black, though dull hue; it is a gritty powder, and has little value as
a pigment. Wood containing much resin, such as the wood of the pine,
rosin, fish oil, asphaltum, in short, all bodies which are at the same
time easily combustible and rich in carbon, produce on the contrary a
handsome, glistening soot, which forms a valuable pigment.

In addition to this difference in composition, soot from different
sources differs also in the size of its particles. Soot forms light
flocks, which adhere to projections in the flues through which the
products of combustion pass; the larger the flocks the sooner they are
deposited, the smaller they are the longer they remain suspended. The
finest particles of soot are called ”flying soot,” which is very highly
prized on account of its fine division.

The manufacture of soot blacks is a very important industry: black
printing ink, which is used in such enormous quantities, is made from a
soot black; in addition, all the best black paints and lacquers. Soot
black was formerly made in the most primitive manner, and is still, to
some extent, as will be seen from the account of the manufacture of
rosin black. The process used for printing blacks is more rational, but
is still capable of great improvements. The principles of a rational
manufacture of soot blacks will be briefly indicated.

=The Manufacture of Soot Blacks on the Large Scale.=—The principle
of the arrangements necessary for making soot blacks is very simple.
An apparatus is required in which substances very rich in carbon can
be burnt at the lowest possible temperature; this apparatus must be
connected with suitable arrangements for retaining the soot carried
away with the products of combustion. The soot works as at present
arranged are developments of the crude arrangements still used in
districts in which there is an abundant supply of pine wood containing
much resin. The apparatus used in such localities consists of a low
masonry flue connected with a long pipe built of wooden boards. In
order to give this pipe a rough surface upon which the soot can
readily deposit, it is lined in several places with coarse linen, to
the projecting fibres of which the soot adheres. In the flue the very
resiniferous wood is burnt, especially the roots of pines, which are
very rich in resin; these burn with an unrestricted air supply with a
bright flame, but when the air is restricted, they give off a large
quantity of a very heavy smoke. The operation is commenced by first
making a good fire of dry split wood, the object of which is to heat
the flue and thus prevent the deposition of soot in it in the later
part of the process. If a deposit of soot formed in the flue it might
take fire, the fire would then spread further into the wooden flue and
a considerable loss of soot would occur. When the stonework has become
so hot that deposition of soot in it is no longer to be feared, the
materials are introduced from which the soot is to be made. As has been
said, pine roots are generally used, the chips of pine wood are also
used in some districts, and in general such combustible materials as
produce a large quantity of smoke.

The combustion in the flue must be conducted with care, it should
proceed at the lowest possible temperature, but this should not sink
below a certain minimum. If the fire is too strong the greater portion
of the carbon is burnt, which would otherwise be obtained as soot; the
yield of soot is very small, and generally only ”dust soot” would be
obtained, without any quantity of ”flying soot”. On the other hand, if
too little air enters, so that the combustion takes place at too low
a temperature, a large yield of soot is obtained, but it is of poor
quality. Soot obtained by combustion at too low temperatures has not a
pure black but a perceptibly brown colour; it has not the flocculent
nature of the best product, a small weight of which occupies a very
large volume, but is greasy and has a high specific gravity. The
lower the temperature used in producing soot black, the further is
the chemical change removed from that of ordinary combustion, and the
nearer to that of dry distillation. Soot produced with an insufficient
supply of air contains a considerable quantity of liquid and solid
products of dry distillation, which give it a brown colour and the
above-mentioned greasy nature.

In many soot works the air supply is regulated in a most primitive
manner: the workman places against the openings by which air is
admitted to the burning materials a larger or smaller number of bricks
according as the combustion appears to be too rapid or too slow.
Unfortunately there is no clear sign to indicate the temperature most
favourable for the production of the greatest quantity of soot, the
appearance of the flame alone can be taken as a guide. If the flame
appears pure white and very luminous and shows at its end no thick
black smoke the combustion is very complete, and but a very small yield
of soot is to be expected. On the other hand, if the flame continually
threatens to go out the air supply is too small and a liberal amount of
soot will be formed, but mixed with a large quantity of the products
of dry distillation. As far as it can be described in words the flame
should appear as follows: The colour should be a murky red, similar to
that of the flame of a bad tallow candle; in shape the flame should be
a long drawn out tongue, from the point of which a thick black smoke is
clearly seen. The soot deposits in the long board flues in the form of
flocks or dust. The properties of the soot which is deposited in the
different parts of the long flue vary according to the distance from
the place of combustion, near to which a good black soot is deposited,
requiring, however, long grinding with oil or gum solution to form a
good paint. At a greater distance the soft and very fine ”flying soot”
is deposited; it is the most pure black and is regarded as the best
quality. The soot deposited farther on is more and more brown, and
has a more greasy nature the greater the distance from the place of
combustion.

The flue for the reception of the soot must be made so long that hardly
any smoke is perceptible at the end, which it is convenient to connect
with a chimney with a good draught, regulated by a damper. In this case
the combustion and the speed at which the products enter the flues are
twice regulated.

The material of which the flues are constructed through which the
products of combustion are led influences the manner in which the soot
is deposited; the portions nearest to the grate must be of stonework,
but when the smoke has cooled to a certain extent different materials
can be used in addition to wood; flues made of sacking stretched over
laths are used. The fine flying soot adheres to the roughnesses of the
coarse sacking, and is easily loosened by striking the wall of the flue.

As we have said, the arrangements of most soot works are very
primitive; they obtain only a fraction of the quantity of soot which
they could produce by a proper regulation of the draught in the
combustion chamber and in the flue in which the soot deposits. We
shall now briefly describe the conditions which should be regarded in
constructing a soot works; these conditions have not been found to be
observed in any works we have seen, in this branch of chemical industry
so-called practical experience is alone regarded, and consequently the
”practical” soot burner loses large sums, literally up the chimney. A
black works can be properly designed by any one who knows sufficient
chemistry to understand the process of combustion. Of this most
educated men have some understanding, but the practical soot burner
appears to have but hazy ideas on the subject, for one occasionally
finds the arrangements for making soot in direct opposition to the
proper disposition.

For the sake of the uniformity of the product and of safety against
fire, the flues in which the soot is deposited should be entirely of
masonry; the joints of the bricks should be smoothly cemented over,
so that soot cannot collect in quantity in them. The end of this flue
should be connected with a high chimney provided at the top with a
well-fitting damper, so that the draught in flues and chimney can be
regulated at will or completely stopped. Such an arrangement, perhaps
somewhat costly, has many important advantages: it is fire-proof,
and when once warm remains so for a long time, since bricks are bad
conductors of heat. When the flue is hot no water condenses in it; all
the water formed in the combustion remains in the form of vapour, and
is carried away by the chimney. A further advantage is that it is not
often necessary to enter the flue to collect the soot; burning may be
continued for a long time, and a large quantity of soot taken at once
from the flues or soot chambers.

The soot collects on the walls of the chambers in flocks, which finally
become so heavy that they fall off and collect on the floor. The entry
to the soot chambers should be through a single iron door, tightly
closed whilst the chambers are in use, and cemented round. If this door
does not fit air-tight the combustion cannot be exactly regulated by
the damper on the chimney. The soot is removed from the chambers by a
workman, who sweeps off with a soft brush into a sheet-iron vessel the
soot adhering to the walls and lying on the ground. It is extremely
important that soot and nothing else should be collected; the brush
used for loosening the soot should be so soft that it does not rub off
mortar from the brickwork; the workman’s shoes should be provided with
felt soles, so that no particles are loosened from the floor of the
chamber and mixed with the soot. The admixture of the smallest quantity
of sand would be extremely harmful in the ensuing grinding of the soot;
the mills would be damaged.

The flues for collecting the soot in well arranged works are very
similar, but the apparatus used for burning the materials which produce
the soot varies greatly according to the nature of the material. Some
quantity of the soot black used in the arts is still made by burning
pine roots and chips, but for the finer qualities American rosin
is largely employed. Earth-wax or ozokerite and the hydrocarbons
obtained from petroleum and in the distillation of shale are materials
frequently used in making soot blacks of very good quality. For
the finest qualities, such as are used for fine printing inks,
copper-plate inks and black lacquers, soot obtained by burning fish
oils or cheap fatty oils is most commonly used. The great differences
in the physical nature of these materials demand the use of different
apparatus for their combustion. The space allotted to soot blacks in
this work would be far exceeded if the construction of all the forms of
apparatus were described; the most important only will be given.

[Illustration: FIG. 30.]

When rosin is the raw material, the combustion can be conducted in flat
spoon-shaped vessels placed before a narrow opening into the flue. Fig.
30 shows an arrangement very successful in practice. The spoon-shaped
iron vessel, G, stands in a second, G₁, which is filled with water:
this prevents the fused rosin from becoming too hot. If the temperature
in G rose too high, dry distillation would take place along with the
combustion of the rosin, and the soot would be largely contaminated
by the products of this distillation. This may proceed so far that in
place of the fine flocks of soot a greasy mass is deposited in the
flues, consisting of a mixture of soot with products of distillation,
from which the soot could be obtained only with great difficulty. The
soot and gaseous products of combustion pass through the opening, O,
into the flues, R R. This opening is only several centimetres wide,
but is nearly as long as the combustion vessel. Above the vessel,
G, is a movable iron cover, D, in which are slides by which the air
supply is regulated. The cover is only taken off when fresh material is
introduced.

The air supply cannot be sufficiently regulated by the slides in the
cover; these must be used in conjunction with the chimney damper. The
combustion is observed through a thick glass plate let into the cover.
At the commencement of the operation slides and damper are completely
opened. When the chimney is seen to emit a thick black smoke the flues
are filled with the products of combustion, and the speed with which
they pass through the flues must be regulated. The current of air is
decreased until only a little visible smoke escapes from the chimney,
and the flame is no longer white, but a murky red. It should be noted
that the first soot obtained from a new works is never of the best
quality, the production of this is only gradually attained. The cause
of this is that the new masonry is damp and gives up water to the hot
gases, so that they are cooled and their velocity disturbed; the soot
will also be damp. Thus a greasy soot is obtained. It is of very little
use to allow the installation to remain unused for some months after it
is completed; the brickwork would only dry superficially, and when it
was first heated the presence of the water would become evident.

In order to dry the whole erection to such an extent that the moisture
from the brickwork has no effect on the regular course of the
operation, it is advisable to commence with materials of little value
and to allow a portion of them to be lost by the use of a stronger
draught than is generally used, so that the flues can be freed from
all moisture as soon as possible. This applies equally to all kinds of
arrangements for this purpose.

[Illustration: FIG. 31.]

Lamps are used to burn liquid fats and mineral oils. These lamps
naturally have a different construction to those used for lighting,
which are constructed with the object of burning all the carbon in
the oil, so that the temperature may rise as high as possible and the
carbon burn at a high white heat. The lamps used in the soot black
manufacture are designed to burn only as much carbon as is absolutely
necessary to maintain the flame, at the same time the temperature of
the flame must be kept low, so that no portion of the soot is again
burnt. These lamps have flat burners and are enclosed in a sheet-iron
casing provided with an air regulator, which must work very accurately,
otherwise air will enter between the joints and the working of the
regulator will be deceptive. So that the material to be burnt shall
not be too strongly heated, which would be accompanied by great loss
when mineral oil is used, the reservoir should be placed outside the
iron casing which surrounds the burner.

Fig. 31 shows the construction of a soot lamp. The flat burner, B,
is placed in the cylindrical sheet-iron mantle, H, which is bent
above at not too sharp an angle. The products of combustion are led
into a chamber, K, from which they pass into the flues where the
soot is deposited. The form of the upper portion of the cylinder is
of importance; if this is bent at right angles, soot accumulates on
the angle, and when a mass has formed it falls off, and is partly
burned in the flame. If the cylinder is given a proper bend no soot is
deposited in it, but it is all carried away into the soot chambers.
The air regulator, S, is placed at the bottom of the cylinder; it must
turn easily. The larger the slits are made by the rotation of this
regulator, the more oxygen enters the flame, and the more vigorous
is the combustion. A small, well-fitting door is placed in the lower
part of the cylinder, through which the wick can be reached; opposite
to this a glass plate is inserted, so that the flame may be observed
without opening the door. The wick is raised or lowered by means of the
screw, R.

The oil reservoir, O, must be outside the cylinder: in the older
arrangements it is so placed that the wick sucks up the oil; the
workman who is in charge of the lamps must then take particular care
that the reservoirs always contain the proper quantity of oil, if he
neglects to keep the oil at the proper level the wick is charred; it
then sucks up too much oil, which cannot completely burn and is chiefly
distilled, so that the soot is oily and cannot easily be treated
afterwards. The most diligent and attentive workman who is in charge
of a large number of lamps may easily allow one of them to run short
of oil. With the oil reservoir above depicted, a new quantity of oil
flows only when the level of the liquid sinks below the line, U. As
soon as a small quantity is used air enters the reservoir, O, in place
of which oil runs out until the opening, U, is again closed by the
liquid. This construction of lamp only works well when thin oils are
burned, and great care must be taken that the lamps are kept constantly
clean.

