BLACK PIGMENTS.
Carbon occurs in nature in many different forms—the diamond, graphite
(black lead), and purified lamp black are, from the chemical point of
view, one and the same substance, namely, carbon. In colour making the
non-crystalline form of carbon, which is pure black, is alone used;
almost all black pigments used in painting are composed of tolerably
pure carbon. In whatever way and from whatever materials carbon is made
for use as a pigment, the manufacturer must always endeavour to obtain
it as pure as possible, and in a condition of the finest division; the
depth of colour and the covering power depend on these two conditions.
Carbon only shows a pure black colour when it is pure; if it contains
relatively small quantities of impurities, it has a more or less brown
shade.
At first sight it appears to be a very simple matter to obtain black
pigments from organic materials; it is simply necessary to expose them
in the absence of air to a temperature high enough to decompose them,
carbon is then left as a residue. In spite of the apparent simplicity
of this operation there are many difficulties. The preparation of good
black pigments demands considerable practice.
There are two methods by which carbon can be made for use as a pigment.
One has been indicated, the heating of organic materials in the absence
of air, _i.e._, their dry distillation; the second, which produces the
best qualities of black, consists in burning substances very rich in
carbon with a very small supply of air. The product of this process,
commonly known as soot, is very different in external appearance from
carbon made by dry distillation. The latter forms an easily powdered
mass, which only acquires lustre by grinding with oil, whilst soot
consists generally of very light flocks, which, in consequence of their
greasy nature, exhibit a peculiar velvety lustre.
In commerce there are many qualities of black, the names of which are
generally chosen quite arbitrarily. The so-called ivory black is an
example of this. At some period it was observed that ivory produced a
very fine black when carbonised; this black was made for a long time
from refuse ivory. When it was found that a similar black could be
made quite as well from much cheaper materials, it was no longer made
from ivory. The name, however, remains in trade to the present day: it
has come to be regarded as a description of quality. By the name of
ivory black a fine black pigment is understood; it is immaterial to the
consumer whether it is made from ivory or not: to him the quality of
the pigment, but not its source, is important.
According to the method of production, the black pigments can be
divided into charcoal blacks and soot blacks.
CHARCOAL BLACKS.
This term is applied to the pigments obtained by heating organic
substances in the absence of air. The charcoal blacks have two valuable
properties: they are easily made, and they exhibit a pure black shade
which it is far more difficult to obtain with the soot blacks. It is
very difficult to convert these pigments into the requisite state
of fine division. They cannot be levigated on account of their low
specific gravity, which causes them to settle very slowly in water.
It only remains to convert the charcoal into a very soft powder
by grinding, but even then the pigment has but little covering
power, because it is not possible to destroy the organic structure
of the substance carbonised. When the finest charcoal powder, made
by carbonising any organic substance, is examined under a powerful
microscope the structure of the particles is at once recognised, so
that it is almost always possible to decide whether the charcoal is
of plant or animal origin. A very careful examination may show what
particular part of the plant has been carbonised. If a manufacturer
succeeded in producing by direct carbonisation a black with the
covering power of a soot black, that black would soon be the only one
found in commerce.
The charcoal blacks come on the market under various names—ivory black,
bone black, vine black, Frankfort black, Paris black, etc. The ivory
blacks are the best quality, but recently true vine black, _i.e._, made
from grape residues, has come into use.
=True Charcoal Black.=—In charcoal-burning wood is burned in great
piles with restricted air supply; the greater part of the carbon
contained in the wood remains behind as charcoal, which is used for
fuel. The charcoal made from hard wood, such as maple or beech, is not
well suited for pigments; the lighter and more porous the wood, the
more pure is generally the shade of the charcoal, and the more easily
it can be ground. When, therefore, charcoal is to be used as a pigment
it should be made from wood of a porous nature; the wood of the lime,
black-alder and spindle tree is particularly adapted for this purpose.
Spent tan-bark is a very cheap, and at the same time suitable material
for this purpose. It generally consists of oak bark, and has lost the
greater part of the salts it contained by long contact with water;
also the woody substance of the bark has suffered a change which is
favourable to carbonisation. This is especially the case when the bark
has been stored for some time; a considerable quantity of the humins
previously mentioned has formed in it, and these are easily decomposed
at a low temperature.
