“If coke be applied instead of coal, the arches, or empty spaces in the clamp or pile, as well as the strata of the fuel, may be considerably smaller: the heat produced in this case is more uniform and more intense, and a saving of 30 per cent. at least is gained.
“In the baking my own plaster-stone I also employ coke. The calcination of the stone for manure I perform in a common reverberatory furnace, and the men who conduct the process (who are otherwise averse to every thing new) are much pleased with the steadiness of the fire, and little attendance which the process requires, when coke is used instead of coal.
“These are the few facts I wish to state, with regard to the useful application of this species of fuel, which, no doubt, hereafter will become an object of economy of incalculable advantage to individuals, if its nature be better understood than it is at present.”
The quantity of coke obtainable from a given quantity of coal varies according to the nature of the coal employed. One chaldron of Newcastle coal produced, upon an average, in the gas-light manufacture, from one chaldron and a quarter to one chaldron and a half of well formed coke. If the carbonization of the coal has been carried to its utmost point, the coke produced, has a brilliant silvery lustre. Such coke is excellent for metallurgical operations, because it stands the powerful blast of the bellows, but for culinary and other purposes of domestic economy, the carbonization should not be carried so far, because, the coke then produced, kindles more readily and makes a more cheerful fire.
Coal-tar, Oil, and Pitch.—Another, valuable product obtainable from pit-coal, is coal-tar.[30] This substance is deposited, in the purification of the coal-gas, in a separate vessel destined to receive it.
[30] In the year 1665, Becher, a German chemist, brought to England his discovery for extracting tar from coal, this distillation he performed in close vessels. It is not mentioned in the records of the time, whether Becher obtained, or rather collected, any other articles than the tar.
The coal-tar is so called from its resembling common tar in its appearance, and most of its qualities.
Several works have been, at different times, erected both in England and on the continent, to procure from coal a substitute for tar; but they turned out unprofitable speculations. In 1781, the Earl of Dundonald invented a mode of distilling coal in the large way, which enabled him not only to form coke, but, at the same time, to save and collect the tar. Even this process however, for which a patent was taken out, has gained very little ground. Its object was still too limited; for though some of the ingredients of coal were procured, they were procured at an expense that nearly balanced the profits; and no attention whatever was paid to the coal gas, which constitutes the most important part of coal.
Coal-tar may be used with advantage for painting and securing wood that is exposed to the action of air or water. The wood being warmed, the tar is applied cold, and penetrating into the pores, gives the timber an uncommon degree of hardness and durability.
One chaldron of Newcastle coal produces in the gas-light manufacture from 150 to 180lb of tar, according to the circumstances under which it is produced. See [page 94].