Lamps of all constructions have some drawbacks, they must always be
cleaned, and losses of oil occur in filling. These defects are obviated
when, instead of providing each lamp with its own reservoir, a single
one is used for a large number of lamps, which are automatically fed
from it. In this case the attendant has only to regulate the air supply
to the lamps, and to see that the mechanical arrangement by which the
oil flows to the different lamps is working properly. When the lamps
are automatically fed, the burners must be firmly fixed, and all in
the same horizontal plane. A pipe connects each burner with a main
pipe running under the lamps, which latter pipe is connected with the
reservoir, and this in its turn is connected with another reservoir
placed a little higher. In the pipe connecting the two reservoirs is
a tap, which is opened by a float in the lower vessel as soon as the
level of the liquid in it sinks a little, and which remains open until
the level of the liquid has again risen to a certain height. The float
in the reservoir connected with the lamps is arranged so that the level
of the liquid is slightly higher than the burners. Under the slight
pressure oil continually flows to the burners, and it is not difficult
to regulate its flow so that all which reaches the burner is burned.

In using a new oil it is not easy at first to regulate the flow so
that absolutely all is burned without any dropping off the burner; to
prevent the loss of this portion, the lower end of the air regulator
is conical, at the apex of the cone is a small tube and under this
a vessel in which the unburnt oil is caught. In Fig. 32, S is the
air regulator, T the vessel to catch the unburnt oil, L the pipe
leading from each burner to the common pipe H, G the vessel in which a
float regulates the flow from a larger reservoir, so that the liquid
always remains at the same level. When tar oils are used, or thin
mineral oils, the pipes which convey the oil to the burners may be
made tolerably narrow, but when viscous oils or fish oils are burned,
narrow tubes would offer too great resistance to the flow, so that
it is always advisable to use fairly wide pipes. Viscous oils become
considerably more fluid at higher temperatures, it is therefore well
to place the reservoirs in the same room as the lamps; in winter the
temperature of this room is kept fairly high by the burning of the
lamps, and thus the oils remain fluid.

FIG. 32.

In recent years hydrocarbons of very low boiling point have been
placed on the market at low prices; these produce a black of very good
quality. On account of the low boiling point of these very inflammable
liquids, particular care is necessary in burning them; they are very
fluid and retain their fluidity at a low temperature. The reservoirs
for them should be placed for safety outside the lamp room, and
should be closed by an air-tight lid, with only one small opening
through which air can enter. When these low boiling liquids are used,
particular care must be taken to regulate the flow to the lamp, or
large quantities will evaporate without being burnt.

The materials used in the manufacture of soot black differ considerably
in chemical constitution. Rosin, animal and vegetable fats, distilled
oils and volatile hydrocarbons are used. Each of these substances gives
different decomposition products when heated, to which proper regard
must be had, since the quality of the black is dependent on them. The
more difficultly volatile are the products of distillation, the higher
must be the temperature at which the combustion is conducted, otherwise
the soot will contain a considerable quantity of these products of
distillation. In consideration of the great variety of materials used
for making soot black, it is impossible to say exactly in what manner
each should be treated: this must be left to the practical experience
of the manufacturer.

CHAPTER XLI.

MANUFACTURE OF LAMP BLACK.

The variety of soot black known as lamp black is the best; it is used
for making copper-plate inks and black coach paints. Its price is
much higher than that of ordinary soot black; for the best qualities,
twenty times the price of an ordinary black is paid. The materials
already mentioned are used for making lamp black, fish oils and rancid
vegetable oils, and in recent times mineral oil and tar oils. In regard
to vegetable oils it is to be observed that it is advisable to use
very rancid oil, which gives a larger yield of soot; experience has
shown that a very rancid oil requires a larger quantity of oxygen to
burn without a smoky flame than a sweet oil. This indicates that a
portion of the carbon in the rancid oil is present in such a form that
it requires a higher temperature to burn it than is required for the
carbon in sweet oils. The use of rancid oils for making soot blacks has
thus two advantages, it is much cheaper and produces a larger yield.
The only disadvantage, and not a very important one, attending the
use of rancid oil is that the free fatty acids in these oils strongly
attack the metallic parts of the lamps. This is especially the case
with copper and brass; only such portions of the lamps as is absolutely
necessary should be made of these metals; all other parts, and in
particular the oil reservoirs, should be made of tin plate.

Since the development of the coal-tar industry oils distilled from
coal tar have come into commerce at low prices. These oils consist of
carbon and hydrogen and are equally volatile with the essential oils,
to which class turpentine belongs. There are two kinds of these tar
oils, light and heavy; they differ both in specific gravity and in
the boiling temperature, which ranges between tolerably wide limits.
When the burning qualities of these oils are examined, they show a
great difference in the quantities of oxygen necessary for complete
combustion with a white flame without smoke. The more oxygen an oil
requires to burn with a non-smoky flame, the more suited it is for the
manufacture of blacks. Generally speaking, the difficulty of completely
burning these oils is in proportion to their specific gravity and the
height of their boiling point.

By the distillation of rosin, oils are obtained which also consist of
carbon and hydrogen and form a useful material for the manufacture of
soot blacks. The mineral known as earth-wax or ozokerite, a substance
intermediate between asphaltum and petroleum, can also be used, but
since it is a solid it must be burnt in troughs.

In burning light oils the arrangements can be very simply made; wicks
are not required and thus a considerable outlay is spared. In place
of the burner, shallow dishes are used into which the oil enters from
below, replacing that burnt away. It is necessary to cool these dishes
continuously from below, otherwise they would soon become so hot that
the greater part of the oil would evaporate without burning, and it
would also be very difficult to regulate the flame properly.

Black can be obtained from very resiniferous coal by burning in
furnaces with a regulated air supply. The product is, however,
generally largely contaminated by ash, and can only be used for common
purposes. Very resiniferous lignite gives a better product than coal;
the black can be partially separated from the ashes by shaking upon
water and then stirring, when the particles of ash sink to the bottom
and the light black floats on the top.

However carefully the manufacture of soot black is conducted, it
will always contain, in addition to carbon, varying quantities of
the products of distillation, partly solid and partly liquid. In
consequence of these admixtures the soot will not be pure black, but
will show a more or less brown tinge, which is clearly observed when
the black is smeared on white paper. At a certain thickness of the
layer of soot it will be seen that the colour is not black, but an
impure brown. When such soot, as it is taken from the chambers, is
chemically examined, it is found that it gives up a large quantity of
soluble matter to different chemical reagents. By proper treatment
it is possible to remove the admixtures almost completely, so that
nearly chemically pure carbon remains. Such pure carbon can be made by
boiling lamp black with strong caustic soda solution so long as the
liquid is coloured, and when caustic soda can dissolve nothing more,
the residue is boiled with _aqua regia_ until this no longer takes
up soluble matter. The black is then washed with water until free
from every trace of acid, and the residue dried. By this treatment
the soot is converted into a powder of the purest black hue which it
is possible to obtain. It is now no longer soot, but chemically pure
carbon in the non-crystalline form; heated upon platinum foil it burns
to pure carbonic acid without producing smoke or smell. In practice
the purification of the soot is not carried to the extent of producing
pure carbon; this would be accompanied by a diminished yield of the
pigment without increasing the commercial value of the product: the aim
of the manufacturer is simply to produce a substance of a pure black
appearance from the brown soot. To remove the brown substances present
in the crude soot the solvent action of caustic soda solution can be
used. The soot is several times boiled in iron pans with strong caustic
soda solution in order to dissolve the products of dry distillation.
It is superfluous to repeat this operation until fresh caustic soda
remains colourless. The treatment may be stopped when the solution
acquires a slight brownish colour. When the soot has been purified so
far it has lost its brown shade and now appears as a velvety black
powder, very soft and very light, and distinguished by extraordinary
covering power.

It is not so easy as it appears at first sight to judge of the quality
of a black pigment by its appearance. To the unaccustomed eye a pigment
may appear to be of an unexceptionable black, which to the expert
appears decidedly brown; only long practice can give the eye the
requisite keenness. In addition to the test already given—of smearing
on white paper—there is another especially to be recommended to the
inexperienced for discriminating between pigments. A small quantity of
the black under examination is intimately mixed with a white pigment;
white lead or zinc white is very suitable. If the mixture has a pure
grey shade, the black may be regarded as of good quality, but if it
contains brown substances the mixture has an indefinite, dirty shade
instead of a pure grey. This is a sure sign that the black requires
further purification.

Although caustic soda is now very cheap, its use for the purification
of soot black is tolerably costly, because it entails much labour. This
process is therefore only used for the finest qualities which are to be
employed for copper-plate inks and black coach colours. For inferior
qualities the method of calcination is used, which produces, when
properly carried out, a black of such purity that it can be used for
the preparation of even the finest black paints.

_Calcination of the Soot._—The substances which give the soot its brown
colour are products of dry distillation, and hence are all volatile at
a certain temperature; they can be separated from the soot by heating
it in the absence of air. The temperature necessary completely to
volatilise these compounds is fairly high. The soot must be heated to a
good red heat to obtain a safe result.

When the soot is heated too quickly or to too high a temperature it
undergoes an alteration which affects the quality of the product.
By too long or too vigorous ignition the soot changes its flocky
consistency to a sandy nature; it will then require much longer
grinding with oil to produce a uniform mixture than is the case with
the light flocculent soot, which very readily mixes with oil.

The soot is ignited in sheet-iron boxes with a coating to protect the
metal from burning. This coating is best made from clay and hair. A
very thin paste of clay and water is painted uniformly over the boxes
with a brush; when the first coat is dry a second and third are given.
When once the metal is completely covered with clay, several coats
are given of clay mixed with chopped tow until the layer is several
millimetres thick. The coating carefully made in this manner is very
durable, and the boxes can be used for a long time, whilst without
the coating they would very soon be burnt. Particular care must be
given to the manufacture of the boxes themselves; the bottoms must fit
very accurately, and should be coated with clay in order to ensure an
air-tight joint. The lids also must fit accurately, and when they are
closed must equally be made tight by clay.

The soot is at first loosely packed into the box and each portion then
pressed with a rammer so that it tightly fills the boxes. In the lid is
a very small opening, through which the volatile products can escape.
The heating begins quite gently at the back, and proceeds gradually to
the front; the boxes are finally brought to a good red heat, at which
they are maintained for about half an hour; at this temperature the
substances mixed with the carbon are almost entirely volatilised, and
the soot acquires its true black appearance. The soot itself attains
a red heat in the boxes and in consequence of its loose nature it
very readily burns. The precautions given above must be observed in
order to protect completely the soot against the action of the air. The
boxes should not have the smallest opening besides that in the cover
necessary for the escape of the volatile matters; through an opening
invisible to the naked eye so much oxygen may enter during the cooling
of the boxes that a considerable quantity of carbon will burn to
carbonic acid.

To avoid losses through the carbon burning precautions must equally be
taken in the cooling of the boxes. When the ignition is finished they
are drawn out of the furnace by tongs and placed upright on a stone
floor; as the cool air enters, and in contact with the red-hot carbon
would burn a portion of it. This can be prevented by a simple artifice:
a red-hot coal is placed on the small opening in the lid; this converts
the oxygen of the air which enters the boxes into carbonic acid. When
all the boxes are taken out of the furnace the doors of the room in
which they are placed are opened, so that they are cooled in the
draught. Finely divided carbon takes fire at a temperature far below a
red heat, so that the boxes should not be opened until their contents
are quite cold. If they were opened whilst hot the soot might take fire.

=Pine Black.=—Under this name a poor quality of black comes into the
market, which is much used for ordinary black paints, etc. The pine
black formerly brought into the market was what its name purported; it
was made from the roots of the pine in the primitive fashion already
described. The black was sold without further purification: it was a
soft light powder, very variable in shade on account of the absence of
control over the process: it varied from pure black to a dark brown.
At present, under the name of pine black many substances are sold,
generally without purification, made from the most different materials,
frequently from rosin, rosin residues, and other cheap materials.

CHAPTER XLII.

THE MANUFACTURE OF SOOT BLACK WITHOUT CHAMBERS.

A certain temperature is necessary for the combustion of every
substance; if a cold body is placed in the flame of a candle or an
oil lamp it becomes covered with soot, because it cools the flame to
such an extent that the carbon is no longer heated to the temperature
necessary for its combustion, and is, therefore, separated in a finely
divided condition. Use is made of this phenomenon in a method of making
soot black which has many advantages, of which the most important is
that expensive buildings are not required, and the manufacture can be
conducted in restricted space.

[Illustration: FIG. 33.]

