The ordinary fuel for the smith's forge is coal, and the kinds to be preferred, are such as are dense and free from metallic matters, as these are generally accompanied with sulphur, which is highly detrimental. In London, the Tanfield Moor coals, from the county of Durham, are preferred, and the lumps are broken quite small before use.*
The fire is sometimes made open, at other times hollow, or like a tunnel; and the larger the fire is required to be, so much the more distant is it situated from the tuyere iron. Before lighting the fire, the useful cinders are first turned back on the hearth, and the exhausted dust or slack, is cleared away from the iron back and thrown into the ash-pit; a fair-sized heap of shavings is then lighted, and allowed to burn until the flame is nearly extinguished, when the embers are covered over with the cinders, and the bellows are urged; a dense white smoke first rises, and in two or three minutes the flame bursts forth, unless the fire be choked, when the poker is carefully passed into the mouth of the tuyere. The work is now laid on the fire, and covered over with green or fresh coals, which are beaten around the tuyere and the work, the blast being continued all the while: the whole mass will soon be in a state of ignition. A heap of fresh coals is always kept at the outside wall of the fire, and they are gradually advanced at intervals into the center of the flame, to make up for those consumed.
In making a large hollow fire, after a good-sized fire has been lighted in the ordinary way, the ignited fuel is brought forward on the hearth to expose the tuyere iron, into the central aperture of which the poker is introduced. A mass of small wetted coal is beaten hard round the poker to constitute the stock, the magnitude of which will depend on the distance at which the fire is required to stand off, and a second stock is also made opposite the first, the two resembling two hills with the lighted fuel lying between then the durability of the fire will depend on the stocks being hard rammed, which for large works is often done with the sledge- hammer The work is now laid in the hollow just opposite the blast-pipe, and covered on its two sides and top with thin pieces of wood, and a heap of wetted coals is carefully banked up around the same, and beaten down with the slice or shovel: when carefully done, the heap is made to assume the smooth form of an embankment of earthwork. The bellows are blown gently all the time, and the work is not withdrawn until the wood is consumed, and the flame peeps through at each end of the aperture, so as to cake the coals well together into a hard mass; after which the work may be removed or shifted about without any risk of breaking down the fire *.
Williams's compressed peat charcoal has been strongly recommended on account of its freedom from sulphur, one of the greatest enemies in nearly all metallurgie operations.
Sometimes when a fire is required only for hardening, the centering of the arch is made entirely of wood, either in one or several pieces: and in this manner it may be built of any required form, as angular for knees, circular for hoops, and so on (although such works are usually done in open fires, which resemble the above in all respects, except the covering-in or roof): small-coal is thrown at intervals into the hollow fire to replace that which is burned, and by careful management, one of these combustible edifices will last half a day, or even the entire day, without renewal. Occasionally, the stock around the tuyere iron will serve with a little repair for a second day, if when the fire is turned back at night, that part is allowed to remain, and the fire is extinguished with water.
When a small hollow fire is required, the same general methods are less carefully followed, and an iron tube, introduced amidst the coals, makes a very convenient muffle or oven for some purposes.
In forging, the iron or steel is in almost every case heated to a greater or leas degree, to make it softer and more malleable by lessening its cohesion; the softening goes on increasing with the accession of temperature, until it arrives at a point beyond that which can be usefully employed, or at which the material, whether iron or steel, falls in pieces under the blows of the hammer, but which degree is very different with various materials, and even with varieties bearing the same name.
* In localities where wood is scarce, small iron rods are placed around the principal mass, often designated the heat; the small rods are first withdrawn when the fire has burned up, to allow room for the removal of the work.
Pure iron will bear an almost unlimited degree of heat, the hot-short iron bears much less, and is in fact very brittle when heated; other kinds are intermediate; of steel, the shear-steel will generally bear the highest temperature, the blistered-steel the next, and the cast-steel the least of all; but all these kinds, especially cast-steel, differ very much according to the processes of manufacture, as some cast-steel may be readily welded, but it is then somewhat less certain to harden perfectly.
Without attempting any refined division, I may add, the smith commonly speaks of five degrees of temperature; namely -
The black-red heat, just visible by daylight:
The low-red heat:
The bright-red heat, when the black scales may be seen:
The white-heat, when the scales are scarcely visible:
The welding-heat, when the iron begins to burn with vivid sparks.
Steel requires on the whole very much more precaution as to the degree of heat than iron; the temperature of cast-steel should not generally exceed a bright-red heat, that of blistered and shear-steel that of a moderate white-heat. Although steel cannot in consequence be so far softened in the fire as iron, and is therefore always more dense and harder to forge, still from its superior cohesion it bears a much greater amount of hard work under the hammer, when it is not over-heated or burned; but the smallest available temperature should be always employed with this material, as in fact with all others.
It has been recommended to try by experiment the lowest degree of heat at which every sample of steel will harden, and in forging, always to keep a trifle below that point. This proposal however is rarely tried, and still less followed, as the usual attempt is to lessen the labour of forging, by softening the steel so far as it is safely practicable.
Iron is more commonly worked at the bright-red and the white-heats, the welding-heat being reserved for those cases in which welding is required; or others in which, from the great extension or working of the iron, there is risk of separating its fibres or laminae, so as to cause the work to become unsound or hollow, from the disrupture of its substance; whereas these same processes being carried on at the welding temperature, the work would be kept sound, as every blow would effect the opera' of welding rather than that of separation. The cracks and defects in iron are generally very plainly shown by a difference in colour at the parts when they are heated to a dull-red; this method of trial is often had recourse to in examining the soundness both of new and old fargings.
When a piece of forged work is required to be particularly sound, it is a common practice to subject every part of the material in succession to a welding heat, and to work it well under the hammer, as a repetition of the process of manufacture to ensure the perfection of the iron: this is technically called, taking a heat over it, in fact, a heat is generally understood to imply the welding heat. For a two-inch shaft of the soundest quality, two and a half inch iron would be selected, to allow for the reduction in the fire and the lathe; some also twist the iron before the hammering to prevent it from becoming "spilly"
The use of sand sprinkled upon the iron is to preserve it from absolute contact with the air, which would cause it to waste away from the oxidation of its surface, and fall off in scales around the anvil. If the sand is thrown on when the metal is only at the full red heat, it falls off without adhering; but when the white-heat is approached, the sand begins to adhere to the iron, it next melts on its surface, over which it then runs like fluid glass, and defends it from the air; when this point has been rather exceeded, so that the metal nevertheless begins to burn with vivid sparks and a hissing noise like fireworks, the welding temperature is arrived at, and which should not be exceeded. The sparks are however considered a sign of a dirty fire or bad iron, as the purer the iron the less it is subject to waste or oxidation, in the course of work.
In welding two pieces of iron together, care must be taken that both arrive at the welding heat at the same moment; it may necessary to keep one of the pieces a little on one side of the most intense part of the fire, (which is just opposite the blast,) should the one be in advance of the other. In all cases a certain amount of time is essential, otherwise if the fire be unneccssarily urged, the outer case of the iron may be at the point of ignition before the center has exceeded the red-heat. In welding iron to steel, the latter must be heated in a considerably less degree than the iron, the welding heat of steel being lower from its greater fusibility, but the process of welding will be separately considered under a few of its most general applications, when the ordinary practice of forging has been discussed, and to which we will now proceed.