Black Lead

Black Lead (or BLACKLEAD), GRAPHITE, or PLUMBAGO, a mineral consisting chiefly of carbon, but containing also more or less of alumina, silica, lime, iron, &c., to the extent of 1 to 47 per cent., apparently mixed rather than chemically combined. Black lead is the popular name, and that by which it is generally known in the arts, though no lead enters into the composition of the mineral; graphite is that generally preferred by mineralogists. It sometimes occurs crystallised in flat hexagonal tables; but generally massive, and more or less radiated, foliated, scaly, or compact. It is of a grayish-black colour, with a somewhat metallic lustre, and a black and shining streak, and is perfectly opaque. It is greasy to the touch, and is a perfect conductor of electricity. It occurs in beds and masses, laminæ or scales in the schistose rocks (gneiss, mica-schist, clay-slate, &c.), and is sometimes in such abundance as to give its name to the schist (graphite-schist) in which it appears. It occurs also now and again in fissures in granite, or in scattered scales in various other igneous rocks, as in syenite in Norway, in porphyry in the Harz, &c. Thick vein-like masses of black lead are met with in Siberia, Spain, Canada, New Brunswick, United States (mines at Ticonderoga, N.Y., supplying almost the whole output), Ceylon, and elsewhere; the once extensive supplies of Borrowdale in Cumberland are now exhausted. It is far more incombustible than even anthracite (or blind-coal), burning with much difficulty even before the blowpipe, on which account it is much used for the manufacture of crucibles or 'melting-pots,' which withstand a great heat. These are not, however, made of mere black lead, but of black lead in powder, mixed with half its weight of clay. Black lead is employed for making Pencils (q.v.). It is also extensively employed to give a black gloss to iron grates, railings, &c., and to diminish the friction of belts, machinery, and rifle cartridges.