Among the graphites now in the market, it may be said that the supply for refractory purposes, blacklead, and antifriction, comes in a great measure from Bavaria and other parts of Germany, and from Ceylon. The graphite of Passau (Bavaria), so extensively used in apparatus subjected to great heat, contains only 35-42 per cent. of pure graphite, the residue being argillaceous. The better varieties for pencil-making are also principally of Bavarian and Bohemian production, and form the best substitutes for the renowned Cumberland mineral, which owes its value rather to the peculiar state of aggregation of its particles than to chemical purity, as it is much lass pure than some of the Ceylon graphite which does not approach it in price. The value of the comparatively pure crystalline graphite of Ceylon and America, containing very small proportions of earthy matters, is placed at about 201. per ton. The former is too fragile for pencils. That found in the neighbourhood of Ronda in Grenada, and near Malaga in Spain, is hard and difficult to grind.
It now remains to describe the process above alluded to by which impure and waste graphites may be rendered available for many uses. The graphite is first reduced to very fine powder, then compacted by moderate pressure, and enclose.' in thin paper glued all over and pierced in one place by a small hole, to permit the escape of air when placed under an exhausted receiver. In this way, the air is removed ; and the orifice is closed. Within 24 hours, it is subjected to pressure again, and the block formed is capable of treatment as a natural solid body.
In order to remove the impurities from inferior graphites, the finely pulverized mineral is mixed with a proportion of nitric acid or an alkaline nitrate, chlorate, chromate, or bichromate, preferably chlorate of potash, in weight about z to a of the quantity of mineral. To this is applied sulphuric acid, sp. gr. 1.8, in quantity about equal to twice the weight of mineral, and the whole is thoroughly blended. The mixture, prepared in an iron vessel, is heated to a moderate degree, by which chlorous gas is copiously evolved. As soon as these fumes subside, the vessel is allowed to cool, and the oxidized and sulphated mass is thrown into water and washed by decantation. It is then dried, and heated to redness in a furnace, which causes it to swell up and disintegrate. The resulting powder has only to be agitated in water in order to separate the graphite, which is porous, and floats on the surface, from the silica, peroxide of iron, and other impurities, which, being heavier, sink to the bottom. The graphite thus prepared is absolutely pure.
The product obtained from amorphous graphite is not so fine as that from lamellar or foliated, and cannot be levigated with the same facility. To complete its purification, a little fluoride bf sodium is added to the mixture in the iron vessel, as soon as the chlorous fumes cease to he evolved. The hydrofluoric acid, set free by the combination of the sodium with the sulphuric acid, immediately attacks the silica present, and carries off this impurity as gaseous fluoride of silica.
Ceylon graphite is very difficult to purify. After twice treating as above, there remained 0.42 per cent. of an incombustible residue, which was reduced by a third operation to 0.12 per cent. Bohemian and Styrian grapbites purified in this way yielded mere imponderable traces of white incombustible matter.
For making crucibles, the first process which the graphite undergoes is that of grinding in " cannon-ball " mills, shown in plan and section in Fig. 804: a is a heavy iron saucer-shaped receptacle, having an aperture in the centre, across which are arms, connecting it with the central shaft b. This shaft is rotated by pulleys. Above the saucer, is a disc c, in which are four recesses. In these recesses, and resting on the saucer below, are four 32-1b. cannon balls ; and, attached to the middle of the disc, is a sleeve d, enclosing shaft b, and carrying a pulley, by which it is rotated in a direction relatively opposite to that of shaft b. A casing surrounds the mill, through which the graphite enters, emerging below through the funnel e, whence it is taken away by an elevator. When the graphite enters, the centrifugal force generated by the swiftly rotating parts throws it out ward, so that it may be at once acted upon by the balls. Wear by the latter on the disc is prevented by steel pins f.
It will be obvious that, under this condition, the heavier particles of the material will approach nearest the circumference, while the finer ones will arrange themselves in the order of their weights toward the centre. Consequently the finest ground graphite will always be that which is escaping from the mill, while the grinding parts constantly act on the coarser portion. In this way, the grinding operation is greatly facilitated, and, at the same time, the graphite is reduced to a degree of fineness unattainable in ordinary forms of mill.
The graphite thus prepared is mixed with a small proportion of China-clay, varying according to the use for which the crucible is intended. To every 10 parts of graphite, is also added 7 parts of a grey clay which is imported from Klingeuberg, in Bavaria, besides a little ground charcoal. These ingredients ere mixed dry ; water is afterwards added, and the compound passes to a cast-iron cylinder, capable of holding about 3 tons. Here thorough stirring is done by means of arms arranged radially on a central vertical rotating shaft. Each arm, besides having four vertical bevelled blades, is made flat above and bevelled below, so that the mass undergoes a kneading, which secures its rapid and homogeneous mixture. The material emerges in the form of thick mud, and is at once moulded either by hand or machinery, the operation being performed in earnest exactly the same way as by potters. The same may be said of the subsequent baking. The above description applies more especially to the Dixon (American) crucibles. The Plumbago Crucible Co., of Battersea, mix Stourbridge clay with their graphite ; their crucibles show, on analysis, per cent. of carbon, of earthy matter, and 2.08 of water.