Core, the solid product left behind when the volatile matters are expelled by distillation from bituminous coal. There are two kinds: gas coke, obtained from the retorts of gas works after the gases have been separated; and oven coke, which is made in ovens or pits, and which is considered by manufacturers as the only true coke, gas coke being merely cinder. Oven or pit coke is made upon a large scale at mines of bituminous coal, for the purpose in part of saving the fine refuse coal by converting it into a valuable fuel, and in part of converting the lump coal into a form better adapted for metallurgic operations, and for the use of locomotives when the flame and smoke of bituminous coal would be objectionable, as upon underground railways and in populous streets. It was formerly the opinion of some engineers that the calorific qualities of bituminous coal exist undiminished in the coke, notwithstanding that the gases expelled in the process of making the coke possess also a considerable heating power.

Mr. Josiah Parker states, in vol. ii. of the "Transactions of the Institution of Civil Engineers," that he has "found that 75 lbs. of coke, produced from 100 lbs. of coal, evaporated as much water as 100 lbs. of the self-same coal." He also cites the experience of Mr. Apsley Pellatt in his glass furnaces, which were especially well adapted for showing the relative calorific value of coke and coal, provision being made in them for the full combustion of the volatile products of the coal. Of late years, however, after much discussion upon the subject, bituminous coal has come into use in place of coke upon the principal railway lines in England, the use of the latter being retained only when flame and smoke require to be avoided. Coke has never been used on American railroads, and now that locomotives are so constructed that crude coal can be burned with facility and economy, the occasion will probably never arise. In consequence of its freedom from sulphur, coke is much better adapted to metallurgic processes, and therefore special attention has been directed in Europe, not only to the best methods of preparing it, but to the selection of that kind of coal which is best adapted to the purpose; and it has been found in practice preferable to incur considerable expense to procure it of the best quality, some companies even preferring to obtain it from England rather than use the cheaper but inferior qualities made from the coals of France and Belgium. The cause of the superiority of the English coke is attributed to the coal beds of England containing but few seams of slate interstratified with the coal; so that this is obtained clear of the impurities which in the French and Belgian coals add largely to the proportion of ash, and render it necessary to subject the coal to processes of washing and sorting before coking.

Iron pyrites is the principal objectionable material in coal for melting metals, and to get rid of it is the chief problem of the coke maker. A protracted application of heat expels a great part of the sulphur, with the formation of bisulphide of carbon and carburet of iron. The bisulphide-of carbon, being volatile, passes off, and the carburet of iron which remains does no injury unless there is silica present, which is not the case in good coking coal. In the north of England it has been found that when the coal contains much pyrites, if it is first treated with a very strong brine the elimination of the sulphur is very greatly facilitated. The tendency to vitrifac-tion possessed by clays and lime salts renders these substances objectionable in coke, and therefore good coking coal is only found in certain districts. (See Coal.) - Two methods of coking are in use: one in ovens constructed for the purpose, and the other in large open heaps, upon the ground. The ovens are built of fire brick or stone of various sizes, and frequently arranged several together in one stack, in which case dimensions of 12 ft. square and 10 ft. high are found most convenient.

They are arched over at the top, a hole being left for the exit of the gases, and another for the introduction of the coal, closed by an iron door in front. About two tons of coal are introduced through the latter, filling the oven to the springing of the arch, and leaving room above for the coal to swell. The charge is ignited by the heat left in the walls by the process just previously completed. Air is allowed to enter in proper quantity at the lower door, and the gases escape at the top. In 24 hours the air holes below are closed, and for 12 hours more the gases pass out at the top. The upper hole is then closed with a slab of stone or iron and covered with sand, and left for 12 hours more to partially cool down the charge. After this the door below is opened, the coke taken out quickly, quenched with water, and carried off in iron wheelbarrows. Where it is an object to save the coal tar, the ovens are provided with a flue at the top, through which the volatile products are conducted into a receptacle in which the liquid matters are condensed. The product of bituminous coal in coke, gas, and tar varies with its quality. As it approaches anthracite in quality, the yield of coke is large and of gas small, while the reverse is the case with the fat or highly bituminous coals.

From 50 to 75 per cent, is the general range of the yield of coke. Upon some of the railroads in England the size of the ovens is stated to be 80 ft. square, and the charges about 8 tons each, spread in a thickness of about 4 ft. The duration of the process is 96 hours. With the same coals and ovens, by making the charges lighter and increasing the quantity of air admitted, and thus raising the temperature, so as to complete the process in 48, 24, or even 12 hours, the coke will be obtained lighter and more friable according as it may be desired. It has been found that the higher the heat of the oven the larger the yield of coke. This fact seemed for a long time anomalous, but is explained by well known chemical laws. When coal is melted, its hydrogen and carbon combine in the form of bicarburetted hydrogen, which in passing up through the red-hot coal above is decomposed into solid carbon and light carburetted hydrogen. Thus one half of the carbon of the gas is saved. The principle is illustrated by passing bicarburetted hydrogen through a red-hot tube, which after a while will become filled with a solid carbonaceous deposit. - Coking in the open heap is the most common practice at the mines of bituminous coal of the United States, and this is the oldest method.

The coal is piled up in long ranges, extending sometimes 200 ft. in length, with a width of 12 ft. at the base, and a height of 6 ft. The piles are made so that along the whole length an air passage extends through the centre on the ground. The largest lumps are placed in the middle portion, and smaller and smaller pieces toward the outside. Stakes are set up at intervals along the central line of the heap, which reach down to the base; when the pile is completed, these are taken out, and the passages they leave serve for the introduction of burning coals to fire the heap along its whole length. Whenever the thick black smoke and flame cease in any portion, and this begins to show appearance of ashes, the fire is kept down by the application of coke dust or ashes; and this goes on until the whole heap is thus covered. It is then left for a few days to cool. Portions exposed to the full action of the wind require, as in the process of making charcoal, a thicker cover of dust or ashes to check the consumption of the product. When sufficiently cool, the coke is drawn out. The process is not an economical one, much of the inside coal being always consumed to waste before the inner portions have been coked.

A method has been adopted at the Clyde iron works in Scotland, by which a part of this waste is obviated. A mound is built up of a circular form around a central chimney of brick, which may be 3 ft. square at the base and rise 3 or 4 ft. from the ground. Openings of the size of a brick are left at intervals in its sides, for the passage of the gases, and from the lowest of these the coal around is so piled that flues extend through it to the circumference of the heap. The diameter of the mound maybe 20 ft., and its ashes 4 1/2 ft., sufficient with the cover of ashes and cinders to reach above the top of the chimney. The heap is lighted by burning coals thrown into the chimney, from which the flames reach through the aperture. In four or five days, when the mound has become thoroughly on fire, the apertures on the outside and the top of the chimney are closed with plates and ashes, and the heap is left to cool for three days, after which the coke is drawn out. It is the practice now in Europe to utilize the heat produced in making coke. At some chemical works salt is made by using the waste heat; in blast furnaces the air has been heated from the same source, and in many others the heat from the burning of the escaping gases is used to increase the heat of the coking oven itself.

Many of these are in use on the continent of Europe. - A species of coke called "charred coal" is now used in place of charcoal in the manufacture of tin plates. It is made by spreading fine coal on the red-hot floor of a reverberatory furnace to the depth of four or five inches. Much gas is given off and ebullition takes place, producing a spongy mass which is removed after an hour.