ANTHRACITE (Gr. , like coals, from
, coal), the most condensed variety of
mineral coal, containing the largest proportion
of carbon and the smallest quantity of volatile
matter. Excepting the diamond, anthracite is
the purest form of carbon in its natural state. The best specimens contain 95 per cent. carbon, but the average production of the purest beds of this coal will not exceed 90 per cent., and generally not more than 80 to 87 per cent. carbon. The volatile matter in the dense, hard varieties is almost exclusively water and earthy impurities, but in common varieties the volatile portion consists of water, hydrogen, oxygen, and nitrogen; while the ash or incombustible matter contains oxide of iron, iron pyrites, silica, alumina, magnesia, lime, &c. The gradation of anthracite is arbitrary; there is no fixed limit in the descending scale at which anthracite becomes semi-anthracite. A coal containing 80 per cent. carbon may be and often is termed anthracite, while other coals containing 85 per cent. carbon are truly semi-bituminous. The superior density, irregular fracture, and general appearance of anthracite are distinguishing features to common observation; while water and ash take the place of hydrogen and oxygen, or bituminous matter. But anthracite which contains only 80 per cent. carbon, with 20 per cent. water and incombustible matter, is the lowest grade of commercial coal, and of little value as fuel.
—The constituents of
anthracite, as determined by ordinary analyses, and generally published, are only approximate.
They are generally made from picked specimens, by many men and many methods, each giving widely diverse results even from the same coal, and the mere aggregates of carbon, volatile matter, and ash, while the distinguishing features and chemical constituents are seldom given. The change from anthracite to semi-anthracite is gradual and imperceptible in the coal beds of the prominent anthracite fields. There is no fixed point at which the one terminates or the other commences. The same uncertainty is manifest in all published analyses of mineral coal. No commonly adopted limit is assigned to the various gradations. Those called semi-anthracite in one place are termed anthracite in others, and vice versa. The same indefinite relations are observable between semi-anthracite and semi-bituminous, and between semi-bituminous and bituminous coals; while the gradations of all carbon compounds are alike indefinite and unsettled, down through cannel coal, bitumen, asphaltum, petroleum, naphtha, and carburetted hydrogen gases. The uncertainty, however, exists in the mean and not the extreme varieties. Hard, dense anthracite could not be mistaken for any other class; and while light, volatile semi-anthracite might be readily termed semi-bituminous, it could not be mistaken for anthracite. The following table gives the average aggregate constituents of the prominent varieties from the chief anthracite districts of the world:
The above table is compiled from the best
available sources; and though the analyses
are generally from hand specimens, and therefore
not commercially useful, they are
characteristic, and indicate the chief constituents
of the prominent anthracites.—The
anthracites of Pennsylvania are generally
denominated white-ash or red-ash coals, but
the color of the incombustible residue varies
from pure white to gray, rose-pink, pink, light-red, brick-red, and brown; and this variation of color is as marked in the ash of bituminous
and all intermediate varieties of coal as in
anthracite. The color of the ash is obtained
from the oxide of iron, and is no criterion of
the character or value of the coal, because
these colors exist, from white to brown, in the
lowest, oldest, and hardest anthracite, as well
as in the upper, latest, softest, and most volatile
semi-anthracite and bituminous coals.