Graphite , (Gr. to write), a mineral commonly called black lead or plumbago, but which titles are incorrect, as it contains no lead. Its composition is similar to that of anthracite coal, containing usually from 90 to 95 per cent. of carbon, with from 4 to 10 per cent. of iron, and traces of silica, alumina, lime, and magnesia. Specimens have been found in Ceylon said to contain 98.55 per cent. of carbon. It occurs in beds and imbedded masses and laminae, in granite, gneiss, mica schist, and crystalline limestone, and sometimes in greenstone. It is sometimes the result of alteration by heat of the coal formation, and is an ordinary artificial product of the destructive distillation of coal in the retorts of gas works. It is found in nature in both a crystalline and amorphous condition, opaque, of a metallic, steel-gray color and lustre, and giving a peculiar, shining, greasy streak on paper. Its specific gravity is 2.09, rising somewhat above this as impurities increase.
Its hardness ranges between 1 and 2. Crystallized graphite occurs in six-sided tables, belonging to the hexagonal system, cleaving perfectly in the direction of the base, and having the basal planes striated parallel to the alternate sides; but the mineral is more commonly found in foliated or granular masses. It is found associated with olivene and sphene at Ticonderoga, N. Y., and in beds of gneiss at Sturbridge, Mass., usually in a scaly and granular, but sometimes approaching a crystalline form. It is also found at North Brookfield, Brimfield, and Hinsdale, Mass., at Brandon, Vt., and at Grenville, Canada, where it is associated with sphene and tabular spar. It occurs near Amity, Orange co., N. Y., in white limestone, associated with spinel, chondrodite, and hornblende ; at Rossie, St. Lawrence co., with iron ore, and in gneiss; in Bucks co., Pa., near Attleboro, associated with tabular spar, pyroxene, and scapolite, and also in syenite at Mansell's black-lead mine near the same locality. There is a large deposit at St. John, X. B. The mine at Borrowdale in Cumberland, England, has long been celebrated for yielding graphite of a superior quality for making black-lead pencils, one of its principal uses.
The mine has been known since the time of Queen Elizabeth, and probably furnished the first lead pencils ever made, as their invention cannot be traced back as far as the discovery of the mine. It is in a mountain, 8 m. S. of Keswick, 2,000 ft. high. The mineral occurs in small nests in trap. The pieces are about the size of the fist. The mine became so valuable as to be an object of plunder, being reached underground from neighboring mines, and being once forcibly taken possession of at the surface. The graphite was of so pure a quality that it required but little preparation for the market; and much of it was sawed up in its natural state for pencils. The mine is now nearly exhausted, and has not been worked for many years. Graphite has been found in Germany, France, Austria, and South America, and in enormous masses in N. E. Siberia. Besides furnishing a material for writing pencils, it is used for making crucibles, and linings for small furnaces; as an ingredient in lubricating compounds for machinery ; for giving a smooth surface to the moulds of metal castings, and for polishing stoves and iron castings generally ; and also for a coating to wax or other impressions of objects designed to be electrotyped, for the purpose of forming a good conducting surface for the galvanic current.
It has also been employed by Graham as a diaphragm in his dif-fusiometer or instrument for observing the comparative rate of diffusion of gases. (See Crucible, and Pencil.)