Graphite
The nature of graphite, sometimes called plumbago or black lead, is not generally understood. Eminent writers on friction have declared that graphite is the best natural lubricant known, and scientific and mechanical papers have advocated its use for many purposes. Incompetent, if not unscrupulous, parties have attempted to meet the demand by putting on the market graphite productions that are totally unfit for the uses specified. Graphite is one of the forms of carbon. It is not affected by heat or cold, or any known chemical. As it comes from the mine, however, it contains from 50 to 80 per cent of silica, sulphur, and other impurities, and the process of completely freeing the graphite from impurities requires very expensive machinery and the most skilful manipulation. Only manufacturers having such facilities can hope to produce an absolutely pure article. The impurities in much of the graphite now in the market take on the appearance of graphite by contact, and such impurities are sometimes undetected even by the expert, unless chemical tests are employed. This is especially true of amorphous graphite, commonly called black lead, which is graphite without any particular form, and usually mixed with clay.
Pure graphite, and even black lead, is useful in many ways; but to be useful in the highest degree the graphite should be carefully selected with a view to the use intended. Graphite suitable for lead-pencils is not the most suitable for lubricating, although it has lubricating qualities. Again, graphite suitable for stove-polish would not answer for crucibles, although it might be equally pure and stand the heat equally well.
Graphite varies greatly in its construction and usefulness, and the best results are only brought about through experience, knowledge, and proper mechanical facilities. See BLACK LEAD; FACING.
Graphite in Pig-iron.--Pig-iron contains from 2 to 6 per cent of carbon, some portion of which is held in chemical combination with the iron, the rest being distributed throughout the mass mechanically. The latter is called graphitic carbon, or graphite. This graphite is seen as scales, which may be detached from the mass when the pig-iron is reduced to powder, and may also be found among gray-iron borings that are subjected to a process of grinding and sifting. If gray iron that has been melted in contact with an excess of carbon is allowed to cool slowly, the carbon crystallizes out and forms graphite; this is commonly called Kish in the foundry, and it is always seen to gather when the iron is melted under conditions answering to those described above. See KISH; CARBON.