A convenient arrangement for obtaining black by this process is
represented in Fig. 33. The thin-walled hollow cast-iron cylinder is
turned smooth on the outer surface, and is surrounded by a sheet-iron
cover at the distance of a few centimetres; it rotates in bearings,
which, like the spindles, are hollow; cold water is thus led through
the cylinder from a tank at a higher level. Below the cylinder are
placed, near to one another, smoking lamps, and at the side of the
cylinder is a broad brush of soft hair which continuously removes the
soot deposited on the surface, which then falls over an inclined plate
into the collecting vessel. The cylinder is kept in slow rotation by
any mechanical arrangement. The lamps are devised not to produce a
hot flame, but when this comes in contact with the cold surface of
the cylinder, it deposits a ring of soot on the rotating cylinder
which is then removed by the brush. The cylinder is kept cooled by
the water which runs through it. The soot which is collected shows a
tolerably strong brown shade in consequence of the rapid cooling of the
flame, which causes the formation of a large quantity of products of
distillation; the soot will always require ignition.

[Illustration: FIG. 34.]

It has been proposed further to simplify the manufacture of soot black
by this method, by burning coal gas under the cylinder from numerous
small openings in a pipe. Coal gas indeed gives a very fine deep black,
but the yield is so small that this process could never be adopted with
advantage.

The apparatus constructed by Thalwitzer for the manufacture of oil
black (Fig. 34) consists of a plate, _A_, with a rim, _a_, fastened to
a vertical axis, _b_. This is carried by bearings in _B_, and is kept
in rotation by the cog-wheels, _d_ and _f_, moved from the shafting,
_O_. The plate is cooled by water supplied by the pipe, _g_, it flows
away by _h_ and the annular vessel, _D_. The lamps, _e_, are connected
with the common oil reservoir by _n_. _H_ is a scraper fastened to _B_
by _x_.

CHAPTER XLIII.

INDIAN INK.

Indian ink consists of purified soot mixed with gum, as binding medium,
and a little camphor and musk. It is generally believed that Indian ink
is made in China by a process which is still a secret; this opinion
is supported by the fact that the ink made in Europe is almost always
inferior to the Chinese. It is quite possible that a substance which
produces a particularly fine black is used in China to prepare the soot
used for the ink, and which we do not yet know or do not use on account
of its high price. But it appears that the difficulty of producing an
ink equal in quality to the Chinese lies less in the quality of the
soot —for by the methods of purification already given we can make
almost chemically pure carbon—than in the extremely careful mechanical
treatment of the soot with the other constituents of the ink.

The genuine Chinese ink has a peculiar smell, which, when not hidden
by musk, distinctly recalls the smell of burning camphor; the camphor
tree is a native of China, so that it is not impossible that the soot
of the wood of this tree is used for the ink. The soot from camphor
alone would be too expensive, so that it is probable that camphor wood
is used from which camphor has been prepared, or that camphor soot is
mixed with the soot of a cheaper substance. In fact several varieties
of Chinese ink show distinctly the smell peculiar to soot produced by
fat. The binding material is animal glue, which is most thoroughly
ground with the carbon; the secret of making good Indian ink lies in
the careful grinding with the binding medium.

According to Piou, who lived for a long time in China and learned the
process there used, the black employed for the ink is made from the
resins of the pine and other trees. The soot is sieved through silk,
boiled with glue and water, and kneaded by hand with a little oil until
the mass has become completely homogeneous. It is then left for some
days, heated, and pressed into the moulds. This description is not very
satisfactory; the products obtained are far inferior to those made by
the ordinary European process.

The viscous mass obtained by long treatment of the black with the
binding medium is slowly dried, and again ground when it has become
thick. When a completely homogeneous paste has been obtained it is
made into sticks which are pressed into moulds, and then very slowly
dried so that they do not crack. Small cracks always occur; they are
filled in by a brush dipped in the thick paste, finally the sticks are
entirely or partially covered with gold leaf.

In order to impart to the ink the odour characteristic of the Chinese
ink, it is sufficient to use good lamp black, made odourless by
extraction with caustic soda, and to add a little solution of camphor
in turpentine during the grinding. If it is desired to impart the musk
smell which several Chinese varieties possess, a small quantity of a
spirit extract of genuine musk may be added.

The size solution is made by long boiling isinglass with water; it
must be so concentrated that it forms a strong jelly on cooling. A
small quantity of acetic acid is added, so that the grinding may not
be hindered by the viscous nature of the size, which the acid prevents
from gelatinising. When the mixture has been made by long grinding,
the mortar is warmed to 40° or 50° C., whereupon the acetic acid is
soon volatilised, and the mass rapidly becomes very thick.

=Neutral Tint Black.=—A mixture of Indian ink, Chinese blue, and a very
small quantity of madder lake forms the colour known as ”neutral tint,”
the shade of which is a peculiar greyish violet. By alterations in the
proportions of the constituents different shades of ”neutral tint” are
produced.

APPENDIX.—BLACK MINERAL PIGMENTS.

=Chrome Copper Black.=—When copper chromate is strongly ignited in air
and then treated with boiling nitric acid a glittering black compound
is obtained, which shows up well when printed on fabrics with albumin.
This and all similar pigments are distinguished by great durability.

=Chrome Black.=—When a mixture of chromium oxide with varying
quantities of ferric oxide is strongly heated, pigments are obtained
of all shades from dirty yellow and green to the deepest black. This
pigment is largely used in porcelain painting to produce a black, which
also can be obtained by other but much more costly methods. The best
mixture to produce a deep black contains one part of chromium oxide to
four parts of ferric oxide.

CHAPTER XLIV.

ENAMEL COLOURS.

An enamel is a glass distinguished from ordinary glazes by a much lower
melting point, and generally by opacity. Most enamels are coloured by
additions of metallic oxides.

Without going in detail into the art of enamelling, some observations
may here be given which appear indispensable for an understanding of
the manufacture of enamel colours. The enamels are glasses, but not
every glass can be used for an enamel, since the constituents of the
glass react with the substances by which the enamel is coloured, and
with certain colouring matters quite different shades to the intended
might be produced. Of equal importance with the composition of the
glass used in the preparation of the enamel is the temperature at which
the glass melts. Many colours bear but a low heat; they decompose at a
somewhat higher temperature and produce quite the wrong colour. This is
especially the case with the enamels coloured purple red by gold.

The enamel colours are fixed by mixing them with an easily fusible
glass (the flux), applying them to the object to be enamelled, whether
metal, glass, or porcelain, and heating the painted article in muffles
until the glass melts, and either dissolves the colouring matter or
encloses it unaltered. According as the enamel melts as a whole, or the
flux alone melts and encloses the colouring matter, so transparent or
opaque enamels are produced. The former are really coloured glasses,
the latter are glasses in which is enclosed the sintered colouring
substance.

Whilst enamels were formerly of importance only for artistic purposes,
they have recently attained great industrial employment; not only is
earthenware now covered with enamel, and thus made to resist the attack
of chemical reagents, but boiler tubes are lined with enamel to prevent
the formation of scale.

The colours used for enamels must in all cases be metallic oxides; for
yellow, silver and antimony oxides; for red, gold, copper oxide and
ferric oxide; for blue, cobalt compounds; for green, copper oxide or
chromium oxide. Two operations are required in the production of enamel
colours, the preparation of the flux and its fusion with the real
colouring substance. In most cases the latter operation takes place
simultaneously with the fixation on the enamelled article, but the
enamels may be melted and cast into lumps, which are then powdered.

=White Enamels.=—These are always ordinary crystal glass, to which tin
dioxide or potassium antimoniate has been added. Particular care must
be taken that only pure raw materials are used, for almost unweighable
quantities of iron compounds are sufficient to impart to the enamel a
yellow tinge. In almost all cases the glass must be decolourised by the
addition of pure pyrolusite (manganese dioxide), which at a red heat
gives up a portion of its oxygen to the ferrous oxide contained in the
glass, producing ferric oxide, which has much less colouring power. Too
large a quantity of pyrolusite imparts a blue tinge to the enamel.

The tin dioxide used in the glaze is made by direct oxidation of tin
in air. It has been found that tin burns much more easily in air when
it is mixed with lead; 20 to 40 parts of tin are melted with 100
parts of lead, and the alloy heated in shallow vessels in the air;
it takes fire at a certain temperature, and it is only necessary to
remove continually the layer of oxide in order to oxidise the whole
of the metal within a short space of time. The mixture of oxides
obtained in this manner is freed from particles of metal by grinding
and levigation; when it is fused with the glass, the lead oxide enters
into combination, whilst the tin oxide is embedded in the colourless
glass. When paste enamels are required the mass is melted in shallow
crucibles, poured into water and broken up to a coarse powder, which
is again fused. In some cases this operation must be repeated several
times to obtain a quite homogeneous product, for tin dioxide is very
heavy and sinks to the bottom of the fluid glass; this is readily
seen if the fused enamel is allowed to cool in the crucible. When the
crucible is broken the lower portions of the fused mass are dense
white, whilst the upper are merely milky white. By repeatedly fusing
the mass it is endeavoured to obtain it uniform. In fusing the enamel
care must be taken that it is heated in a vessel from which the fire
gases are completely excluded, since the smallest quantity of coal or
of ferruginous ashes coming into contact with the melted mass would
injure its colour. If the melting point of the enamel is too high it
can be lowered by the addition of a small quantity of pure quartz sand.

A fine white enamel is obtained by using litharge or red lead. The
mixture contains 60 parts of quartz sand, 30 of alum, 35 of common
salt and 100 of red lead. It is advisable to add a small quantity of
finely-powdered talc to the sand. In consequence of the considerable
proportion of alumina contained in this enamel it is difficult to melt,
and can be heated to very high temperatures without injury to the shade.

When antimony oxide is used a glass free from lead must be taken; lead
glass does not give a pure white with antimony oxide. A very good white
enamel is obtained by fusing 3 parts of crystal glass with 1 part of
sodium antimoniate.

=Coloured Enamels= owe their colour to metallic oxides. Enamels
coloured by metallic oxides, in which the oxygen is firmly united,
can be fired without great precaution, but if they contain oxides
easily decomposed, great care is required to obtain a fine colour. In
enamelling metals a white under-glaze is generally used beneath the
coloured enamel; it consists of a refractory white enamel. By this
means it is more easy to obtain pure colours.

=Yellow Enamels= are coloured by silver, antimony oxide combined with
litharge, or ferric oxide; from the latter a red can also be obtained.
To produce a yellow enamel by means of silver the article is first
enamelled white at a low temperature; silver oxide is then applied
where required, and the article again heated. It then frequently
happens that the surface has a metallic appearance owing to the
reduction of a part of the silver oxide to metallic silver. When this
coating is scraped off the enamel beneath is found to be coloured a
fine yellow.

The antimony yellow is obtained by mixing 1 part of antimony oxide, 1
part of alum and 1 to 3 parts of white lead, according to the depth of
colour required. These finely-powdered materials are intimately mixed
with 1 part of sal ammoniac, and heated in an open vessel with stirring
until the yellow colour appears. The vapours of the ammonium chloride
indicate the proper temperature for the operation. When this substance
is completely volatilised the temperature should not be further raised,
or the mixture would fuse.

By means of ferric oxide a fine and durable yellow is produced; the
quantity employed must not be too large, or a red colour will result. A
very high temperature is required for burning in this colour. The alum
used in making the yellow enamel colours serves to prevent the oxides
from fusing.

=Red Enamel.=—Ferric oxide is generally used. When a purple red is
required it is obtained either directly from metallic gold or from
purple of Cassius.

Red iron enamel is made by slowly heating 20 to 25 parts of pure
ferrous sulphate with 10 parts of aluminium sulphate until all the
water of crystallisation is expelled, when the temperature is gradually
raised to an intense heat. The shade depends upon the temperature: the
higher it is the darker is the colour. Tests are taken from time to
time of the mixture, and rapidly cooled. Hot ferric oxide is black,
consequently the shade can only be judged with complete certainty in a
cooled portion. Although the temperature required for the preparation
of this colour is very high, yet it ought not to rise so far that the
magnetic oxide is produced, a small quantity of which would cause the
enamel to appear dirty red, since it gives to the flux a blackish
green colour. The red ferric oxide colour should not be dissolved in
the glass of the enamel, but should be embedded in it. Thus, when the
enamel is burnt on, the temperature should not rise so high that ferric
silicate is formed, otherwise a yellow or even completely black enamel
results.

Purple red is obtained by means of gold; formerly purple of Cassius
was exclusively employed for this purpose, but an equally bright red
can be obtained from gold chloride. The colour obtained from gold will
bear only a very low heat; it must be mixed with a very fusible glass,
brought on to the article to be enamelled, and heated just sufficiently
to melt the mass. The following mixture is used for gold red and other
delicate colours: 3 parts of calcined borax, 3 parts of quartz sand and
1 part of chalk. A very small quantity of gold is sufficient to produce
a deep red, the amount of gold preparation used for a pale red or a
medium purple must be carefully weighed.

=Blue Enamels= are always coloured by cobalt oxide. Any cobalt
compound can be used, since at a red heat the silica of the glass
displaces other acids, and produces cobalt silicate. It is most simple
to take cobalt nitrate. This salt is readily obtained completely pure,
and absolute purity is of great importance, for only thus is a pure
blue produced.

