out by the swelling of the coal: c, Fig. 36, is a round hole, left in the top, through which the disengaged gases may escape; this hole, from twenty to twenty-four inches in diameter, is left open until all the bitumen of the coal is driven off, after which it is to be shut by a cast iron plate, and luted with clay. The interior of the furnace is to be built with fire brick and fire clay mortar; the rough wall either with stones or common brick. Such an oven has a capacity of from seventy-five to eighty bushels; it will furnish from Pittsburg slag coal nearly one hundred bushels of coke. After it is finished by the masons, and ready for use, some wood is thrown on the hearth towards the door, which is to be repeated each time the oven gets cold; the door is then walled up; and the coal thrown in through the hole at the top, and spread uniformly over the hearth. Eighty bushels of coal will cover the bottom about twelve inches high, which will rise to fifteen inches after being burnt. The top arch, therefore, should be sufficiently high from the bottom to permit the swelling of the coal, and the breaking up of the solid mass of coke. The height of the arch from the bottom is generally from twenty to twenty-four inches; in the centre, thirty inches. When the coal is properly spread, fire may be applied at the door and top; after the first and second heats, it needs no kindling, for the bottom and sides of the oven are sufficiently warm to kindle the coal. After ten or twelve hours, the bituminous gases are mostly expelled; the top can then be closed, that the oven may cool; eight or ten hours will be sufficient for this purpose. Though the coke may be redhot, there is no danger of its further burning. The door is now opened, and the hot coke removed in iron wheelbarrows. This is frequently quite a hard task; and a set of long and strong crowbars, besides some long iron scrapers, are needed in every establishment of this kind, to facilitate operations, and to prevent any delay of the regular work. The coke of the first heat is generally raw at the bottom, and spongy at the top; but the second and following heats improve as the oven gets hotter. We may say, generally, the hotter the oven, the better the coke. Good coke ought to exhibit a uniform crystalline texture throughout the whole mass, and when cold, should sound like fragments of stoneware.

One of the most common arrangements of coke ovens, in the Old World, is exhibited by the following diagrams: Figs. 37 and 38. The oven here represented is that of the Northumberland and Lemington Iron Works upon Tyne. Four ovens are shown to be in a line; this arrangement is preferred because it keeps the heat

together, and saves masonry. The hearth of the oven is an oblong square, ten by twelve feet; and from the bottom to the arch is three feet high. The rough walls are of common brick, two feet thick,

and the lining of fire brick. At the centre, on the top, is a round hole, two and a half feet in diameter, through which gases escape. The doors are three feet square. On the top is a cast iron frame, with two rips; between these rips a door slides. This door is of wrought iron, filled with brickwork, which, sliding upon the above frame, may cover the gas hole, or be withdrawn at pleasure. The same arrangement is used to shut the doors, with this difference,

Fig. 38.

Ground-plan of English coke ovens.

that the door is vertical, and balanced by a lever. In the brick filling of the door, are a number of draft-holes, through which air can have access to the burning coal.

This arrangement, in its principle, differs not in the least from the Pittsburg arrangement. But the execution of the latter is more perfect than that of the former, though, at the same time, more expensive, and less suited to our country. This oven is charged with from two to three tons of coal. The manipulations are the same as those at the Pittsburg ovens. In countries, Germany and France, for instance, where coal tar is of value, and its gathering yields profit, coke ovens have a different form. As coal tar may be

highly valuable in some parts of the United States, we shall describe one of these ovens. Fig. 39 represents a cross section of a

coke oven which, for many years, has been in operation in Silesia, eastern Germany, and may be considered of an improved form. This oven is about nine feet high; it is of a cylindrical form, and is four feet in diameter; the interior is built of fire brick, and the exterior of common brick or stones, bound with iron hoops. The coal is put in partly through the door a, and partly through the top hole b. Care is taken to lay coarse coal at the bottom. The bottom part of the oven forms a kind of grate, for the holes c, c, c are left open, in which iron pipes are walled in; there are seven of such holes in the bottom. The holes in the side wall d, d, d are draft-holes, secured by iron pipes. The top is covered with an iron plate in which the lid e fits. The tar and gases are conveyed by the iron pipe f, into a reservoir, or tar barrels. This pipe is conducted through cold water, that the tar, during summer, may con

