Graphite

Graphite is generally of organic origin, having being formed from carbonaceous material, probably without previous separation of amorphous carbon, by pressure of overlying rocks and a sufficiently high temperature. The presence of about 1 per cent, of hydrogen in natural graphite confirms this view of its formation.

Graphite is the most stable form of carbon at high temperatures. Thus the tips of the carbon electrodes of the arc lamp gradually become covered with a growth of graphite by vaporisation; and all kinds of carbon are vaporised, and thus converted into graphite, in the electric furnace. Likewise diamond is changed into graphite when placed between the poles of an electric arc. It has been shown by Arsem, however, that other forms of carbon are not always converted into graphite when heated in closed crucibles to 3000°-3300° C. When various carbon compounds are decomposed at high temperature the carbon remains as graphite; graphite also results from the decomposition of carbides, cyanogen, acetylene, and carbon disulphide in the electric furnace.

Graphite of exceptional purity is prepared artificially by the Acheson process, by passing an electric current between carbon electrodes in a furnace built of fire-bricks and lined with carborundum. The intervening space is filled with sand and coke or anthracite. It is found that petroleum-coke yields a fine quality of graphite. The carborundum first formed decomposes, the silicon being volatilised and the carbon left in the form of graphite. Thus:

SiO₂ + 3C = SiC + 2CO;
SiC = Si + C.

This graphite leaves on combustion only 0.05 per cent, of ash.

When carbon crystallises from molten metals, such as iron, it separates as graphite. This fact was first observed by Scheele in 1778. Whilst some of the carbon forms carbide, the excess is held in a state of solution from which it crystallises when the metal cools. Grey cast-iron contains more or less crystallised graphite; and graphite likewise occurs in meteoric iron. It has already been pointed out that when carbon crystallises from molten iron under great pressure it appears as diamond.

Graphite, like diamond, has been known from ancient times, and was early employed, for writing purposes. It was formerly, however, confused with molybdenum sulphide, MoS₂, which resembles it in appearance and in leaving a mark upon paper. The name plumbago, or black lead, shows that graphite was also identified with or thought to contain lead. Scheele, in 1779, distinguished clearly between molybdenum sulphide and graphite, showing the latter to be a mineral form of carbon since it is converted into carbon dioxide by nitric acid; and in 1800 Mackenzie showed that graphite burns like charcoal, producing carbon dioxide.

Graphite occurs in nodules in granite, gneiss, slate, and other rocks, in flakes or powder scattered throughout limestone, and sometimes in large masses. Excellent graphite was formerly found at Seathwaite-in-Borrowdale, Cumberland, and made into lead-pencils at Keswick; but now these mines appear to be exhausted. Graphite is also found in Ceylon, in various parts of the United States, especially California, at Passau in Germany, and in Spain, Bohemia, Moravia, Styria, and Siberia (Irkutsk). Much of the graphite used in the United Kingdom comes from Ceylon; but Bohemian graphite is said to be the best for making pencils, and Ceylon graphite for the manufacture of crucibles. Graphite from these various sources varies much in purity. Passau graphite, for example, contains only about 43 per cent, of carbon, and Styrian graphite about 73 per cent., the remainder being in each case ash with a little volatile matter.