thick on earthenware plates, which stand one above
another at a distance of 5 centimetres, each supported on three feet.
The plates are introduced by means of iron tongs through large openings
in the top of the box; after filling iron plates are screwed on to the
openings, through them pass sheet-iron tubes reaching to the bottom,
in which thermometers can be lowered on wires. The iron boxes stand in
heated chambers, which have openings corresponding with the openings
in the boxes, and shut off from the interior of the furnace. Chlorine
and steam are led into the iron boxes at both ends through lead tubes
reaching to the bottom; the parts of these tubes in the furnace and the
iron boxes are protected by wide sheet-iron tubes, the space between
being filled with clay. The gases evolved, hydrochloric acid, sulphur
chloride and steam, pass from the covers through earthenware pipes into
boxes filled with limestone, upon which water drops, and thence to
the chimney. After filling, the boxes are heated to 280° C. and steam
introduced to remove sulphur; they are then allowed to cool to 160° C.,
when chlorine and steam are led in for three hours. The violet is then
finished, and the residual gases are blown out by a fan.
=Hydrochloric Acid and Air Process.=—The operation is conducted in
the same iron boxes, upon the bottoms of which, beneath the openings,
are earthenware dishes into which hydrochloric acid can be poured
through earthenware pipes. The chimney draught must be strong enough
to draw sufficient air through these pipes into the boxes. After the
temperature has been maintained for seven hours at 220° to 230° C.,
hydrochloric acid being poured in from time to time, the blue is
changed to a dull violet, which becomes brighter when hydrochloric acid
is repeatedly added at diminishing temperatures, 210°, 200°, 180° and
160° C. More recently the iron boxes have been replaced by stone chests
or by chambers above the ultramarine furnaces, the temperature in this
case being kept at about 200° C.
=Ammonium Chloride Process.=—A mixture of ultramarine blue with 5 per
cent. of ammonium chloride is heated during fourteen days in porous
crucibles placed in the upper chambers of the ultramarine furnaces,
when the contents of the crucibles become a handsome violet throughout.
If sodium nitrate is used together with ammonium chloride, the violet
is formed in a much shorter time. After washing, the violet still
contains nitrogen; on ignition or heating with strong alkalis it
loses ammonia. Unfortunately, this fine violet is decomposed by the
continued action of moist slaked lime. The violets made by the first
two processes absorb ammonia when this gas is led over them at 180° to
200° C., and it cannot be removed by washing.
=Pale Blue Ultramarine.=—If the violet is heated in hydrogen at 280° to
290° C., it is converted into a pure, bright, pale blue. This has an
absorption spectrum in which the red is not absorbed, but appears more
brilliant than in the spectrum of ultramarine rich in alumina. Pale
blue is turned violet blue by heating at 300° C., and at a red heat a
dull blue. It is not yet made on a commercial scale, but on account of
its great purity of shade it appears to be valuable for many purposes;
perhaps it may replace alumina cobalt blue. The composition of pale
blue is:—
Calculated for Found.
Na₅ 12·4 per cent. 11·9 per cent.
H₅ 0·54 ” 0·62 ”
Al₄ 11·7 ” 13·1 ”
Si₆ 18·2 ” 19·7 ”
S₄ 13·9 ” 12·7 ”
O₂₅ 43·3 ” 42·0 ” (by difference).
By a comparison of the composition of the violet and pale blue
ultramarines, it is seen that the chief difference is an increase of
hydrogen in the latter.
=Ultramarine Red.=—Since ultramarine violet increases in brightness
and redness of shade in the air, Wunder erroneously believed that
this was due to oxidation, and that, consequently, the violet could be
converted into a red by oxidising agents. Nitric acid vapours led over
ultramarine violet at 170° to 200° C. do not act upon it, but where
drops of nitric acid are spirted over, the violet is changed to red.
Wunder then reduced the temperature to 135° to 145° C. and obtained
the first ultramarine red. Iron is attacked by nitric acid at lower
temperatures, but not at 135° C.; the iron boxes previously described
could therefore be used. It was afterwards found that at a sufficiently
low temperature hydrochloric acid gas converts ultramarine violet into
red. The iron boxes cannot be used for this operation, as they are
attacked at the temperature; the stone chests are used instead. Other
acids also act on ultramarine violet; boric acid gives a reddish-violet.
The violet is spread out on the dishes standing on three feet mentioned
before, and heated to 128° to 132° C. At higher temperatures the violet
is unaltered, whilst below 100° C. it is decomposed. The hydrochloric
acid is poured in from time to time through earthenware tubes into
dishes in which it evaporates.
A mixture of red and blue would appear violet, but would behave
towards reagents in a different manner to real ultramarine violet.
Ultramarine blue is decomposed at 128° to 132° C. by hydrochloric acid
to a gelatinous mass, whilst at this temperature ultramarine violet is
changed into a bright red. From the blue no violet can be obtained by
nitric acid, but the violet gives a red at 135° to 145° C. Analysis
would also indicate the difference. Ultramarine red has the following
composition:—
Calculated for Found.
Na₃ 7·9 per cent. 8·1 per cent.
H₅ 0·57 ” 0·72 ”
Al₄ 12·3 ” 13·3 ”
Si₁₆ 19·1 ” 19·3 ”
S₄ 14·6 ” 15·2 ”
O₂₅ 45·6 ” 43·4 ” (by difference).
It appears that in the red two more equivalents of sodium have been
replaced by hydrogen. The violet is apparently a sodium salt of which
the red is the acid. The violet made by means of ammonium chloride
is also converted by hydrochloric acid gas at 128° to 132° C. into a
handsome red containing nitrogen, and the red is changed by hydrogen at
280° to 290° C. into a lighter pale blue.