a is the fire door; b, the grate; c, the fire bridge; d d, cast-iron hearth plates, resting upon cast-iron beams e e, which are bolted upon both sides to the cast-iron binding plates of the furnace. f is the hearth covered with cinders or sand; g, is the main working door, which may be opened and shut by means of a lever g′, and chain to move it up and down. In this large door there is a hole 5 inches square, through which the iron may be worked with the paddles or rakes; it may also be closed air-tight. There is a second working door h, near the flue, for introducing the cast iron, so that it may soften slowly, till it be ready for drawing towards the bridge. i, is the chimney, from 30 to 50 feet high, which receives commonly the flues of two furnaces, each provided with a damper plate or register. [Fig. 595.], shows the main damper for the top of the common chimney, which may be opened or shut to any degree by means of the lever and chain. k, [fig. 593.], is the tap or floss hole far running off the slag or cinder.
The sole is sometimes made of bricks, sometimes of cast iron. In the first case it is composed of fire-bricks set on edge, forming a species of flat vault. It rests immediately on a body of brickwork either solid or arched below. When it is made of cast iron, which is now beginning to be the general practice, it may be made either of one piece or of several. It is commonly in a single piece, which, however, causes the inconvenience of reconstructing the furnace entirely when the sole is to be changed. In this case it is a little hollow, as is shown in the preceding vertical section; but if it consists of several pieces, it is usually made flat.
The hearths of cast iron rest upon cast-iron pillars, to the number of four or five; which are supported on pedestals of cast iron placed on large blocks of stone. Such an arrangement is shown in the figure, where also the square hole a, [fig. 592.], for heating the rake irons, may be observed. The length of the hearth is usually six feet; and its breadth varies from one part to another. Its greatest breadth, which is opposite the door, is four feet. In the furnace, whose horizontal plan is given above, and which produces good results, the sole exhibits, in this part, a species of ear, which enters into the mouth of the door. At its origin towards the fireplace, it is 2 feet 10 inches wide; from the fire it is separated, moreover, by a low wall of bricks (the fire-bridge) 10 inches thick, and from 3 inches to 5 high. At the other extremity its breadth is 2 feet. The curvature presented by the sides of the sole or hearth is not symmetrical; for sometimes it makes an advancement, as is observable in the plan. At the extremity of the sole furthest from the fire, there is a low rising in the bricks of 21⁄2 inches, called the altar, for preventing the metal from running out at the floss-hole when it begins to fuse. Beyond this shelf the sole terminates in an inclined plane, which leads to the floss, or outlet of the slag from the furnace. This floss is a little below the level of the sole, and is hollowed out of the basement of the chimney. The slag is prevented from concreting here, by the flame being made to pass over it, in its way to the sunk entry of the chimney; and there is also a plate of cast iron near this opening, on which a moderate fire is kept up to preserve the fluidity of the scoriæ, and to burn the gases that escape from the furnace, as also to quicken the draught, and to keep the remote end of the furnace warm. On the top of this iron plate, and at the bottom of the inclined plane, the cinder accumulates in a small cavity, whence it afterwards flows away; whenever it tends to congeal, the workman must clear it out with his rake.
The door is a cast-iron frame filled up inside with fire-bricks; through a small hole in its bottom the workmen can observe the state of the furnace. This hole is at other times shut with a stopper. The chimney has an area of from 14 to 16 inches.
The hearth stands 3 feet above the ground. Its arched roof, only one brick thick, is raised 2 feet above the fire-bridge, and above the level of the sole, taken at the middle of the furnace. At its extreme point near the chimney, its elevation is only 8 inches; and the same height is given to the opening of the chimney.
In most iron-works the sole is covered with a layer of refractory sand, from 21⁄2 to 3 inches thick, which is lightly beat down with a shovel. At each operation a portion of the sand is carried away; and is replaced before another. Within these few years, there has been substituted for the sand a body of pounded slags; a substitution which has occasioned, it is said, a great economy of iron and fuel.