Cobalt oxide produces a more or less deep blue when fused with varying
quantities of glass. In order to produce shades similar to turquoise
and forget-me-not, a white enamel must be used beneath the blue, or
bone ash must be mixed with the blue enamel; this produces a paler
colour, which is also opaque.

=Green Enamel= is coloured by copper oxide or chromium oxide. In the
former case 1 part of copper oxide is used to 30 to 50 parts of glass,
according to the depth of shade required. The enamel made with copper
oxide alone has never a pure green colour; it exhibits a blue tinge. A
pure green is obtained by adding a very small amount of ferric oxide,
which produces a yellow and compensates for the blue shade, so that a
pure green is formed.

Chromium oxide produces a beautiful emerald green without any addition.
The enamel may be exposed to very high temperatures without injury.

Green enamels may also be made by mixing blue cobalt enamel with a
yellow enamel: an unexceptional shade results, but this method is
seldom used, since green enamels are obtained in a simpler manner from
copper and chromium oxides.

=Violet Enamel.=—Manganese dioxide is used exclusively to produce
violet enamel; an extremely small quantity is sufficient to colour a
considerable quantity of glass. A pure violet shade is only obtained
when very pure manganese dioxide is used; the artificially prepared
substance should be used to produce the finest violet: the cost is
considerably greater than that of the mineral pyrolusite, but equally
good results can rarely be obtained from pyrolusite.

=Black Enamel.=—When a large quantity of ferric oxide, copper oxide or
cobalt oxide is fused with a glass, a deep black enamel is obtained.
Generally mixtures of these oxides are used; experience has shown that
a much deeper black is thus obtained than from any one alone.

Now that enamels are not used exclusively for artistic purposes, but in
considerable quantities for earthenware and other technical purposes,
the manufacture of enamel colours has attained considerable importance;
works already exist occupied almost exclusively with this special
branch of manufacture.

By Lacroix’ process enamel colours are made which can be applied to
porcelain without further admixture with a flux; for example, a very
fine blue is made by dissolving 300 parts of pure alumina and 100
parts of cobalt carbonate in nitric acid, evaporating the solution to
dryness, igniting the residue and fusing it with 300 parts of quartz
sand free from iron, 900 parts of crystallised boric acid and 1,800
parts of red lead. A blue glass is obtained which easily melts, but
is difficult to powder; but if poured when fluid in a thin stream
into cold water, it forms thin threads, which are very brittle in
consequence of the rapid cooling, and may be easily converted into a
soft powder.

CHAPTER XLV.

METALLIC PIGMENTS.

Metallic pigments do not always consist of metals; the name is also
applied to compounds which possess a pronounced metallic lustre; mosaic
gold, the preparation of which has been previously described, is an
example of such a pigment. Another variety of metallic pigment is made
by heating finely-powdered alloys; a layer of oxide is thus produced
upon the surface of the metallic particles, this layer produces a
shade of the colour of the metal or alloy. Metallic pigments are only
unalterable when they are composed of metals which are not changed
by the action of the air. As a matter of fact, this property is not
possessed by any of the metals used for this purpose; even gold and
silver are blackened by the action of the sulphuretted hydrogen in the
air. This alteration proceeds, however, very slowly when the metal is
enveloped by a layer of a binding medium, which is always the case when
the metallic powder is used for painting. There are many manuscripts,
the initial letters of which have been coloured by gold or silver,
in which the metals retain their peculiar lustre after the lapse of
centuries.

Metallic paints made from yellow alloys have never much durability:
they always contain copper, which readily alters; articles painted with
imitation gold soon lose their lustre, and in the course of time become
green.

=Shell-Gold.=—This very expensive artists’ paint is made by rubbing
gold-beaters’ refuse with gum solution upon a stone slab until a
completely homogeneous mixture results, which is then rapidly thickened
over the fire and generally allowed to dry in small shells. The
preparation so made is known as shell-gold: it is sold at very high
prices.

The operation of grinding the gold is very lengthy; it considerably
increases the cost of the already expensive material. The process may
be considerably shortened if the gold is obtained in a very finely
divided state by a chemical operation. For this process coins or broken
jewellery is heated in hydrochloric acid, and nitric acid gradually
added. The gold dissolves in the mixture. If it was alloyed with silver
a white precipitate of silver chloride is formed, which is filtered off
after largely diluting with distilled water. The solution is boiled
for some time to remove excess of nitric acid. A solution of ferrous
sulphate is then added; the liquid at once becomes bluish black, and
in a short time deposits a brown precipitate composed of chemically
pure gold, which is so finely divided that it is almost without the
characteristic glitter of gold. The precipitate is filtered off, dried
and preserved in stoppered bottles. In order to prepare shell-gold
from this gold powder it is simply necessary to grind it with thick
gum solution in a porcelain mortar; under the pressure of the pestle
the gold rapidly acquires its natural glitter. The grinding need only
be continued until the gold and gum solution are uniformly mixed.
The mixture so made should be at once filled into the shells, since
on standing the gold would soon separate from the gum solution in
consequence of its high specific gravity.

=Shell-Silver.=—Genuine shell-silver can be made by rubbing silver leaf
with gum solution upon a slab exactly as genuine shell-gold. In this
case also the labour may be considerably lessened by converting the
silver into a state of fine division by a chemical process. Silver is
dissolved in nitric acid, which must be quite free from hydrochloric
acid, or insoluble silver chloride would be formed. In the operation
brown suffocating fumes are evolved which attack the respiratory
organs; it should therefore be conducted in the open air or under a
flue with a good draught. The silver solution, which is generally
coloured blue by admixed copper, is diluted with a large quantity of
distilled water. A sheet of copper is then dipped into the liquid and
rapidly moved about in it. The silver then separates as a dark grey
powder; after washing it is chemically pure, and when ground with gum
solution it produces genuine shell-silver.

Shell-silver and shell-gold are rarely used in painting on account of
their cost; they are chiefly used for illuminated manuscripts. When
these metallic pigments are to be used in oil painting, in place of
gum they must be ground with a liquid which mixes with boiled oil or
essential oils. For this purpose copaiba balsam is to be recommended.

=Imitation Silver.=—The imitation silver pigments are made from an
alloy of tin and bismuth, or from a tin amalgam. The latter is most
easily made by melting tin in a porcelain dish and adding one quarter
of its weight of mercury; the mixture is then well stirred, and
allowed to cool. It quickly solidifies to a crystalline mass, which is
tolerably brittle and can be powdered without difficulty. To obtain a
substance of a true silvery appearance the amalgam must be converted to
a powder of a certain degree of fineness. If the powdering is carried
too far the product loses a great part of its metallic lustre, and
acquires a dull grey colour.

The bismuth alloy is made by fusing 100 parts of tin, adding 100
parts of bismuth, and then 10 parts of mercury. It is not absolutely
necessary to add mercury, but the addition has the advantage that the
solid alloy is far more easily powdered. The imitation silver made by
this process has a white metallic colour approaching that of genuine
silver, but not equal to it, especially in lustre. It is, however,
largely used in the industries on account of its low price; for
example, for paper hangings.

CHAPTER XLVI.

BRONZE PIGMENTS.

It would be anticipated from the name that bronze pigments were
composed of an alloy of copper and tin; in reality the alloy is
composed of copper and zinc, _i.e._, brass. The bronze pigments are
made by a similar process to that described for genuine gold and silver
pigments. The waste produced in the manufacture of imitation gold leaf
is ground with a solution of dextrine upon a slab until the mixture is
uniform and separate metallic particles can be perceived only through
a lens. Whilst genuine gold and silver paints are always made in small
quantities, on account of the expensive nature of the material, and
machinery is not employed, the bronze pigments are in different case;
the use of mechanical arrangements is necessary for producing the fine
subdivision, otherwise the bronze would be very dear on account of the
great cost of grinding. The mechanical arrangements required to divide
the alloy are of the ordinary nature, but special machines have been
constructed for the manufacture of bronze pigments, by which the metal
is far more quickly converted into powder than by means of grinding
machinery. These machines consist of metal drums studded on the
interior with a large number of fine needles and capable of very rapid
rotation. When a metal powder already tolerably fine is brought into
these drums, it is rapidly brought to such a condition of fine division
as would be attained by hand grinding only by prolonged and laborious
exertion.

The raw material for the manufacture of bronze powders is produced
in making imitation gold and silver leaf; the waste metal obtained
in beating the sheets is used. The employment of this waste has two
advantages: the metal is already in very thin sheets and is composed of
alloys varying in colour from silver white, through gold, to a bright
copper. In making leaf metal the waste of each colour is kept carefully
separate, so that it simply requires to be broken up to produce bronze
powders of different shades.

Before this waste is brought into the drums mentioned above it must be
subjected to a preliminary grinding in a mortar with a small quantity
of a fatty oil, which serves to bind together the mass. Sufficient oil
should be used to give the mass some degree of coherence; if too much
oil is added the space between the needles of the drum would be coated
with the mass and the process in the drum would require a much longer
time. The uniform mixture of oil and bronze waste is then brought
upon a wire sieve of the finest possible mesh, and the mass is rubbed
through by means of a fine metal brush into a vessel below; thus the
larger particles are retained by the sieve and only those which are
smaller than the mesh pass through. The product of this process is
then brought into the drums, which are rapidly revolved; the small
particles of metal are thrown with great force against the side and
are converted by the fine points with which it is studded into a very
fine powder. The time required for this process depends on the rate of
revolution and on the quantity of powder treated at once. The drums are
stopped from time to time and the contents examined. When the powder
is sufficiently fine it is taken out of the drum. This is most easily
accomplished if the drum is arranged to take apart into two halves.

In most works it is usual to free the bronze powder from the admixed
oil by subjecting the mass to the greatest pressure that can be
produced by a very powerful hydraulic press. The oil which flows
from the press is always green, which shows that chemical action has
taken place. In consequence of the large surface imparted to the oil
it becomes speedily rancid, and then contains free fatty acids which
attack copper very energetically.

[Illustration: FIG. 35.]

The use of hydraulic presses may be avoided by removing the oil
by means of a solvent. Fatty oils dissolve very readily in carbon
bisulphide, but this solvent cannot be used in this case, because
commercial carbon bisulphide always contains dissolved sulphur, which
would blacken the bronze powder. Petroleum ether and benzene are very
suitable solvents for this purpose. On account of the volatility of
these inflammable liquids lights must be absolutely excluded from the
room in which they are used, and in order to avoid loss of solvent
the bronze powder must be treated in closed vessels. The safest plan
is to use a special apparatus of simple construction to dissolve
the oil. Fig. 35 shows the construction of an arrangement suitable
for this purpose. It consists of a cylindrical vessel of tin plate
surrounded by a rim into which fits the edge of the cover. When the
lid is placed on and the rim filled with water, the contents of the
vessel are closed in air-tight and cannot evaporate. The lower portion
of the vessel is conical, and is joined to a pipe in which is a tap,
and which communicates at the side by a tube with the glass vessel in
which the solvent is contained; another tube connects the neck of this
vessel with the cover. When this apparatus is used for extracting the
oil from bronze powder, a filter of strong blotting paper is placed
in the conical portion of the vessel, care being taken that it fits
accurately so that it is not torn by the weight of the bronze. The oily
bronze powder is placed on this filter, the cover set on, and the rim
filled with water. By opening the tap attached to the solvent reservoir
the liquid is allowed to enter the cylinder from below, the air in the
latter passing through the tube in the cover to the reservoir. After
some hours the oil is dissolved, the tap in the cover is then opened
and the liquid run off by opening the lowest tap. If the bronze powder
is not quite free from oil after one treatment with the solvent, the
operation is repeated with a fresh quantity.

When free from oil the powder and filter are removed from the apparatus
and dried; the dry mass forms a solid cake, which is broken up in a
mortar and by a little grinding changed into a fine powder.

The colour of the bronze powder is the same as that of the alloy used,
but the shade is always rather paler than that of the coherent metal;
regard must be paid to this circumstance in making a bronze of a
determined colour: the alloy employed must have a rather deeper colour
than the shade the bronze is to possess.

The manufacture of leaf metal and bronze powder is frequently conducted
in the same works, which also often prepare the requisite alloys; we,
therefore, give a few examples of the composition of the alloys which
produce certain shades. The more zinc the alloys contain the lower is
their melting point, the greater their brittleness and hardness and
the paler their colour. An increase in the copper causes the colour
of the alloy to approach more nearly to that of gold, and increases
the malleability, a property useful in making leaf metal, but not
desirable in making metallic powder. A zinc copper alloy which contains
between 1 and 7 per cent. of zinc has an almost pure red, or even a
dark red colour; an alloy containing 7·4 to 13·8 per cent. of zinc has
a pure golden yellow colour, between 16·6 and 25 per cent. of zinc a
yellow appears. An increase of the percentage of zinc above this point
produces the colour of brass; it is noteworthy that an alloy containing
still more zinc, 33 to 41 per cent., again shows a reddish colour,
which is most developed when the alloy contains equal parts of zinc and
copper. If the zinc is increased still further the shade gradually goes
over to white; this change is already observed in an alloy containing
51 per cent. of zinc, which shows a pure golden yellow colour, and is
very brittle. When the zinc rises to 53 per cent. the colour is reddish
white; at 56 per cent. it is yellowish white, at 64 per cent. bluish
white, and between 75 and 90 per cent. the alloy is bluish-grey.

