" Plumbago," says M. Breant, "has appeared to me in some circumstances to soften the steel, which an excess of carbon would render too harsh ; at least I have obtained excellent results with a hundred parts of steel, one of smoke black, and one of plumbago. But a very remarkable experiment in regard to the advantage which may be derived from it, in working on a large scale, is, that a hundred parts of soft iron, and two of smoke-black, melt as easily as com mon steel." From this announcement by M. Breant, it appears, that he was unacquainted with Mr. Mushat's patent process of converting malleable iron into steel by fusion with charcoal, described at page 787. M. Breant continues, " It must be supposed that the whole of the carbon does not enter into combination. Some of our best blades are the product of this combination. It is evident from this experiment that it is not necessary, in order to obtain very good steel, to begin the operation by cementation with iron. The iron may be treated immediately with the smoke-black, and this would greatly diminish the expense of the manufacture. A hundred parts of the filings of very grey cast-iron, and a hundred parts of similar filings previously oxidized, have pro duced a steel of a fine damask, and fit for the manufacture of bright arms; it is remarkable for its elasticity, a quality not possessed by the Indian steel. The more carbon the steel contains, the more difficult it is to forge. Most of the specimens that I have prepared have not been drawn out, but at a temperature the limits of which are extremely confined." " I am convinced from experience, that the orbicular veins called roma by the workman, which are seen on fine oriental blades, are the result of the manner of forging. If we content ourselves with drawing the steel out lengthwise, the veins will be longitudinal ; if we extend it equally in all directions, the damask has a crys talline appearance. If we render it wavy in the two directions, there will be shades and gradations as in the oriental damask. It will not require long trials to produce any variegated design we desire."—Repertory of Arts.
Having explained the most approved modes of preparing the celebrated damasked steel, as well as the various steels of our now no less celebrated British manufacturers, we proceed to the consideration of another department of the subject.
Case-hardening.—This is a process for converting the surface only of articles made of malleable iron into steel, in order that they may afterwards receive a high polish. The process is extensively applied to the pokers, tongs, and shovels of our domestic fire-grates, and to an infinite variety of our iron manu factures. The following mode, recommended by Mr. Gill, editor of the Techno logical Repository, deserves confidence, from the extensive practical knowledge of that gentleman in the treatment oiiron and steel. This is effected, he says, by inclosing the articles in carbonaceous compounds, either animal or vegetable, and exposing them to heat in close vessels, until the change is completed, and until the surface at least of the articles is converted into steel. For this pur pose, bones, from which the ammonia has been extracted by distillation at a high temperature, and which are afterwards ground to a coarse black powder, are chiefly used. The articles being surrounded with this powder, contained in cast iron vessels, are exposed to a high red heat, in an open fire-place, for several hours, until the surfaces of the iron articles are sufficiently changed to steel ; when, if large enough, they may be taken out, whilst hot, and quenched in water, or if too small and numerous, the whole contents of the vessel, bone-dust and all, may be poured into the water. Any parts of the articles which are required to remain iron after this operation, may be guarded from the action of the carbon, by coating them with clay or loam. Sometimes the water is covered with a layer of oil, two or three inches in depth, to prevent the small steel articles from being cracked in quenching ; and it is very convenient in this case to have a wire sieve suspended in the water, at a proper depth beneath its surface, to suffer the bone ashes to fall through, but to detain the small articles. Other substances are employed in case-hardening; leather, burnt till it can be pulve rised, is considered a good agent; also the hoofs and horns of animals, heated in an oven until they can be beaten to a coarse powder: the latter are preferred by gunsmiths for their work.
Hardening and Tempering.—In giving the requisite degree of hardness to cut ting instruments of steel, two distinct processes are employed ; first, hardening, and afterwards tempering. The hardening is effected by heating the steel to a cherry-red, and immediately plunging it into cold water ; by this process, the steel becomes so hard as to resist, or turn the edge of the hardest file ; likewise so brittle as to be useless for most purposes, but particularly for cutting instru ments. To adapt the steel to the latter, the second process, called tempering, is resorted to, which is a species of annealing, or softening, or as the workmen term it, WI* down, to the degree of hardness which is necessary for the peculiar purpose for which it is designed ; for in proportion as the edge is harder than is required, is its liability to break in use. It is a remarkable fact, that the greatest part of our ordinary cutlery is hardened by a process which, although well known to be a very defective one, is persisted in from the force of habit, or from an indisposition in the workman to make any changes in the cus tomary routine of executing work, in which the remunerating prices are very low. We allude in particular to the practice (which, we are informed, is general at Sheffield,) of hardening direct from the anvil ; that is, the articles are hard ened with the scale on the surface, which is produced by the act of forging. The scale varies in thickness, according to the degree of heat the steel received in forging; it is also a very bad conductor of heat, consequently the transition from heat to cold (by which the effect of hardening is produced) is not so sudden in one piece of steel as in another; and some are scarcely hardened at all, owing to the temperature of the water having scarcely penetrated through the scale. Hence, when such articles are afterwards tempered or " lowered," there are not two alike in temper out of a great number ; a fact which can hardly have escaped the observation of any man who has shaved for several years. Instead, therefore, of the customary mode of hardening the blade direct from the anvil, it has been recommended by an experienced manufac turer, that the blades be passed from the anvil to the grindstone. " A slight application of the stone," he observes, " will remove the whole of the scale or coating, and the razor will then be properly prepared to undergo the operation of hardening with advantage. It will be easily ascertained that steel in this state heats in the fire with great regularity, and that when immersed, the obstacles being removed to the immediate action of the water on the body of the steel, the latter becomes equally hard from one extremity to the other. To this may be added, that as the lowest possible heat at which steel becomes hard is indubitably the best, the mode here recommended will be found the only one by which the process of hardening can be effected with a less portion than is or can be required in any other way." Considerable difficulty has been experienced in giving to articles about to be hardened a perfectly uniform degree of heat in an ordinary fire ; and one of the best means of obviating it is probably that which has been published by Mr. Nicholson, of his adoption, and which he had for some time previous, for justifiable reasons, kept secret : this was to employ a bath of melted lead, heated to moderate redness, and well stirred ; into this the piece was plunged for a few seconds, until, when brought near to the surface, that part did not appear less luminous than the rest. The piece was then speedily stirred in the bath, suddenly drawn out, and plunged into a large body of water. Instead of employing simple water as the cooling medium, a variety of salts added to it have been at different times recommended and boasted of. Not long ago, mercury was cried up, and just now it is the fashion to extol a current of air, grounded, we believe, on the report of travellers, that the sabres of Damascus are hardened by cleaving the north wind with them. Although it is probable that improvements in the present mode of hardening may be discovered, we think it is improbable that they will be found in that fluid which is an inferior conductor of heat, and that cannot be applied with equal uniformity to water.