A certain temperature is necessary for the combustion of every substance; if a cold body is placed in the flame of a candle or an oil lamp it becomes covered with soot, because it cools the flame to such an extent that the carbon is no longer heated to the temperature necessary for its combustion, and is, therefore, separated in a finely divided condition. Use is made of this phenomenon in a method of making soot black which has many advantages, of which the most important is that expensive buildings are not required, and the manufacture can be conducted in restricted space.
Fig. 33.
A convenient arrangement for obtaining black by this process is represented in Fig. 33. The thin-walled hollow cast-iron cylinder is turned smooth on the outer surface, and is surrounded by a sheet-iron cover at the distance of a few centimetres; it rotates in bearings, which, like the spindles, are hollow; cold water is thus led through the cylinder from a tank at a higher level. Below the cylinder are placed, near to one another, smoking lamps, and at the side of the cylinder is a broad brush of soft hair which continuously removes the soot deposited on the surface, which then falls over an inclined plate into the collecting vessel. The cylinder is kept in slow rotation by any mechanical arrangement. The lamps are devised not to produce a hot flame, but when this comes in contact with the cold surface of the cylinder, it deposits a ring of soot on the rotating cylinder which is then removed by the brush. The cylinder is kept cooled by the water which runs through it. The soot which is collected shows a tolerably strong brown shade in consequence of the rapid cooling of the flame, which causes the formation of a large quantity of products of distillation; the soot will always require ignition.
Fig. 34.
It has been proposed further to simplify the manufacture of soot black by this method, by burning coal gas under the cylinder from numerous small openings in a pipe. Coal gas indeed gives a very fine deep black, but the yield is so small that this process could never be adopted with advantage.
The apparatus constructed by Thalwitzer for the manufacture of oil black (Fig. 34) consists of a plate, A, with a rim, a, fastened to a vertical axis, b. This is carried by bearings in B, and is kept in rotation by the cog-wheels, d and f, moved from the shafting, O. The plate is cooled by water supplied by the pipe, g, it flows away by h and the annular vessel, D. The lamps, e, are connected with the common oil reservoir by n. H is a scraper fastened to B by x.