“Bituminous” coal, or ordinary “soft coal,” burns readily, with a smoky flame, and is the coal most commonly used for manufacturing purposes; in fact, the bulk of the coal mined throughout the world belongs to this rank. It includes a good many varieties, some of which are extensively used in making coke, while others, such as “cannel” coal, have been in great demand for use in gas-works. Nowadays, however, the widespread introduction of “water-gas,”[2] which does not require any particular kind of coal, has diminished the demand for “gas coals.”
[2] Made by forcing steam over glowing coal or coke. See Monograph No. 4.
“Sub-bituminous” coal, or “black lignite,” is common in some of our western coal fields. It is a clean and useful domestic fuel when used near the mines, but is not very satisfactory for shipment, as it shrinks and crumbles under the effects of “weathering” and is liable to spontaneous combustion.
“Lignite” is the least valuable of coals, and is the form of coal which is the least altered from the original peat. The Geological Survey applies this name only to those coals which are distinctly brown and either markedly woody or claylike in appearance. Lignite, as it comes from the mine, contains from thirty to forty per cent. of moisture, and it “slacks” or falls to pieces much more rapidly than sub-bituminous coal when exposed to the air. It is hardly suitable for transportation.
For commercial purposes coal is also classified according to size. The coal as it comes out of the mine, without any sorting into sizes, is known as “run of mine,” and the semi-bituminous coals are commonly shipped in this form. Most coals, however, are passed over bars or gratings, which constitute screens of different degrees of fineness; each screen permits all the lumps below a certain size to fall through, and thus the coal is divided into the different standard sizes. The sizes of anthracite, from the smallest to the largest, are: rice, buckwheat, pea, chestnut (or nut), stove, egg, broken (or grate), steamboat, and lump. Bituminous coal is divided into slack, nut and lump (the largest size). A mixture of lump and nut is called three-quarter coal.
The Modern History of Coal
Press Illustrating Service
MODERN MINING MACHINE
for undercutting coal. The “cutter bar” is shown in front, filled with “cutting teeth” set in a chain that travels around. See illustration opposite