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ovens are generally arranged six or eight in a row, either against the side of a hill, or in a mound of earth artificially raised to a level with the top, for the convenience of shooting the coals in at the aperture; the doors, or openings in front, are level with the inner floor.
When these ovens are once heated, the work goes on night and day without interruption, and without any other expense of fuel. It is conducted thus :— Small refuse coal, of good quality, is thrown in at the circular opening at the top, in quantity sufficient to fill the oven to the springing of the arch; this charge is then levelled with an iron rake, and the door-way built up with loose bricks. The heat which the oven acquires in a former operation is always sufficient of itself to light up the new charge; the combustion of which is accelerated by the atmospheric air, that rushes in through the joints of the loose bricks in the door-way. In two or three hours, the combustion gets to such a height, that the attendant finds it necessary to check the influx of the atmospheric air: the door is, therefore, now plastered up with a mixture of wet soil and sand, except the top row of bricks, which is left unplastered all night. Next morning, (when the charge has been in the oven twenty-four hours) this row is completely closed also; but the chimney remains open till the flame isgone, which is generally quite off in twelve hours more: this aperture is then covered with a few loose stones, upon which is heaped a quantity of sand or earth. All connexion with the atmosphere is now cut off, and in this condition the whole remains for twelve hours more, to complete the operation. The door-way in front is then opened, and the cokes,
HARD AND SOFT COKES. 413
which appear in large rhomboidal pieces, having a form similar to that presented by starch, are raked out into iron wheelbarrows, or low waggons, to be carted away. The whole operation takes up fortyeight hours; and as soon as the cokes are removed, the ovens are again filled with coal for another burning: about two tons are put in for each charge.* The cokes thus produced are, as just stated, very large, ponderous, extremely hard, of a light gray colour, very sonorous, and shine with a metallic lustre, appearing very different in this respect from those commonly produced by the burning of whatever description of coal under ordinary circumstances. They are used in the smelting of iron ore, in the steel casting furnaces, and in various manufactures that require an intense and long continued heat. Cokes of a similar quality are produced without the aid of ovens by piling large coals in long heaps about three feet wide, two feet high, and ten yards in length: the row being lighted by the application of burning cinders, and the access of atmospherical air regulated, and finally excluded, by covering up the mass with sand and ashes: in the conversion by this process, the fuel loses about half its weight.
When coke is required to be more of the nature of charcoal, the process is conducted in a different manner. If small coal be used, it is thrown into a large receptacle, similar to a baker's oven, built of brick, with a tall wide chimney to throw off the smoke, and previously brought to a red heat. Here the door is kept constantly open, because the heat of the oven is of itself sufficient to dissipate the bitumen of the coals, the disengagement of which is promoted by frequently stirring the coal with a long iron rake: water is also occasionally thrown upon the mass during combustion. The coke made by this method, though the same kind of coal be used, is very different from that produced by the former operation: this being intensely black, very porous, and as light as pumice-stone. It is used for a variety of purposes; but when intended for the iron and steel forges, great care must be taken that the coal shall be free from all heterogeneous mixtures, especially sulphur and pyrites.
* Chemical Catechism, p. 453, edit. 1822.
The appearances presented by different descriptions of coal when passing into the state of coke, has already been intimated by the terms open-burning and caking: in both kinds, the process of incineration ends in yielding a residuum of ashes,—but the different effects of fire upon them are extraordinary; the pieces of coal, in the one instance, igniting and consuming, almost as independently as so many lumps of wood; and in the other, not only coagulating, but as it were, tumifying and expanding, somewhat in the manner of borax when exposed to heat. One of the free-burning coals of South Wales is remarkable for the swelling or branching which takes place during combustion, and in the process of coking, which is effected in the open air. This arborescent appearance, so different from that which takes place in other bituminous coals under similar circumstances, had led to the local term, "G16 spagod," or branching coal. In some varieties (from the Clyngwernon seam, for instance,) this property exerts such an effect on the coke as to make it nearly as light and porous as wood charcoal.
In every instance where coals are burnt, there are two products evolved of a most obvious character, or
SMOKE, SOOT, AND ASHES. 415
rather the same product presented under two conditions, smoke and soot,—the former, turned to no profitable account, the latter of some value in agriculture and the arts. "Were it possible," says Peckstone,* "to collect the dense smoke thrown out by burning coals, deprive it of its combustible parts, and condense it, it might perhaps be applied to the purpose of generating artificial light." This object, however, has not been attempted. Soot, derived from the combustion of pit coal, yields, according to Sir H. Davy, in addition to the charcoal which forms its basis, certain proportions of volatile salts, extractive matter, and an empeurematic oil: to these it owes its efficacy in certain cases, as a manure, being chiefly used in top-dressing some descriptions of land. It may be added, that the ashes of pit coal are used about London in admixture with clay in the making of bricks; and in the northern counties generally, as forming with the night soil, a valuable agricultural compost. Thus, we see, that not only is this fuel of inestimable value for the direct purposes of affording light and heat, but also in a variety of other ways; that even its very refuse is converted by the skill of the chemist, or the industry of the labourer, to economical and other uses.
* History of Gas Lighting, p. 40.
Extent of Home Consumption—Importance of Coal in the generation of Steam—Steam Engines—Manufactures of Earthenware and Glass—Statements of Mr. Pellatt—Gas Works—Iron Works—Consumption of Coal in Sheffield—Manchester—Birmingham—Leeds—Liverpool—London—Consumption in the United Kingdom—Tax on Coals or Hearths proposed—Waste of Coal at the Collieries.
XT would be uninteresting, if not impossible, to specify all the circumstances under which coals for the purposes of fuel are applied in this country; and while it would be an hopeless attempt to ascertain the exact quantity consumed in any given section of the United Kingdom, it were almost equally futile to pretend to estimate with precision the whole amount which may be burned in Great Britain and Ireland in the course of a year. Some details, however, in addition to what has been already stated, as being strikingly illustrative of the importance of this fuel in particular instances, may be given; and also a few of those general statements which, from the data upon which they are founded, must be acknow