The alloys for bronze powders of different shades have the following
composition, according to R. Wagner:—

                      Copper,          Zinc
                     per cent.       per cent.
    Pale yellow         83             17
    Red                 94-90           6-10
    Deep red           100

Bronzes from English, French and Bavarian works contain the following
percentages of copper:—

      ENGLISH BRONZES.
    Orange          9·82 per cent.
    Deep yellow    82·37    ”
    Pale yellow    80·42    ”

      FRENCH BRONZES.
    Copper red     97·32 per cent.
    Orange         94·44    ”
    Pale yellow    81·29    ”

     BAVARIAN BRONZES.
    Copper red     98·92 per cent.
    Violet         98·82    ”
    Orange         95·30    ”
    Straw yellow   81·55    ”
    Speiss yellow  82·34    ”

In each case the remainder of the alloy consists entirely of zinc.

Alloys containing from 1 to 35 per cent. of zinc are only malleable
in the cold. The malleability is at the greatest with a content of
zinc between 15 and 20 per cent.; such alloys are the most suitable
for making leaf metal. Alloys containing between 36 and 40 per cent.
of zinc may be hammered either cold or hot, whilst the former alloys
become brittle on heating. When the percentage of zinc is still further
increased the malleability decreases. The most brittle alloys contain
60 to 67 per cent. of zinc.

The alloys are made in a furnace with a good draught, for copper
liquefies at a very high temperature. To prevent loss of copper by
oxidation the molten metal should not come in contact with air; it
should be covered by a layer of red·hot coal, which prevents oxygen
from reaching it. When the copper is completely melted, which is
ascertained by stirring with a piece of wood, the whole of the zinc
is added. Some skill is required in this operation, otherwise a large
proportion of the zinc will be volatilised, and the vapours will burn
when they come in contact with air, in which case dazzling bluish white
flames are seen over the crucible. The best method is to throw the zinc
into the crucible and immediately stir it into the molten metal with
a wooden rod. The products of the dry distillation of the wood, which
are given off in great quantity at this high temperature, keep the air
from the surface of the metal and prevent the oxidation of the zinc
vapours. The zinc is thoroughly mixed with the copper by stirring with
the wooden rod, the crucible is then slowly cooled, with the precaution
that the surface of the metal is kept covered by red-hot coals so long
as the metal is fluid. When sufficiently cool the metal is poured into
shallow iron moulds, in which it quickly solidifies; it is then rolled
into sheets, which may be converted into thin leaves by hammering in a
similar manner to that in which the gold-beater makes gold leaf.

To obtain bronze powders of different shades alloys of different
colours may be used; the bronze powders may also be shaded by two
methods—either by adding certain colouring matters of very great
colouring power or by partially oxidising the finely divided metallic
powder. In the first process the finely ground colouring matter is
mechanically mixed with the bronze powder. The use of manual labour
would involve a great loss of time; even when quite small quantities of
bronze and colouring matter are mixed in a mortar it is necessary to
grind diligently for a very long time before a mixture of homogeneous
appearance is obtained. In working on a somewhat larger scale it is
therefore advisable to adopt mechanical mixing arrangements. A very
simple apparatus suffices. A sheet-iron cylinder is used which can be
revolved, and provided with a well-fitting slide. In this cylinder are
placed the bronze powder and the colouring matter until it is about
half full; then, after tightly closing the slide, it is set in slow
rotation, which is continued until a test taken out shows a uniform
colour.

When bronze powder is slowly heated in a shallow vessel the colour
begins to darken at a temperature not much above the boiling point of
water. In consequence of the fineness of the particles of the metallic
powder the copper readily takes up oxygen, and is superficially
converted into copper oxide. This oxide is of a darker colour, and thus
by this method the shade of the bronze can be deepened as desired.
This simple operation requires a certain amount of practice to produce
a product of a determined shade. The desired result is most safely
attained when the bronze is spread out quite uniformly in a thin layer
upon a metal plate, which is gently heated from below. The powder soon
begins to darken; by cooling the plate the progress of the oxidation
may be arrested at any moment.

Recently bronze powders have come into the market showing all possible
colours in the deepest shades, by the aid of which very remarkable
colour effects can be produced. These bronzes are made by dissolving
an aniline dye in a little alcohol, pouring this solution over the
powder, and mixing the dye uniformly through the whole of the bronze by
working the mass for a sufficient length of time. In this way bronze
powders are produced which possess a green, red, blue or violet lustre,
according to the colour of the dye used. These colours with metallic
lustre can also be produced by bronzing the article with a white (zinc)
bronze, and then coating it with a varnish in which the required
aniline dye is dissolved. A bronze with a fine golden red glitter is
produced by applying a golden yellow bronze and then a varnish in
which a little aniline red is dissolved. It should be observed here
that these effects, produced by a coat of varnish in which an aniline
dye is dissolved, only turn out well when the dye is used in very
small quantity, for these colours are the strongest with which we are
acquainted, and in colouring power far surpass cochineal carmine, which
is renowned for this property.

When bronze is coloured by dyes the most varied shades can be obtained
with a metallic lustre. According to Conradty a very fine blue bronze
is obtained by boiling white bronze for some hours with a weak alum
solution, washing and drying, and then mixing in a mortar with a
strong solution of aniline blue in alcohol until the solvent has
evaporated. This operation is repeated until the desired depth of shade
is obtained. The bronze is then washed with pure water. Conradty also
recommends that the coloured bronze should be ground with a little
petroleum, and then exposed to the air to allow the petroleum to
evaporate. This operation, for which no chemical reason can be given,
is quite unnecessary. If other dyes or mixtures of them are used in
place of aniline blue, bronzes of corresponding colour are obtained.

However handsome are the bronzes coloured by this process, nearly all
have the disadvantage that the colours have little permanence, and
quickly fade when exposed to light. This is especially the case when
the bronzed article is coated with an oil varnish; if, however, a
spirit varnish is used, or indeed any varnish composed of a resin and a
volatile solvent, the colour of the bronze, protected by the layer of
resin, remains quite unaltered for a long time.

=Electrolytic Copper Bronze.=—Electrolytically precipitated copper may
be used as a bronze pigment; it is most simply made by adding pieces
of metallic zinc to a solution of copper sulphate free from iron and
violently shaking the flask for a long time. The liquid becomes warm,
and the copper separates in the form of a very fine precipitate, which
is collected on a filter and washed with air-free water (boiling water
is best) and then quickly dried. The upper portions of the precipitate
in the filter, which are exposed to the air, have generally a brownish
colour due to the incipient oxidation of the finely divided metal. They
are removed, and the lower portions show the characteristic colour of
pure copper.

In the same way silver can be precipitated from a solution of silver
nitrate in a finely divided state, but the particles of the silver
powder are so very small that they reflect very little light, and
consequently the powder has an unsightly grey colour. When a surface
painted with this silver is rubbed gently with a hard body, the
metallic lustre appears.

=Tungsten Bronze Pigments= are expensive and rarely employed. They
are obtained by fusing sodium or potassium tungstate in a porcelain
crucible and gradually adding tungstic acid until the mass has an
acid reaction. Tin dioxide is then added in quantity sufficient to
neutralise the tungstic acid; the mass is cooled and finely powdered.
According as potassium or sodium tungstate is used, a violet or reddish
pigment is obtained which exhibits the peculiar metallic lustre of a
bronze powder.

Still more costly is vanadium bronze, which is made by adding ammonium
vanadate to a solution of 2 parts of copper sulphate and 1 part of
ammonium chloride with continual stirring, until the precipitate no
longer re-dissolves on stirring. The liquid is then heated for several
hours to about 35° C., when vanadium bronze separates in golden yellow
scales. These are collected on a filter, washed and dried When ground
with oil or gum solution they can be used as a red gold bronze. The
colour is unaltered by the air.

CHAPTER XLVII.

VEGETABLE BRONZE PIGMENTS.

Under this apparently contradictory term, substances come into the
market which produce a peculiar metallic lustre. When applied under
certain conditions, the appearance is similar to that produced by real
bronze. The vegetable bronze pigments are lakes as pure and free from
foreign admixtures as possible. The lakes obtained from red wood or
logwood can be used for this purpose.

From the red woods (_see_ p. 384) a magnificent bronze pigment can be
obtained, which is either pure golden yellow or possesses a greenish
golden metallic lustre not unlike the colour of the wing cases of
the rose-bug. To obtain either shade a pure lake is first made by
extracting red wood with boiling water, adding a little carbolic
acid to the decoction (0·01 per cent. of the quantity of liquid) and
allowing to stand for several weeks. The liquid is syphoned off from
the deposit, heated, and alum added equal in quantity to 10 per cent.
of the wood used. The mixture is then allowed to stand for about a
week, the precipitate is filtered off, washed, and, if necessary,
dried. If the bronze is to be used in the form of water colour, the
precipitate is dried to a thick paste and mixed with about 10 per cent.
of its volume of thick gum solution, so that a viscid mass is obtained
which can just be applied with the brush. When the coating is made so
thick as to hide the surface of the bronzed article, it has when dry
the golden green colour.

In using this lake to prepare a pigment similar to gold bronze it must
be almost completely dried, and then mixed with the liquid obtained
in the following manner: White soap is melted on the water bath with
the smallest possible quantity of water, and when completely dissolved
the same quantity of white wax is stirred in, finally water is added,
so that the cooled liquid has the consistency of a moderately thick
varnish. When this liquid is ground with the requisite quantity of the
still damp lake and the mixture applied to paper, wood, or leather, and
after drying rubbed with a glass ball, it gradually acquires a very
fine golden bronze colour. This method of bronzing is largely used
in the manufacture of wall papers and for colouring fancy leather.
To protect this coating against the action of water, it should be
varnished when dry.

These bronze pigments may also be used in varnish. The lake is then
completely dried and ground with varnish in such quantity as to give a
thick mass which can just be brushed on.

The vegetable bronze is obtained from logwood in a similar manner, a
solution of stannic chloride being generally used to precipitate the
lake when a deep bronze is required, and alum for a pale gold shade.
By using mixtures of the two salts intermediate shades are produced.
The precipitate produced by alum may be shaded by means of potassium
bichromate. Hæmatoxylin forms with chromium oxide a deep bluish black
compound which has such intense colouring power that it is used to
colour writing ink. The ink is prepared by adding a little potassium
bichromate to a decoction of logwood. If a very small quantity of this
dark precipitate is mixed with the lake precipitated from logwood
extract by alum solution, colours are produced possessing the peculiar
metallic lustre and a shade depending upon the quantity of potassium
bichromate added. The addition of this salt must be very carefully
made, as a very small excess is sufficient to render the colour so dark
that it is useless as a bronze. The shade of the precipitate depends
upon the concentration of the liquid and other conditions, so that
it is impossible to give precise quantities. In practice, the safest
and most convenient method is to dissolve the bichromate in a large
quantity of water and add very small quantities of the dilute solution
to the logwood extract along with the alum. After each addition a
portion of the precipitate is rapidly mixed with the above-mentioned
solution of soap and wax, to which a little size has been added, and
then spread upon paper. If the desired shade has not yet appeared, a
little more bichromate is added and another test made, and the process
repeated until the proper shade is attained.

Pigments for colouring wall papers and fancy leather are not easily
made which produce such fine effects at so small a cost as the
vegetable bronze pigments, which deserve the greatest regard from the
colour maker and leather and paper manufacturers.

=Appendix—The Brocade Pigments (”Brocatfarben”).=—Under this
designation powders have been recently introduced characterised by
a strong metallic or glassy lustre, and very suitable for certain
purposes, such as the manufacture of wall papers, since they enable
remarkably fine effects to be produced. They consist of mica in a
tolerably fine state of division. Mica is a mineral which occurs
frequently in nature and which very readily splits into thin sheets;
it occurs in various colours. In thin sheets mica is colourless and
completely transparent; in somewhat thicker pieces it generally shows a
distinct metallic lustre similar either to that of gold or silver. When
ground to fairly fine powder, it has the same gold or silver lustre,
and gold or silver brocade colours are distinguished in commerce. They
are made by grinding mica which has been previously sorted according
to its appearance. The powdered mica can be sold as powder or may be
mixed with a binding substance in order to be ready for use. Gum Arabic
is generally used as the binding medium, but it can be replaced by the
cheaper dextrine. When printed upon paper the brocade pigments produce
the effect of a bronze, and in addition to their cheapness they have
the great advantage of being completely unaltered by the air.

PIGMENTS OF ORGANIC ORIGIN.

CHAPTER XLVIII.

LAKES.

