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.

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.

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.

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.

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.