The manufacture of mineral and lake pigments

by Josef Bersch

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.

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.