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.