The first process employed by our manufacturers in the preparation of steel is to stratify malleable bar iron with pounded charcoal in alternate layers, in a close furnace, to cause a gradual absorption of the carbon by the metal. In the engraving on p. 786, Fig. 1 represents a vertical section of the kind of furnace employed for this purpose; and Fig. 2 represents a plan of the interior structure, without the external conical walls which surround it. a and b are the troughs, called cementing pots, in which the bars of iron are laid to be converted into steel; the pots are made of a peculiar kind of fire-stone, not liable to crack or fuse; their dimensions are usually from 10 to 15 feet long, and from 24 to 30 inches in width and depth. The bars of iron, and the powdered charcoal are laid in the pots in alternate strata; the upper stratum of bars being covered with a thicker layer of charcoal than those underneath; above which is also laid a mixture of sand and clay, to prevent the charcoal from entering into combustion by access of the outward air. c is the external cone, of strong masonry or brickwork, of from 40 to 50 feet high.

Inside this superstructure is a smaller conical dome d, called the vault, which is built substantially of fire-brick, or other material, capable of withstanding an intense heat. The vault rests upon external walls e e, at a distance from those which support the external cone, and the space between them is filled up with rubbish, sand, etc. The cementing pots a and b are supported upon a series of detached courses of fire-brick, leaving spaces or flues between them to conduct the flame under the pots. In the same manner the sides of the pots are supported from the vertical walls of the vault, and from each other by a few detached stones, so as to intercept the heat The vault has a series of short chimneys to conduct the smoke into the great cone c, as shown at f f. In the front of the furnace, an aperture is made through the external building, and another corresponding in the walls of the vault; these openings form the door, at which a man enters the vault to put in or take out the iron; but when the furnace is lighted, these doors are closed by firebricks, luted with fire-clay.

Each pot has also small openings at each end, through which the ends of two or three bars are left projecting in such a manner that, by only removing one loose brick from the external building, the bars can be drawn out without disturbing the process, to examine the progress of the conversion from time to time; these are called tap-holes, and should be placed in the centre of the pots to obtain a fair specimen of their product. The firegrate is shown in Fig. 1 at g, consisting of bars laid over the ash-pit h, which must have a free communication with the open air. The attendant examines the state of the fire from the ash-pit h, which has steps leading down to it, and when he perceives any part of the fire not very bright or fierce, he thrusts a long hooked bar through the grating, and opens a passage for the air. The fire-place has no door, being built open at each end; but a quantity of coals is piled up before each aperture, so as to close the openings, and answer effectually the purposes of doors; from this heap of coals the workman shoves in, with a kind of long hoe, such quantity as may be required to replenish the furnace from time to time, and the renewal of the coals to the heap prevents any air entering the furnace, but such as has passed upwards through the ignited fuel, and contributed to the combustion.

The heat and flames from the fuel are reverberated by the dome d, and have passages underneath and around the pots. The degree of heat at which the carbon begins to be absorbed, is found to be about 70° of Wedgwood's pyrometer; this heat is kept up for about 7 or 8 days, according to the thickness of the bars, and the degree of carbonization required. When the process is completed, the furnace is suffered to cool, which takes 6 or 7 days more; the contents of the pots are removed, and the fresh charges put in, the charcoal serving again, as it is scarcely altered from not having entered into combustion. The bars of iron are now found to be covered with blisters and projections, to have acquired a brittle quality, and to exhibit in the fracture a crystalline structure, which is uniform throughout the bar if the carbonization has been complete. The degree of carbonization is varied according to the purposes for which the steel is intended; and so likewise is the nature and quality of the iron employed for the purpose.

For the finest and most important purposes, the Russian and Swedish irons are always employed for conversion, by reputable manufacturers; and there are certain kinds, of long-established celebrity, to which a very general preference is given; these are distinguished by well-known marks stamped upon them, which the governments of the countries whence they are imported warrant the authenticity of. Amongst the chief of these marks may be particularly mentioned L inside a circle (called hoop L); G, with an L connected to the bottom of the G (called G L); two small circles (called double bullet); a G, and an F underneath, connected to it (called GF); an ellipsis, with some lines across its shortest diameter (called gridiron); J B, the J being made out of the upright line of the B (called J B); a stag's head and antlers (called stein-buck); a crown, with a C close underneath (called C and crown). Of course there are as many marks as there are manufactories or mines, many of which may furnish as good iron, but they are not yet so well established. There is, however, probably some exceptions to this remark, and undoubtedly the Russian iron, branded C C MD, is an exception.

The hoop L, which is the produce of the Dannimora mines, in Sweden, is considered the most valuable; forty pounds sterling per ton being readily paid for it, which is about four times the price of the best English bar iron. However immense this difference of cost to the steel manufacturer, it is of trifling importance compared to that of maintaining his reputation for the production of an unexceptionable article. The paying of threepence per pound more for the raw material, will make no perceptible difference in the cost of the artist's graver, the surgeon's lancet, or even the writer's pen-knife - not to mention watch springs, and a thousand other applications of fine steel, in which, were it necessary to insure perfection of quality, a hundred times the cost of the raw material added to the manufactured article would be gladly paid.

Fig. 1.

Blistered Steel 718

Fig. 2.

Blistered Steel 719