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.
CHAPTER XLIII.
INDIAN INK.
Indian ink consists of purified soot mixed with gum, as binding medium, and a little camphor and musk. It is generally believed that Indian ink is made in China by a process which is still a secret; this opinion is supported by the fact that the ink made in Europe is almost always inferior to the Chinese. It is quite possible that a substance which produces a particularly fine black is used in China to prepare the soot used for the ink, and which we do not yet know or do not use on account of its high price. But it appears that the difficulty of producing an ink equal in quality to the Chinese lies less in the quality of the soot —for by the methods of purification already given we can make almost chemically pure carbon—than in the extremely careful mechanical treatment of the soot with the other constituents of the ink.
The genuine Chinese ink has a peculiar smell, which, when not hidden by musk, distinctly recalls the smell of burning camphor; the camphor tree is a native of China, so that it is not impossible that the soot of the wood of this tree is used for the ink. The soot from camphor alone would be too expensive, so that it is probable that camphor wood is used from which camphor has been prepared, or that camphor soot is mixed with the soot of a cheaper substance. In fact several varieties of Chinese ink show distinctly the smell peculiar to soot produced by fat. The binding material is animal glue, which is most thoroughly ground with the carbon; the secret of making good Indian ink lies in the careful grinding with the binding medium.