WHEN iron has lost all its carbon, and has become malleable, it can be reimpregnated with carbon, to a certain extent, without materially injuring its malleable properties.

The compound of iron and carbon thus produced is called

steel.

To reimpregnate the iron with carbon, it must be put into a close vessel, called a cementing pot, and stratified with powdered charcoal.

The pots are made with a peculiar kind of stone, termed fire-stone, which is found abundantly in the neighbourhood of Sheffield. It possesses the properties of not being liable to crack by the heat, or of entering into fusion. These pots in the interior dimensions are from 10 to 15 feet long, and from 24 to 30 inches square. Each bar of iron is completely covered with powdered charcoal, and the last stratum of it is usually made much thicker than the rest, and kept

close with a mixture of sand and clay, to prevent the charcoal from entering into combustion with the outer air. Two of these pots only are contained in a furnace at a time, and fire is gradually employed till the heat is little short of what would be required to fuse the steel.

A vertical section, and horizontal plan, of the converting furnace is shown in figs. 355 and 356. In both figures the same letters denote the same parts.

CC is the external cone, built in a substantial manner of stone or brickwork. Its height from the ground to its vertex, in order to procure a good draught of air, should not be less than 40 or 50 feet; and to procure a still stronger heat a cylindric chimney of several feet in length is most generally fixed on the top of the cone. The lower part of the cone, which may be made of any dimensions, is built either square or octangular. The sides are carried up until they meet the cone, giving the furnace the appearance of a cone cut to a square or octangular prism at its base, and exhibiting the parabola where every side intersects the cone.

Inside the conical building is a smaller furnace, called the vault, built of fire-brick or stone, which will withstand the action of the most intense heat. DD, in the section, is the dome of the vault, and EE are its upright sides, the space between which, and the wall of the external building, is filled with sand and rubbish. A B represent the two pots that contain the iron to be converted into steel. The space between them is about one foot in width, and the fire-grate is directly beneath it. The pots are supported by a number of detached courses of fire-brick, as shown at e e, in fig. 355, which leave spaces between them, called flues, 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, represented by f, placed so that they may intercept as little as possible of the heat from the contents of the pots. The adjacent sides of the pot are supported from one another by small piers of stonework, which are also perforated to give passage to the flame. The bottoms of the pots are built of a double course of brick-work, about six inches thick; the sides nearest together are built of a single course of stone, about five inches in thickness; and the other parts of the pot are single courses about three inches, the sides not requiring so much strength, because they have less heat and pressure to resist.

The vault has ten flues, or short chimneys, FF, rising from it, two on each side, to carry off the smoke into the great cone, shown in fig. 356, communicating with each side, and two at each end. In the front of the furnace an aperture is made through the external building, and another corresponding in the wall 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 fire-bricks luted with fire-clay. Each pot has also small openings in its end, through which the ends of two or three of the 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 the tap-holes; they should be placed in the centre of the pots, that a fair and equable judgment may be formed from their result of the rest of its contents.

ab, in the elevation, is the fire-grate, formed of bars laid over the ashpit I, which must have a free communication with the open air, that it may convey a current of fresh air to supply the combustion. The ash-pit

should also have steps down to it, that the attendant to the furnace may get down to examine by the light, whether the fire upon the whole length of the grate be equally fierce; and if any part appear dull, he uses a long iron hook to thrust up between the bars, and open a passage for the air. The fire-place is open at both ends, and has no doors. The fire-grate is laid nearly on a level with the floor of the warehouse, before the furnace, and the fireman always keeps a heap of coals piled up before the apertures at its ends, so as to close the opening. This forms a very simple and effective door; and when the furnace requires a fresh supply of fuel, a portion of the heap of coals is shoved in by a sort of hoe, and the heap renewed, to stop any air from entering into the furnace, except that which has passed upwards through the ignited fuel, and by that means contributed to the combustion.

The fire-stones composing all those parts of the furnace which are exposed to the action of the heat, are first hewn nearly to size, and finished by grinding two surfaces together, so that they make very perfect and close joints; when laid together, they are cemented with well-tempered fire-clay, mixed up thin with water. The fire-clay which answers best for this purpose, is that brought from Stourbridge, in Staffordshire, and is the same of which the celebrated Stourbridge crucibles are composed; but very good fire-clay for the purpose is procured from Birkin-lane, near Chesterfield. When the furnace has been once burnt, this clay becomes equally hard with the stone, and is less liable to fly or vitrify in an intense heat than any other known cement.