The pigments commercially designated lakes generally consist of an
organic colouring matter united with a metallic oxide. In isolated
cases other compounds of colouring matters are included under the term,
such as indigo sulphonic acid, and yet more rarely pure colouring
matters are included, for example carthamine red. But the overwhelming
majority of lake pigments are compounds of a colouring matter with
the oxide of a metal. For this purpose the oxides of tin, lead and
aluminium are commonly used.

Lakes are generally made by mixing the solution of the colouring matter
with the solution of a salt of the metallic oxide, and precipitating
the oxide by an alkali. The colouring matter is separated along with
the oxide and forms with it a substance known as a lake. It has not
yet been decided whether the lakes are true chemical compounds of the
colouring matter with the metallic oxide, or whether the colouring
matter is simply held fast by the surface attraction of the finely
divided oxide. In favour of the latter view is the fact that a larger
or smaller quantity of the colouring matter can be united with a given
quantity of the oxide.

The lakes vary greatly in durability; some, such as the madder lakes,
can be counted among the most durable pigments, whilst others have very
little permanence, as for example, the logwood lakes.

The majority of the colouring matters used in the preparation of lakes
are of vegetable origin, but several are derived from animal sources.
The properties of the colouring matters used for this purpose vary
greatly; each material demands a special treatment for the production
of lakes. Therefore, in describing the methods by which lakes are made,
we shall give first the properties of the colouring matter in question,
and then proceed to the preparation of the lake. In the preceding
portion of the book the pigments have been arranged according to their
colour; the lakes will be described in the same order. The yellow, red
or green lakes of different origin will be treated in the same section
in order to facilitate reference to any particular pigment.

White lakes are not known, neither are there lakes which can be
described as black. With these exceptions lakes of all colours can be
made and also of all shades. As an example of this madder lake may be
given, which is found in commerce in a great variety of shades; it
can be made from the palest rose-red to the deepest purple red, or,
more properly, madder red, which is a characteristic shade. The shades
of any lake are obtained by mixing in a white pigment just as in the
manufacture of mineral pigments. Whilst, however, in the latter case
the white pigment must often be mechanically mixed with the colour, the
shading of the lakes is accomplished in their preparation. Generally
speaking, a pale shade of a lake is obtained by increasing the amount
of the salt, the oxide of which is used to precipitate the colouring
matter. The oxides in question are white (with the exception of lead
oxide); thus in the pale shades a small amount of colouring matter is
precipitated upon a large amount of oxide, whilst in the deep lakes
the reverse is the case, a large quantity of colouring matter is
precipitated upon a small quantity of oxide, and the shade thus appears
very deep.

Apart from the nature of the colouring matter contained in a lake,
and considering only the metallic oxide with which the colouring
matter is united, it appears that lakes containing lead oxide have
little durability. The combination between the colouring matter and
the metallic oxide in a lake is so loose that it is easily destroyed
by sulphuretted hydrogen; if a lake containing lead is exposed to the
action of air containing that gas the colour will, in the course of
time, inevitably blacken. The lakes containing tin are also susceptible
to sulphuretted hydrogen; in air containing but a trace of this gas
they quickly lose their brilliance and in time are quite discoloured.

Alumina is not affected by sulphuretted hydrogen, and must thus be
regarded as the most suitable oxide for the preparation of lakes. It
is generally applied in the form of alum, in the selection of which
great care is necessary if fine colours are to be produced. Commercial
alum frequently contains ferric oxide; when the alumina is precipitated
simultaneously with the colouring matter the ferric oxide is also
thrown down, and is mixed with the lake, its dark colour influencing
the shade of the lake greatly to its disadvantage. The effect of the
ferric oxide upon the colour of the lake is so important that it is
impossible, for example, to obtain a pale red lake with alum containing
iron. To avoid the bad results given by alum containing iron it should
be examined before it is used, and if it is found to contain any
considerable quantity of iron it should be rejected for this purpose.

After alum a solution of stannic chloride is most commonly used to
precipitate lakes, but it must be free from stannous chloride; it
produces as a rule darker lakes than alum. Care must be taken that the
stannic chloride solution is free from iron.

The ordinary process for making lakes is very simple: a clear
aqueous solution of the colouring matter is obtained; to this alum
or stannic chloride solution is added in proportion to the amount of
colouring matter dissolved by the water; the metallic oxide is then
precipitated by an alkali. Sodium or potassium carbonate or caustic
alkalis may be used; ammonia is very suitable for this purpose, since
it is free from iron. The precipitant must be cautiously added, it
is introduced in drops when the greater part of the colouring matter
has been precipitated; an excess of the alkaline solution would have
considerable effect on the shade of the precipitate. The precipitated
lake rapidly settles to the bottom of the almost colourless liquid; the
settling is particularly rapid when stannic chloride has been used. The
liquid is then drawn off, the pasty residue brought upon a strainer,
washed several times with water, and dried in the air or in stoves.

When dry, a properly prepared lake forms a mass of little coherence,
and can be readily ground to a soft powder, which may then be ground
with oil or gum solution to produce oil or water paints. At the same
time the materials are added to the pure lake which are used to shade
its colour; white pigments are generally employed for this purpose—the
paleness of the shade is proportional to the amount of white pigment
introduced. Certain lakes are shaded by small quantities of other
colouring matters; for example, the admixture of a small quantity of
a blue pigment with a pure red lake produces a much deeper colour
inclining to purple. If a small quantity of a red or blue colouring
matter is added to a yellow pigment there is produced a deeper yellow
inclining to orange, or a yellow inclining to green, in proportion to
the quantity of the added colouring matter.

It should be observed that it is of great importance for the colour
maker to have a practised eye, sensitive to fine differences of
colour. He may then produce colours faultless in shade, for if by
chance a colour does not turn out well it may be improved by judicious
admixtures. It is quite impossible to give definite rules for the
mixing of colours; the estimation of a shade cannot be taught in words,
it demands long practice. By the use of a system, provided the user
has normal eyes, very slight differences of colour can be readily
estimated. A scale of colours should be made from pigments of pure
and definite shades, in which scale successful mixtures are gradually
inserted, so that a series is produced in which the separate pure
colours blend regularly into one another. Then, when it is required to
produce a pigment corresponding to a certain shade on the scale, with
a little practice the shade can be readily produced, for the scale
indicates which colour predominates.

The so-called sap colours are generally characterised by very pure
shades. They are produced by decomposing a lake by an acid or a strong
base, evaporating the solution of the colouring matter so obtained at
a gentle heat and mixing with gum, starch, or some other thickening
material to produce a mass of such consistency that it can be formed
into balls or sticks.

The decoctions of the dye-woods always contain other substances in
addition to the colouring matters, which are precipitated with the
colouring matter, and somewhat injure its shade. When such a lake
is decomposed the colouring matter is obtained in a purified form,
thus showing its full beauty. This solution of the purified colouring
matter might be again precipitated with a salt solution and lakes be
produced which would be much brighter than the original, but the losses
incidental to this double precipitation would be so great that the lake
would be made much too expensive.

CHAPTER XLIX.

YELLOW LAKES.

Yellow colouring matters are widespread in nature; from them are
obtained many yellow pigments used in dyeing and in the manufacture of
colours. The colours produced by the yellow vegetable colouring matters
are not particularly bright; thus the yellow lakes are used chiefly
to produce cheap pigments, whilst the inorganic pigments are used for
bright yellows.

=Dutch Pink.=—Several species of buckthorn (_Rhamnus_) contain a yellow
colouring matter—xanthorhamnin—which is obtained pure by extracting
the yellow berries with hot alcohol. On cooling, the impure colouring
matter separates; by repeated recrystallisation from alcohol it is
obtained in the form of crystalline needles, which are soluble in water
and alcohol.

The yellow lake known as Dutch pink is prepared from yellow (Persian)
berries, by boiling the crushed berries with water and mixing the
extract with a solution of alum. The lake is then precipitated by the
addition of powdered chalk. As a rule, 500 parts of water are used to
100 parts of berries, 20 parts of alum are added to the decoction, and
the mixture poured upon 75 parts of finely powdered chalk. The liquid
is decanted off, the residue filtered, washed and dried. Commercial
Dutch pink is made from a mixture of the decoctions of yellow berries,
quercitron bark and turmeric, to which the alum solution is added, and
then chalk. The precipitate is made into conical lumps, which are sold
as Dutch pink, and used for ordinary painting and for colouring leather.

=Weld Lake.=—The dyers’ weld (_Reseda luteola_) contains a yellow
colouring matter formerly much used in dyeing; a yellow lake can also
be obtained from it. When weld is boiled with water a deep yellowish
green decoction is obtained, from which yellowish-green flocks separate
on cooling. The lake can be obtained from all parts of the plant except
the root, the flowering shoots giving the largest yield of colour; the
lake is generally precipitated by alumina. Equal parts of weld and alum
are boiled with water until the latter is dissolved and the liquid is
coloured deep yellow; the hot solution is quickly filtered through a
thick linen cloth and soda solution gradually added in small quantities
with continual stirring so long as effervescence follows.

Alum entirely free from iron is required to produce a bright yellow
lake; a small quantity of iron has great influence on the shade. Weld
contains more or less tannin, which gives very dark blue or green
compounds with iron salts, the production of a very small quantity of
which suffices to convert the yellow shade into an ugly dirty colour.

Chalk may be used instead of soda to precipitate the alumina; in this
case the precipitate will contain calcium sulphate in addition to the
alumina compound of luteolin.

Weld lake may be used in oil, or size, or as a water colour.

=Gamboge Lake.=—Crude gamboge is the dried sap of certain East Indian
trees; it is used as an artists’ colour, but only in water; for use in
oil it is converted into a lake. A particular treatment is necessary
to obtain a handsome lake. Gamboge is treated for several days with
water and the soft mass ground on the slab or in a mill until it forms
a uniform paste, which is mixed with water to a thick liquid, which is
then put through a fine sieve. A hot solution of alum is then added
in the proportion of three parts of alum to one part of gamboge, the
mixture is boiled in a wooden vessel and two parts of nitric acid added
whilst well stirring; finally a dilute solution of potash is added
in small quantities until the liquid solidifies to a jelly, which is
spread out in thin layers on filter cloth, well washed with water and
dried at a gentle heat.

=Prepared Gamboge.=—Crude gamboge cannot be used in oil painting
because it never gives a uniform shade, but when the colouring matter
is freed from admixtures and ground with oil, it gives a deep durable
colour of great beauty.

The colouring constituent of gamboge is a resin which is fairly soluble
in strong alcohol, whilst the impurities are insoluble. The pure
colouring matter is obtained without great expense in the following
manner: Coarsely powdered gamboge is placed in a large flask with
strong alcohol; the flask is well closed, placed in a warm position
and repeatedly shaken; the deep yellow solution is carefully poured
off from the sediment, water is then added, when the colouring
matter separates in flocks. Only sufficient water should be added to
precipitate all the colouring matter; test portions of the liquid
should be examined from time to time, the alcohol can then be recovered
by distillation and used again. The colouring matter separated from the
alcoholic solution forms a hard mass on drying; it must be very finely
powdered before it is ground in oil.

There are many other yellow lakes which can be obtained by adding
alum to an aqueous decoction of the colour-bearing material and
neutralising the liquid. Such decoctions are obtained from fustic,
quercitron (_Quercus tinctoria_), young fustic (_Rhus cotinus_), the
root of barberry (_Berberis vulgaris_), annatto (the fruit of _Bixa
orrellana_), turmeric (the root of _Curcuma longa_), etc. The yellow
lakes obtained from these sources are seldom used in painting, the
mineral pigments are preferred on account of their finer appearance.
These colouring matters are largely used in dyeing to produce shades
varying from yellow to brown, the majority are also used in calico
printing.

=Fustic Lake.=—Of the colouring materials mentioned above fustic
produces a lake which is so handsome that it deserves larger use in
painting than it has yet found. ”Old fustic” is chipped immediately
before use and extracted with boiling water; whilst still hot the
extract is mixed with a hot solution of alum; on cooling a fine yellow
precipitate is formed, which when dry is frequently sold under the name
of Dutch pink. (Genuine Dutch pink is made from a decoction of yellow
berries; the majority of the pigments sold under this name are made
from decoctions of mixtures of the different yellow dye wares.)

To obtain the best lake from fustic the alum used must be completely
free from iron, otherwise only a dirty green lake is produced. Since
the least trace of iron damages the shade of the colour, fustic lake
is more frequently made by means of lead oxide. To obtain the lead
fustic lake the decoction is allowed to stand for several days, so that
the morin, which together with maclurin forms the colouring principle
of fustic, may separate; the liquid is filtered from the precipitate,
and lead acetate solution added which has been boiled with litharge to
saturation, so that the most basic acetate is produced. The lead fustic
lake has a deep yellow colour; it is well adapted for use in painting,
but, like all lead pigments, is not specially permanent in air. By
admixtures of levigated chalk or starch paler shades of the dull yellow
lake are obtained.