The charcoal, however obtained, is ground to a soft powder. When it has
reached the proper degree of fineness it should be repeatedly washed
with water to remove the salts. When instead of pure water a very
dilute acid, such as hydrochloric, is used, practically all the salts
dissolve, and the residue consists of nearly pure carbon.
=Vine Black.=—In wine-producing countries a good and cheap black can be
made from the residues of the wine manufacture; this is the so-called
vine black. Either the pressed grapes or the lees separated in the
fermenting vessels may be used.
_Vine Black from Wine Lees._—The lees always contain a considerable
quantity of liquid, they must be thoroughly dried before they are
carbonised; this is most simply done by spreading out the pasty mass in
a thin layer, and exposing it to a temperature of about 100° to 120° C.
In drying the volume considerably diminishes. The lees are changed into
a brown mass which is easily powdered; it is packed into barrels while
still warm, and can be kept for a considerable time without alteration.
Fresh lees can be kept but a short time; they are quickly destroyed by
a rapid fermentation.
The dried lees are carbonised in iron tubes, protected from the
fire by a thin coating of clay mixed with chopped hair so that the
coat more readily adheres to the iron. Old stove pipes or cast-iron
gas or water pipes may be used. The tubes are about one metre long;
they are closed with well-fitting covers, in one of which is a small
opening through which the gases produced on heating can escape. These
covers are fastened on air-tight with clay. The process begins by
affixing the unperforated lid, and packing with a wooden rammer the
dry lees as tightly as possible into the tube; the second lid is then
luted in place. The tubes are placed near one another in a suitable
furnace and first slowly heated at the back, _i.e._, the end closed
by the unperforated cover. At the commencement of the operation heat
must be gradually applied. With too large a fire the products of
dry distillation might be evolved in sufficient quantity to force
the covers from the tube; air would then enter and the contents be
burnt. When the hinder part of the tube is red-hot, the heating is
conducted forward, and finally the whole length of the tube is brought
to a good red heat. The termination of the operation is shown by the
disappearance of the pointed flame of the products of distillation
which protrudes from the opening in the cover of the tube. When this
occurs, the fires are drawn; the tubes are left to cool in the furnace
until they can be taken out by the hands protected by wet cloths. The
covers are at once taken off and the hot contents emptied into a large
tub filled with water, which is thus soon raised almost to boiling
point. The time required for washing is thus considerably shortened.
The charcoal falls from the tubes in lumps, which soon fall to a fine
powder in the water. Wine lees consist of a mixture of yeast cells and
small crystals of tartar (a mixture of the tartrates of potassium and
calcium). On ignition these salts are converted into carbonates, by
which the particles of carbon are united. Potassium carbonate is very
readily soluble in water; in contact with the warm water it dissolves
in a very short time, the carbon particles then fall apart.
When the charcoal has settled to the bottom of the vessel the liquid
is drawn off. It can be utilised in a colour works, since it is a
fairly strong solution of potassium carbonate. The charcoal is mixed
with calcium carbonate, arising from the decomposition of the calcium
tartrate, and with the other insoluble salts which yeast ashes contain
in some quantity. These salts would prevent the proper grinding of the
charcoal; they are therefore removed by treatment with hydrochloric
acid after the water has been drawn off as completely as possible.
A small quantity of hydrochloric acid, diluted with an equal volume
of water, is poured over the charcoal, an effervescence of carbonic
acid follows, the easily soluble calcium chloride is formed and the
remaining salts are also dissolved.
The charcoal made in this way is very pure. After washing, grinding and
drying, it forms a pigment whose shade leaves nothing to be desired.
The drying must be conducted at a low temperature; charcoal in such a
fine state of division is very easily inflammable.
_Vine Black from Pressed Grapes._—The material contains the stems
and pressed remains of the grapes after the must has been expressed.
After-wine, spirits or vinegar can be obtained before it is used for
vine black. The process is exactly the same as for wine lees. The
charcoal taken from the tubes has rather more coherence than that made
from lees and must be ground. The black is a very useful pigment, but
is inferior to the black from lees. In regard to the latter, it should
be stated that it is a particularly good black pigment, and when finely
ground is as well suited for the preparation of the finest blacks, such
as are used in copper-plate printing, as the far more costly soot black.