The flame arising from the ignited fuel upon the grate passes upwards between the pots, and strikes the dome of the vault, from whence it is reverberated down upon the pots, and ultimately escapes through the flues or chimneys of the vault. By this means every part of the pot is exposed to the same degree of heat, which is of great importance.

In order to ascertain when the cementation is perfect, one or two of the bars, having their ends, as before described, projecting from the pots, are taken out of the furnace, and examined.

The blisters upon the surface of the steel, caused by the carbonic oxyd, is, in general, adopted as a criterion to judge if the metal be sufficiently converted; but this is found frequently to be fallacious, and well it may, for the size of the blisters depend more upon the degree of heat to which the bar has been exposed, than to any other cause.

The time usually required for the conversion of iron into steel is about seven days and nights; and a similar number

Carbonic oxyd is the union of the two gases which arise from the small portions of carbon and oxyd of iron, of which the iron was possessed, and which is dissipated by the heat of the furnace during this long process

of days and nights is allowed for the gradual cooling of the furnace.

The steel when taken from the converting furnace is found on its surface to be covered with blisters; and on being broken is found to be full of cavities within, for this reason it is called blistered steel.

To make it sound and tenacious, it is put into a furnace, and moderately heated, and is then exposed to the action of the tilt-hammer, which we have already described. This is called sheer-steel.

The steel is made of different degrees of hardness, by giving it more or less carbon, according to the different degrees and duration of the heat applied.

The steel used in the manufacture of coach-springs contain the smallest portion of carbon; a somewhat greater quantity is used in the different branches of cutlery, and in the make of agricultural implements; and the greatest dose of all is required for files, which cannot be too hard, provided the steel be sufficiently malleable to be worked.

Cast-steel, which is entirely free from the defects of blistered steel, and is, in some degree, preferable to sheersteel, is made, by placing small portions of the bars of blistered steel into a crucible, capable of containing about 30 pounds weight..

These crucibles are made of Stourbridge clay, mixed with a small portion of powdered charcoal, which makes them much less liable to crack in the heating or cooling. They are furnished with covers, which are more fusible than the body of the vessel, and, on that account, soon enter into a state of partial vitrification; by which means they become closely luted at the time the steel is at a temperature sufficiently high to be destroyed by the oxygen of the atmosphere.

The fuel employed for melting steel should consist of the hardest cokes, which will give a great heat for a longer continuance than the soft cokes.

When the metal is fused it is taken from the furnace, and poured into iron-moulds, which form it into ingots of an. octagonal shape, about 30 inches long.

These ingots, like the bars of blistered and sheer steel, are again heated, and drawn into bars by the operation of the tilt-mill. By means of this machinery the ingots of caststeel can be drawn into bars one-third of an inch square; and by the hands it can be drawn into rods of a much smaller size.

The manufacture of steel has been greatly improved within

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a short period, and it can now be fused with so small a portion of carbon, as will admit of its being welded either with iron or another piece of steel.

The most singular property belonging to steel is that of its hardening by being heated red-hot, and suddenly cooled: and the hotter the steel be made, and the colder the fluid into which it is plunged, the harder will be the steel. Water is generally employed for this purpose; and spring water is considered to be the best. File-makers state, that the salt which is inevitable in their hardening water, makes the steel harder, and they sometimes put sulphuric acid into it for the same purpose.

In hardening steel in thin plates, such as saws, particularly when of cast-steel, quenching in water would cause them to crack, and make them so hard as not to be useful. They have, in consequence, recourse to some substance which is not so good a conductor of heat. Oil, with tallow and bees' wax, and resin dissolved in it, is generally employed for these articles. If the steel be heated red-hot, it mostly returns to its original state. This, however, is sometimes not the case with thin plates of cast-steel. In giving various degrees of heat from the hard state, it becomes more soft and less elastic.

In the year 1789, Mr. David Hartley took out a patent for a method of tempering steel by the aid of a pyrometer, or thermometer, applied near to the surface of the article, and at the same time recommended the use of heated oil, in which (he says) many dozens of razors or other tools might be tempered at once with the utmost facility, and the various degrees of heat necessary for different purposes might speedily be determined by experiment. (See Nicholson's Journal, vol. i. quarto.) An improvement of this principle has been since suggested by Mr. Parkes, by providing a bath of oil or of some kind of fusible metal for the tempering of every species of edged tool, which contrivance would, in his opinion, give to this operation a greater degree of certainty, than has ever been experienced by those who have conducted such manufactories.

WIRE MANUFACTURE.

WIRE is made of various ductile metals; but as the manufacture of the whole is very similar, we shall confine ourselves principally to a description of the manufacture of iron wire, which is by far the most extensive article of commerce,