=Quercitron Lake.=—Lakes are seldom made from quercitron alone,
although they possess a deep yellow colour; they are more frequently
employed in dyeing when they are produced directly upon the fibre.
Quercitron lake can be made by treating the aqueous decoction of the
ground bark with tin crystals and a little alum solution. With tin
crystals alone a darker yellow lake is obtained than when alum is used
at the same time. Different shades may thus be produced.

Extract of quercitron bark comes into commerce as a greyish yellow
powder under the name of flavine, which has great colouring power, and
may be used with advantage in the place of aqueous extracts of the bark.

Since the discovery of the aniline dyes the applications of all lakes
have been largely extended, since the dyes enable brighter shades to be
obtained without the many processes necessary when natural colouring
matters are used, and do not require so much skill on the part of the
workman. This applies not only to yellow colouring matters, but to the
many other colouring matters formerly used by the dyer. The principal
reasons which have so quickly brought the new dyes to the front lie in
their greater beauty and in the diminished labour required in their use.

=Purree or Indian Yellow= must not be confounded with the yellow cobalt
pigment known also under the latter name. It is a compound of magnesia
with an organic acid—euxanthic acid. It is obtained from the urine of
cows fed upon mango leaves. It comes into commerce in lumps weighing
about 50 to 60 grammes, which are dark brown on the exterior, but on
fracture show a fine orange yellow colour. Indian yellow is little
used, and is not likely to attain importance. Many cheaper colouring
matters equal in shade are known.

=The Colouring Matter of Saffron.=—The dried stigma of the flowers of
the saffron, a species of _iris_, has been long used for colouring
foods. It contains a very handsome yellow colouring matter of a
characteristic shade; formerly this was largely used in silk-dyeing,
and especially for dyeing glove leather. It is practically now no
longer used; considerably cheaper aniline dyes produce an equally good
shade. On account of its high price (1 kilogramme of saffron contains
about 60,000 stigmata) this material cannot be used for lakes,
although they leave nothing to be desired in fineness of shade, but
they have no great permanence.

=The Colouring Matter of Gardenia Grandiflora.=—The fruits of this
plant, which are imported from Southern Asia, contain a beautiful dark
yellow colouring matter, which, according to Rochleder, is identical
with the colouring matter of saffron. It is readily soluble in water;
lakes of various shades can readily be obtained by the addition of
salts to the aqueous solution. Alum gives a pure yellow, lead acetate a
yellowish red, and stannous chloride a dark orange red lake.

CHAPTER L.

RED LAKES.

COCHINEAL AND CARMINE.

The purple of the ancients was obtained from the shellfish _purpura_;
it was so costly that it was regarded as an attribute of royalty.
According to history the Tyrians discovered this colour; at any rate
they were able to use it for dyeing. In the writings of the ancients
Tyrian purple garments were regarded as the greatest luxury. The art of
obtaining a purple from this source was lost; more recently it has been
re-discovered, but the colour is found in no way to correspond with
what we regard as a bright shade.

When the Spaniards conquered Mexico they found an insect cultivated
which produced a splendid red colouring matter. This insect, the
cochineal, is parasitic upon certain cacti, especially _Cactus
coccinellifer_ and _Cactus opuntia_. The females only are used as
colouring material; the males are very small, and much fewer in number.
The cultivation of the proper species of cactus and of the cochineal
has spread to most tropical countries. The females, which are attached
in enormous numbers, are brushed off and killed by heating upon hot
plates. They then appear as grains of the size of millet seeds, with
a wrinkled surface covered with a silvery grey dust. This variety of
cochineal is the best. Black cochineal also comes into commerce; it
has a brownish black colour. It is produced by killing the insects in
boiling water, by which the grey dust which covers the living insect is
removed. Cochineal produces when ground an ugly reddish brown powder.

The animal nature of cochineal cannot be recognised by the naked
eye, it is seen on examination under the microscope; this has given
opportunity for the most incredible adulterations of the costly
substance. Cases have been known in which a paste chiefly composed of
flour has been pressed into the form of cochineal, the grains coloured
by some cheap red colouring matter, powdered with the dust from boxes
in which cochineal has been packed, and placed upon the market as
cochineal. The expert would not be deceived by this gross fraud, but
another method of adulteration is far more difficult to recognise;
genuine cochineal is brought into commerce from which the greater part
of the colouring matter has been extracted, and which has been again
powdered with the grey cochineal dust. Such extracted cochineal does
not produce the same brownish-red powder as the fresh substance.

In the cochineal insect there is a very large quantity of colouring
matter: it may reach 50 per cent. of the weight of the dry insect. The
colouring matter, which is known as carmine (the same name is also
applied to its lake), is soluble in water with a fine red colour. When
cochineal is extracted with water the operation must be often repeated,
and each time fresh quantities of colouring matter are dissolved.
The cochineal may be exhausted by repeated boiling with water, but a
large quantity of liquid containing little colouring matter is then
produced. In order to obtain a strong solution of the colouring matter
the cochineal must be powdered; this is difficult on account of the
softness of the material, which does not give a fine powder, but a
pasty mass. The object is best attained by grinding the cochineal
through a mill similar to a coffee mill, but since the grooves of the
steel cone, which effects the crushing, may be easily clogged, the mill
must be arranged so that the cone can be taken out and cleaned.

The colouring principle of cochineal is an acid, carminic acid, which
was obtained by its discoverer, Warren de la Rue, in the following
manner: 1 part of powdered cochineal is boiled with 40 parts of
water for 20 minutes; after standing, the liquid is poured off from
the sediment, and mixed with a solution of 6 parts of lead acetate,
acidified by 1 part of acetic acid. The precipitate, which consists
of impure lead carminate, is filtered from the colourless liquid and
carefully washed; whilst still wet it is suspended in water, through
which sulphuretted hydrogen is passed; the lead carminate is decomposed
into lead sulphide and carminic acid, which dissolves in the water. The
colouring matter is not yet quite pure; the treatment with lead acetate
and sulphuretted hydrogen must be repeated, the solution then obtained
is evaporated at a low temperature, the residue dissolved in boiling
alcohol, phosphoric acid added to decompose traces of lead carminate
still present, then ether, finally the clear liquid is separated from
the precipitate and evaporated. By this process carminic acid is
obtained in a state of complete purity; unfortunately the process is
too complicated for practical application. Pure carminic acid is a
purple red mass, which transmits red light at the edges and forms a
pure scarlet red powder when ground. The aqueous solution gives, with
alum and ammonia, a characteristic precipitate, the colour of which
is the purest carmine-red; lead, zinc and copper salts produce purple
red precipitates. The composition of carminic acid is expressed by the
formula C₁₇H₁₈O₁₀.

CHAPTER LI.

THE MANUFACTURE OF CARMINE.

The colouring matter of cochineal is used in the form of a lake under
the name of carmine, the finest and most expensive colour used in
painting. However simple the preparation of carmine may appear, it
is not easy to obtain a product of great beauty; certain conditions,
still unexplained, play an important part in the process. Until not
long ago the manufacture of fine carmine was regarded as a secret; this
it is no longer, and with proper care any one may produce an entirely
satisfactory product. Many recipes have been given, some of the best
will be mentioned.

Whatever method is used to obtain carmine certain precautions must
be taken, without which it is impossible to obtain a bright colour.
Alkalis and alkaline earths in very small quantity affect the shade of
carmine, so that spring water should never be used in its preparation.
Pure rain water, or, still better, distilled water, should be employed.

The decoction of cochineal is difficult to filter. Paper cannot be
used, because the pores are so rapidly stopped up that new filters
would be continually required. Fine silk is the most serviceable; it
should not be washed with soap—the alkalinity of the small quantity of
soap the fabric would retain would affect the colour. The decoction is
made in a well-tinned copper pan, all other vessels should be of glass
or porcelain, which are most easily cleaned, and great cleanliness
is the prime essential to the success of the process. The greatest
care is required to prevent the contact of the liquid with iron during
the whole process, the smallest trace of this metal would result in a
discoloured product.

The essentials of the manufacture of carmine on the large scale are
that the colouring matter is dissolved in water, and precipitated by
the addition of an aluminium salt, generally alum, absolutely free from
iron. The more slowly the carmine separates the finer is its colour.
It is generally observed that the last portions to precipitate show
the brightest shade. This is because the foreign substances occurring
with the carminic acid in the decoction are thrown down with the first
portions of the precipitate. In the process of Frau Cenette, famous for
the beauty of the product, a solution is made from which the whole of
the carmine is separated in about three weeks. During this long time
the majority of the substances which have been dissolved in the water
together with the carminic acid are decomposed; the liquid acquires an
unpleasant smell, and is covered with mould. In the author’s opinion
so good a product is obtained by this process because the greater part
of the impurities is decomposed: the nearer the product approaches the
pure compound of carminic acid and alumina, the purer and brighter will
be the shade of the carmine.

It is known that light has considerable influence on the beauty of
carmine. During dull winter days it is quite impossible to produce so
fine a product as in summer. Instead of alum a tin solution may be
used to precipitate the colouring matter; the shade of the product is
different to that of alumina carmine.

CENETTE’S METHOD.—1 kilogramme of finely powdered cochineal is boiled
with 75 litres of water for two hours; 90 grammes of saltpetre are
added, the liquid boiled for three minutes, then 120 grammes of salt of
sorrel (acid potassium oxalate) are added and the liquid again boiled
for ten minutes. The liquid is then completely clarified by standing,
drawn off from the residue by a syphon, and brought into shallow glass
dishes which are placed, protected from dust, in a bright light in
a uniformly warm place. During several weeks the carmine separates,
the last portions being always brighter than the first. The addition
of potassium oxalate has the object of assisting the separation of
the carmine, for acid salts separate the carmine from solutions; the
saltpetre may reasonably be omitted.

The majority of the recipes for carmine, which are frequently sold at a
high price, differ but little from the above; acid potassium tartrate
is used instead of the oxalate, but the latter is to be preferred
because of the slight solubility of the tartrate. It is important
not to use too strong decoctions of cochineal, and to add only small
quantities of alum. The clear liquid is placed in shallow glass dishes.
After a few days the nature of the deposit should be examined: if a
considerable quantity of a red precipitate has formed, the liquid is
poured off into other dishes, in which carmine again separates during
the following days, and usually of a brighter colour than the first.
For example, 125 grammes of cochineal are boiled with 5 litres of water
during fifteen minutes, 30 grammes of very finely powdered alum are
added to the boiling liquid, which is again boiled for a few minutes,
allowed to clarify and cool. The greater part of the carmine is then
obtained in a few hours, but the liquid still separates carmine after
several days.

According to another formula 500 grammes of cochineal are boiled with
30 litres of water, 60 grammes of cream of tartar are added, then
30 grammes of alum, boiling is continued for several minutes, and
the liquid then allowed to cool. Carmine may be obtained in a very
short time by means of tin solution. The process is similar to that
just given. The liquid which would be set aside for the spontaneous
deposition of the carmine is, however, returned to the pan, and a
solution of pure stannous chloride added in drops, so long as the
solution is still clearly red. The carmine separates at the bottom of
the pan; the liquid drawn off, even when it appears almost colourless,
produces a further small quantity of carmine in a few days.

According to J. J. Hess, a brighter carmine is obtained when the fat of
the cochineal is previously extracted by ether or benzene.

Proposals have often been made to deepen the colour of prepared
carmine; this must be done with the greatest care, for it is very easy
to produce a less handsome instead of a finer pigment. The carmine is
moistened with distilled water containing about 5 per cent. of ammonia
solution.

Carmine readily dissolves in ammonia. This property may be applied to
test its purity, pure carmine should dissolve without residue in 5 to
6 times its quantity of ammonia. Any considerable residue denotes an
intentional addition of some adulterant. Starch, vermilion, and cheaper
lakes are used for this purpose. The red solution obtained by treating
carmine with the above quantity of ammonia may be used as a red ink.
This solution can also be used to purify carmine; when it is allowed to
stand in an uncorked bottle for some time the ammonia escapes, and the
greater part of the carmine is deposited as a very fine powder.

Carmine solution is made by dissolving carmine in the necessary
quantity of ammonia, adding glycerine equal in quantity to the ammonia,
driving off the latter by heat and diluting the liquid. This solution
is well adapted for colouring confectionery, but cannot be employed in
painting and writing, since the glycerine would prevent it from drying.

Carmine is extensively used in painting, and for many purposes cannot
be replaced by another pigment, _e.g._, the cosmetic known as vegetable
rouge can be made from no other pigment. Carmine may be used in all
methods of colouring; it is quite harmless for confectioners’ purposes.

=Munich, Vienna, Paris, or Florentine Lake.=—This fine deep red lake
differs from carmine in containing a much larger quantity of alumina,
thus possessing the character of an ordinary lake. Occasionally carmine
lake is intentionally mixed with light white substances, such as
magnesia, to obtain paler shades or cheaper pigments.