The black from grape residues, or from grapes themselves (the poor
grapes removed in pruning the vine are used), can be employed with
advantage for many purposes for which the best black is not necessary.
The greater number of the pigments sold under the names of Frankfort
and Paris black consist of this substance.
=Bone Black or Ivory Black.=—Bone black, for of ivory black only the
name now exists, is distinguished by a peculiar quality which it owes
to the structure of the raw material from which it is made. This raw
material is animal bones, which largely consist of incombustible
materials (bone ash); these form a delicate framework whose interstices
are filled by organic matter. When bones are carbonised, the carbon
resulting from the decomposition of the organic matter is deposited on
the incombustible framework of bone ash and thus acquires a very great
surface.
Carbon is well known to possess very powerful absorptive properties,
which are especially developed in bone black, in consequence of its
fine division. The use of granular bone black (known as ”char”) in
sugar works, and generally for decolourising liquids, is due to the
peculiar state of division of the charcoal. For these purposes bone
black is used in enormous quantities; it is made in special works.
It does not fall within the scope of this work to give a detailed
description of the manufacture of bone black, and we must here restrict
ourselves to matters of special interest to the colour maker.
The bones are coarsely powdered and freed from fat by boiling; they are
then generally carbonised in iron retorts, a number of which are built
in a furnace in a vertical position; a valve at the bottom serves to
empty the retorts. In a well arranged apparatus the products of the dry
distillation of the bones, chiefly consisting of ammonium carbonate,
are collected. The retorts are carefully closed so that air cannot
enter, otherwise a portion of the carbon would burn and the black would
acquire a grey shade instead of the pure black which it should possess.
Bone black makers who work for sugar factories carbonise by preference
the densest bones; these produce a black of the most powerful
decolourising properties. This property is of no advantage to the
colour maker, who simply requires a very dense black. The bones
of young animals, and especially certain bones, contain a larger
proportion of cartilage than the hollow bones which produce the best
”char”. In making bone black to be used as a pigment just those bones
should be chosen which are of less value for ”char”.
On the small scale, bone black can be made by carbonising in crucibles;
these hold about 16 kilogrammes of bones, they have a projecting rim,
so that when one is placed upon the other it serves as a lid for the
first. Piles of these pots are formed, the top one being covered by a
well-fitting lid. The piles are heated in a furnace in such a manner
that the flames can freely circulate between them. At first a small
fire is applied; as soon as dry distillation of the bones begins, which
is recognised by pale white flames at the rims of the crucibles, the
fire is damped, because the burning products of distillation produce so
much heat that the crucibles are soon at a good red heat.
When the flames disappear the fire is maintained for about fifteen
minutes longer, and when the crucibles have cooled to some extent
they are taken out of the furnace and immediately emptied into a
sheet-iron cylinder, in which the charcoal remains until quite cold.
If the black comes in contact with air whilst still hot a portion of
the carbon burns and the product has little value either as a pigment
or for decolourising purposes. When quite cold the black is ground and
levigated. The bone ash it contains makes these operations easy. Bone
black contains at the most 12 to 13 per cent. of carbon; the remainder
consists of bone ash and water absorbed by the hygroscopic carbon from
the air.
Bone black which has been partially burnt in the process has a greyish
tinge; it may also have an ugly shade of brown: this is the case when
it has not been heated to a sufficiently high temperature, so that
it still contains some quantity of organic matter. Such black can be
made usable by again heating, but it would always be desirable to test
a small portion before the whole was ground and levigated; a uniform
heating of the finely-powdered black would be attended with difficulty.
Finely ground bone black has numerous uses as a pigment; by a simple
process it can be converted into almost pure carbon, which can be used
as an excellent black pigment for all purposes for which blacks are
employed. Bone black consists of bone ash upon which fine particles
of carbon are deposited. Bone ash is easily soluble in hydrochloric
acid; if finely ground bone black is treated with this acid and the
residue washed with water until the washings are neutral, a residue
of extremely soft and pure carbon is obtained, which has a deep black
colour and, in consequence of its fine division, very great covering
power.