The fine metal obtained by the coke is puddled by a continuous operation, which calls for much care and skill on the part of the workmen. To charge the puddling furnace, pieces of fine metal are successively introduced with a shovel, and laid one over another on the sides of the hearth, in the form of piles rising to the roof; the middle being left open for puddling the metal, as it is successively fused. Indeed, the whole are kept as far separate as possible, to give free circulation to the air round the piles. The working door of the furnace is now closed, fuel is laid on the grate, and the mouth of the fire-place as well as the side opening of the grate, are both filled up with coal, at the same time that the damper is entirely opened.
The fine metal in about twenty minutes comes to a white-red heat, and its thin-edged fragments begin to melt and fall in drops on the sole of the furnace. At this period the workman opens the small hole of the furnace door, detaches with a rake the pieces of fine metal that begin to melt, tries to expose new surfaces to the action of the heat, and in order to prevent the metal from running together as it softens, he removes it from the vicinity of the fire-bridge. When the whole of the fine metal has thus got reduced to a pasty condition, he must lower the temperature of the furnace, to prevent it from becoming more fluid. He closes the damper, takes out a portion of the fire, and the ribs of the grate, and also throws a little water sometimes on the semi-fused mass. He then works about with his paddle the clotty metal, which swells up, with the discharge of gaseous oxide of carbon, burning with a blue flame, as if the bath were on fire. The metal becomes finer by degrees, and less fusible; or in the language of the workmen, it begins to get dry. The disengagement of the oxide of carbon diminishes, and soon stops. The workmen continue meanwhile to puddle the metal till the whole charge be reduced to the state of incoherent sand; and at that time, the ribs of the grate are replaced, the fire is restored, and the register is progressively opened up. With the return of the heat, the particles of metal begin to agglutinate, the charge becomes more difficult to raise, or in the labourers’ language, it works heavy. The refining is now finished, and nothing remains but to gather the iron into balls. The founder with his paddle takes now a little lump of metal, as a nucleus, and makes it roll about on the surface of the furnace, so as to collect more metal, and form a ball of about 60 or 70 pounds weight. With a kind of rake, called in England a dolly, and which he heats beforehand, the workman sets this ball on that side of the furnace most exposed to the action of the heat, in order to unite its different particles; which he then squeezes together to force out the scoriæ. When all the balls are fashioned, (they take about 20 minutes work,) the small opening of the working door is closed with a brick, to cause the heat to rise, and to facilitate the welding. Each ball is then lifted out, either with tongs, if roughing rollers are to be used as in Wales, or with an iron rod welded to the lump as a handle, if the hammer is to be employed, as in Staffordshire. Thus we see that the operation lasts in whole from 2 hours to 21⁄2; in a quarter of an hour, the fine metal melts at its edges, when the puddling begins, in order to effect its division; at the end of an hour or an hour and a half, the metal is entirely reduced to a sand; a state that is kept up for half an hour by continual stirring; and finally, the balling operation takes nearly the same time.
The charge for each operation is from 31⁄2 to 4 hundred weight; and sometimes the cuttings of bar-ends are introduced, which are puddled apart. The loss of iron is here very variable, according to the degree of skill in the workman, who by negligence may suffer a considerable body of iron to scorify or to flow into the hearth and raise the bottom. In good working, the loss is from 8 to 10 per cent. In Wales, the consumption of coal is estimated at one ton for every ton of fine metal. About five puddling furnaces are required for the service of one smelting furnace and one finery. The hearth of the puddling furnace should be exposed to heat for 12 hours before the work begins on the Mondays; and on the Saturdays, the old sole must be cleared out, by melting it off; and running it out by the floss-hole.
Mr. Schafthault obtained, in May, 1835, a patent for the conversion of cast into wrought iron, by adding a mixture of black oxide of manganese, common salt and potter’s clay, in certain small portions, successively to the melting iron in the puddling furnace.