Whilst only the finest varieties of cochineal can be used to make
fine carmine, the cheaper sorts can be used for Florentine lake. The
cochineal used for carmine is boiled but a short time with water,
so that the residue contains considerable quantities of colouring
matter, which may amount to half of that originally present. The same
materials are used as in the preparation of carmine, and in this case,
too, stress should be laid upon their purity. The weight of the alum
is usually 10 to 15 times that of the cochineal; a little stannous
chloride and cream of tartar are also added to brighten the shade. All
these materials are boiled with the cochineal, soda is added to the
clear solution until effervescence no longer occurs, the separated lake
is then washed. The lake may be made with magnesia instead of alumina,
it is added in the form of magnesium sulphate; the more magnesia is
used the paler is the lake. The proportions in which the materials are
employed vary with each manufacturer. The following quantities have
always given the author a favourable result: Cochineal 10 parts, alum
150 parts, water 250 parts; or cochineal 10 parts, magnesium sulphate 5
parts, alum 0·5 part.

In making Florentine lake a fairly deep colour should be produced.
This may be mixed without difficulty in the wet or the dry state with
a white pigment, thus producing any desired shade, even to the most
delicate rose-red.

=Ammonia-Cochineal.=—This preparation, which is chiefly used by dyers,
is obtained by treating cochineal in a well-closed flask with strong
ammonia, which dissolves the colouring matter. After about a month alum
equal to about 3 per cent. of the amount of cochineal is added without
separating the undissolved residue from the solution, and the whole is
evaporated at a gentle heat in a tinned pan until it becomes a stiff
paste on cooling. The mass solidifies more readily if a small quantity
of starch paste is added. ”Cochineal paste” is cochineal which has been
treated in this manner; it is generally brought into the market in the
form of cakes or small slabs.

In addition to the true cochineal other species of the same insect
contain a red colouring matter and have a restricted use in dyeing. The
most important of these is the Polish cochineal (_Coccus polonicus_),
which lives on the roots of the _scleranthus_. It has not been proved
that these insects contain the same colouring matter as true cochineal,
but it is certain that the colouring matters of Polish and also of
Russian cochineal (_Coccus euvæ ursi_) are far inferior in beauty to
that of true cochineal.

CHAPTER LII.

THE COLOURING MATTER OF LAC.

From the punctures of the insect _Coccus lacca_ on certain East Indian
trees, especially those belonging to the genus _Ficus_, flow at the
same time resin and colouring matter in such quantities that the
insects are frequently enclosed and large red masses are formed on the
trees. The insects live in some measure in these masses, the females
lay their eggs in the spaces and the larvæ are said to feed on the red
sap contained in the mass. When the larvæ have left the incrustation,
generally in November, it is broken off from the branches and
collected. It is now known as stick-lac; it consists of large masses
of resin of a fine deep red colour. In the interior the cells of the
insects may still be perceived. On chewing, it becomes soft and colours
the saliva a deep violet. When stick-lac is boiled in water a portion
of the colouring matter dissolves. Good stick-lac contains 10 per cent.
of colouring matter and 80 per cent. of resin.

Seed-lac has the same origin as stick-lac, but frequently the greater
part of the colouring matter has been extracted; it rarely contains
more than 2·5 per cent., and is consequently of little value for
colouring purposes.

=Lac Dye.=—The colouring matter of stick-lac is generally separated
from the resin in India and comes into the market under the name of
lac dye. It is made by stirring coarsely powdered stick-lac in large
vessels for several hours with warm water. Almost the whole of the
colouring matter dissolves, whilst the resin remains as a ruby red mass
which is melted and brought into the market under the name of shellac.
The solution of the colouring matter is evaporated in shallow vessels
in the sun or boiled down in pans; the residue is made into cakes. At
present soda solution is used instead of pure water to extract the
colouring matter; a larger yield is obtained. Lac dye usually contains
45 to 50 per cent. of colouring matter, 25 per cent. of resin, and, in
addition, earthy substances which are to be regarded as intentional
admixtures.

In the preparation of lac dye by Stephen’s method coarsely powdered
stick-lac is boiled with soda, and the remainder of the colouring
matter contained in the resin extracted by repeated boiling with
water. All the extracts are then united and precipitated by alum. The
colouring matter separates as a lake which still contains a large
quantity of resin.

According to the patented process of Henley, the colouring matter of
seed-lac is extracted by hydraulic presses. The lac is filled into
press bags which are placed between iron boxes heated by steam. On
applying pressure the melted resin, shellac, comes through the bags,
whilst the colouring matter remains behind. If this method were
employed on the large scale, presses similar to those used in stearin
candle works would be suitable.

When lac dye is treated with hydrochloric acid the colouring matter
dissolves; the solution can be used to dye wool, which it colours a
beautiful red. The colouring matter is very similar in appearance to
that of cochineal, but has the appearance of much greater fastness. It
appears also to be similar in composition to carminic acid, but little
is known as to its chemical constitution.

When lac dye is to be used for painting it must be freed from resin;
this is accomplished by treating the finely powdered mass for a long
time with boiling alcohol and separating the solution of resin from
the undissolved colouring matter, which is then dried. In this way a
lake is produced known commercially as Vienna red and little inferior
to carmine lake. Unfortunately the price of lac dye, although it is
imported in large quantities from India to England, is so high that it
can only be used for artists’ colours.

CHAPTER LIII.

SAFFLOWER OR CARTHAMINE RED.

The safflower or bastard saffron (_Carthamus tinctoria_) grows wild
in Southern Europe, and is also cultivated. It contains two colouring
matters, yellow and red. The former is not used in dyeing, but is
employed for colouring liqueurs, since it is innocuous. The red
colouring matter is used for colouring artificial flowers and for fine
cosmetics; formerly it was employed in dyeing, but is now rarely used
for this purpose, since carthamine red has little permanence and can be
replaced by cheaper dyes.

The yellow colouring matter, which is not used, is generally removed
by treating the corolla of the safflower, which contains the colouring
matter, with water. After this treatment it is known as washed
safflower. In the dried corolla of the flower up to 36 per cent. of the
yellow colouring matter is found, whilst only 0·4 to 0·6 per cent. of
the red is contained.

It appears that the colouring matter of the safflower has been used
from the most ancient times. The Chinese employ it to obtain a handsome
cosmetic, the Tyrians are said to have used it in dyeing. In Europe it
was not cultivated before the seventeenth century, when it came into
use for dyeing.

In order to obtain pure carthamine red—and this is the form in which
it is generally used—safflower is treated for a long time with water
containing a little acetic acid until a yellow solution no longer
results. The residue is then treated with soda solution for several
hours, when the colouring matter dissolves; the solution is filtered
and neutralised with acetic acid. Cotton is then introduced. Carthamine
red is a colouring matter which at once dyes animal and vegetable
fibres when these are brought in contact with its solution. Thus the
whole of the colouring matter is precipitated on the cotton, which is
coloured a deep red. After twenty-four hours the cotton is taken out,
washed, and treated with soda solution, in which the colouring matter
dissolves. When this solution is carefully neutralised with citric acid
the colouring matter separates in the form of fine flocks. These are
collected, dissolved in strong alcohol, the solution evaporated to a
small volume, the colouring matter again precipitated by the addition
of a large quantity of water, and washed with pure water until the wash
waters begin to be coloured red. The pure carthamine thus obtained is
spread upon small cups, in which it dries to a beautiful red mass which
in somewhat thicker layers has a fine green lustre. It is brought into
commerce as ”cup red” or ”plate red”.

The small quantity of colouring matter contained in the safflower,
as well as the complicated process for obtaining it, make it evident
that this colouring matter will be among the most expensive found
in commerce. The high price is, however, neutralised by its great
colouring power, and by the fact that nothing can in all cases replace
it for colouring the best artificial flowers.

=Safflower Carmine.=—This substance, used by dyers, is a solution of
carthamine red in soda solution. It is only necessary to add an acid
after once introducing the fabric in order to fix the colouring matter
at once upon the fibre.

The shades produced by carthamine red are distinguished by a delicacy
of tint not produced by other dyes; all shades between the deepest red
and the palest rose-red may be obtained from it. Unfortunately it is
very susceptible to the action of alkalis; one careless washing of an
article dyed with carthamine is sufficient to remove the greater part
of the colour.

On account of its cost and little durability carthamine red is not
used in painting. It is principally employed in the artificial flower
industry for colouring flowers. Pure carthamine or a mixture with
finely powdered steatite is applied by rubbing.

=Alkanet.=—The roots of alkanet (_Alkanna tinctoria_) contain in the
bark a fine red colouring matter, from which also a violet lake can
be obtained, though this is rarely made. The plant is largely grown
in Southern Europe and also around Vienna. The colouring matter is
obtained pure by macerating the root for a long time with water and
then treating with strong alcohol, which dissolves the colouring
matter, together with a large quantity of resin. The alcohol is
distilled off, the residue extracted with ether, and the ethereal
extract treated with a large quantity of water, which extracts the
colouring matter. This is left in the pure state, when the solution is
slowly evaporated.

According to Carnelutti and R. Nasini alkannin is obtained by
extracting the root of _Anchusa tinctoria_ with petroleum ether,
evaporating, treating the residue with weak caustic potash, filtering,
and shaking the filtrate several times with ether. The solvent is
removed, the colouring matter precipitated by carbonic acid, dried over
sulphuric acid, and dissolved in ether. The filtered solution is then
allowed to evaporate. Alkannin is then obtained as a dark brownish-red
mass, which is easily powdered. It has a metallic lustre, and readily
dissolves in alcohol, glacial acetic acid and chloroform.

When alum is added to the aqueous solution of the colouring matter a
beautiful violet precipitate separates, which when dry is very suitable
for artistic purposes. Alkanet is little used. It is completely
replaced by much cheaper colouring matters.

CHAPTER LIV.

MADDER AND ITS COLOURING MATTERS.

The root of the madder (_Rubia tinctoria_) contains a red colouring
matter which is distinguished over all other colours of vegetable
origin by great fastness, on account of which it occupies a most
important position in dyeing and colour making. The use of madder in
dyeing has been much restricted by the discovery of artificial methods
of making alizarine, the most important colouring matter of madder.
Thus the following account of madder and its products is chiefly of
historical interest only.

Madder is cultivated in many countries. The roots, which vary from the
thickness of a quill to that of a finger, are cleansed from adherent
earth, carefully sorted according to size, and dried. In commerce a
distinction is made between stripped and unstripped madder. The cortex
of the root contains but little colouring matter and is generally
removed by mill-stones moving at some distance apart. The stripped
roots are finely ground, and are then known commercially as madder.
Merchants distinguish a large number of varieties, which are designated
according to the place of production. In addition to ground madder
there are a number of madder preparations which contain the colouring
matter in concentrated form, and are consequently used in smaller
quantity. The most important of these preparations are garancin,
garanceux, and the madder extracts.

”Refined madder” is made from the natural substance by subjecting it
to a process of fermentation, by which the substances accompanying
the colouring matters are largely decomposed, and the product becomes
relatively richer in colouring matter.

”Flowers of madder,” which have double the colouring power of madder,
are made by treatment with very dilute sulphuric acid. Madder is
stirred with five or six times its quantity of water to which about 1
per cent. of sulphuric acid has been added. After leaving the mixture
in a fairly warm place for five to six days alcoholic fermentation
takes place, and many substances which would be injurious in dyeing
are decomposed. The residue becomes in consequence richer in colouring
matter. When the fermentation is finished the residue is filtered
off, subjected to powerful hydraulic pressure, and the mass broken up
and very thoroughly dried at a temperature of 60° to 70° C. If it is
not thoroughly dried it soon becomes mouldy. This method of treating
madder, due to Julians, is simple, and has the advantage that from the
alcoholic liquid spirit may be obtained. In some respects this process
is more rational than the one immediately following: certain substances
which are destroyed in the manufacture of garancin are made useful, and
also the process requires little outlay.

=Garancin= is obtained from madder by treatment with sulphuric acid,
which decomposes the glucoside, and also sets free the colouring
matter united to lime and magnesia, which would otherwise be lost. In
addition, the warm acid radically attacks the nitrogenous compounds,
the greater portion of which is destroyed. Thus the residue contains
relatively more colouring matter, and the removal of substances which
accompany the colouring matter considerably facilitates the dyeing
process.

Although the preparation of garancin was recommended in 1828, it was
much later before the prejudice which prevented its general employment
was completely overcome, and garancin recognised as a very valuable
material for the dyer. The simplest process for preparing garancin is
as follows: Madder is several times washed with water, then pressed,
and the residue mixed with dilute sulphuric acid in a lead-lined
vessel. To 100 parts of madder 50 parts of sulphuric acid and 50
parts of water are taken, the mixture is heated by steam to 100° C.,
and maintained at this temperature for half an hour. By the action
of sulphuric acid of this strength considerable charring occurs, and
in particular the cell walls of the madder are attacked, so that in
the succeeding process the colouring matter is readily dissolved; in
consequence of the charring, the garancin acquires a deep brown to
black colour. When the action of the acid is finished, it is drawn off
and the residue washed with water until it is free from acid, when it
is dried. The process here described is the original method; it has