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.
CHAPTER XL
MANUFACTURE OF SOOT PIGMENTS.
The soot which is formed in the incomplete combustion of organic substances containing a large proportion of carbon is a mixture of different substances, of which carbon is the chief; in addition to this, we find in soot almost all the products which result by the dry distillation of the substance from which it is formed. Soot from hard wood contains different compounds to soot from soft wood; again, rosin soot is of a different nature to that obtained in the incomplete combustion of fats. We can thus define soot as very finely divided carbon, mixed with the products of dry distillation.
The soot of hard woods, which contain little or no resin, has a deep 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.