dense; but in winter, the atmospheric air is sufficiently cool to condense all the tar which escapes from the oven. Coarse coal is put in the bottom part; upon this is thrown slack coal. If the oven is about two-thirds filled, fire is kindled, and the door a shut with brick and clay mortar. The fire may be safely left to burn, and the top plate e may be put on, and luted with clay. After eight or ten hours, the upper row of holes d will appear brightly red, and may be shut. After that time has again elapsed, the second row from above may be shut. Twelve hours more may elapse, when the lower holes, becoming bright, may be closed. By this time, tar almost ceases to be produced. Then, after shutting carefully all the holes the oven may be left to cool. This cooling will take place in the course of the next twelve hours.

The charge in such an oven amounts to two tons of coal. Two charges may be made during one week, if the coke is drawn in time. The coke thus produced is very hard and compact, and may be considered superior to any other; but the manipulations in this oven are both expensive and troublesome. One bushel of coal furnishes three-fourths of a bushel of coke; and one hundred pounds of coal produce fifty-three pounds of coke, and five gallons of tar. The bituminous coal of upper Silesia is referred to. One hundred pounds of this coal furnish from forty-five to forty-seven pounds of coke, when burnt in the open air; while one bushel of coal furnishes nearly one bushel and a quarter of coke.

In the neighborhood of St. Etienne, in France, a kind of double coke oven is in use, which is worthy of notice. Its form is, in the main, the same as that of any other coke oven, but differs in being of larger dimensions, and in having two doors for drawing, instead, like other ovens, of but one.

a

Fig. 40.

a

French coke oven.

Fig. 40 represents a double oven; a, a are two opposite doors. The bottom is formed of hard rammed fire clay; its top and sides are of fire brick; the rough wall either of common bricks or stones.

The chimney b is eighteen inches in diameter, and the whole arch is covered with sand, to keep in the heat. Its arrangements, in other respects, are the same as those of other coke ovens. It contains a charge of from three to four tons of coal. The hearth ought to be sufficiently large to take this amount of coal, piled ten inches high. The centre of the arch may be four feet from the bottom.

d. Coking in Iron Retorts.-By the distillation of coal in iron retorts, no coke can be made serviceable for the manufacture of iron. Coke thus made is always light, spongy, and never free from bitumen or sulphur; qualities which render it unfit for an iron manufactory. We shall, therefore, dispense with the consideration of this method.

IX. General Remarks on Coking.

The making of good coke is, to the manufacturer of iron, a very difficult task. Good coke ought to be silvery white and compact; it ought to sound like good crockery ware, and should be free of bitumen, hydrogen, and sulphur. Good color and compactness may be secured in various ways; but the other qualities are not so easily secured. Hard, compact coke will be obtained from large piles, either in ovens or in the free air, if the fire is brisk and the covering heavy; but coke made in that way always contains more sulphur and hydrogen than it should contain. A large body of coal, under a slow fire, furnishes light spongy coke, but more free of sulphur and bitumen than under a quick fire. A medium heat serves better than either extreme. Where the body of coal is kindled, the heat ought to be kept as low as possible; the longer the heat is applied, the better will be the result. By this means most of the sulphur, as well as the hydrogen, will be expelled. When we have ascertained that no more sulphur escapes, the heat may be raised by giving free vent to the gases through the air-holes. If a current of steam can possibly be passed through the glowing coal, during the first stages of coking, it should be done. In the yard, where we coke in piles, this may be easily effected, by keeping the ground moist, and laying the coal as closely as possible upon it. But in ovens this is more difficult, because in them it is not so convenient to change and regulate the fire.

If the bottom of the ovens is made of clay laid upon sand, and if we are able to regulate the moisture of this sand, a great deal may be effected. In this manner the French oven, Fig. 40, is constructed. This is the only practicable method by which watery

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