The bluing of steel appears to effect its elasticity in a manner not easily explain ed. This operation consists in exposing steel, the surface of which has been first brightened, to the regulated heat of a plate of metal, or a charcoal fire, or the flame of a tamp, till the surface has ac quired a blue colour. Now, if this blue coal be removed by grinding, the elastici ty is completely destroyed, and may be restored by bluing the steel again. Rub bing with sand or emery-paper, glazing, or burnishing, equally impairs the elasti city, in proportion as it destroys the blue coat. Saw-makers first harden their plates in the usual way, in which state they are brittle and warped ; they then soften them by blazing, which consists in smearing the plate with oil or grease, and heating it till thick vapours arc emitted, and burn off with a blaze ; and after this they may be hammered flat ; lastly, they blue them on a hot iron, which renders them stiff and elastic, without altering their flatness.
The Damascus sword blades have long been celebrated for their excellence, but it is not known how they are made. Mr. Stoddart took six small bars of good mal leable iron, and the same number of sheer steel ; laid them alternately on each other ; welded them together ; forged them into a stout flat plate, which was twisted spirally into a cylinder, hammer ed flat, and again welded; hammered this flat, doubled it throughout its length, inserted in the fold a slip of good steel to form the edge, and by another welding heat consolidated the whole into one mass. This being forged to a proper shape, cracked in different places on be ing cooled in water after heating; but Mr. Stoddart conceives, that, by using more pieces, repeating the twisting, and not quenching in water, the process would succeed.
Every species of iron is convertible in to steel by cementation ; but good steel is not to be made except from iron of the best quality, which possesses a certain stiffness and hardness as well as mallea bility. Swedish iron, as we have before remarked, is the best for this purpose. M. Duhamel tried a great number of the irons of France, Sweden, and Spain. He found the second to be the best ; but he likewise obtained excellent steel, superi or to those of Styria and Carinthia, which are the best German steels, by using cer tain iron made in France. But this iron was selected without fault ; and, in some instances, the loup or piece at the smelt ing furnace was fused and forged a se cond time ; a process, which, though at tended with loss of weight and additional expense, he recommends as absolutely necessary for making steel iron from ores of indifferent quality. The white spa those iron ores afforded him the best iron for the purpose of cementation ; and these also are the ores which afford the best steel by fusion in Styria, Carin thia, and Tyrol. He informs us that the English use no other cement than mere charcoal, which he also finds perfectly adequate to the purpose; and, moreover, that the quality of the steel is not affect ed by the different kinds of charcoal made use of. He remarks, nevertheless, that it may be advisable to add from one fourth to one-third of wood ashes, espe cially where the iron is not of to good a quality as to afford steel possessing tena city of body, as well as hardness. These
ashes, which he used with success, pre vent the steel-making process from being effected as rapidly as it would otherwise he, and give the steel pliability, without diminishing its hardness. It is remarked, that, in the case of this management, the blisters on the surface of the steel are smaller and more numerous. He like wise tried sea-salt. Fifty pounds of salt are sufficient for a furnace of steel of twelve thousand weight. The salt is pulverized, and sprinkled on the bars of iron when put into the furnace. He found that this ingredient likewise con tributes to give body to the steel. In the arrangement of the bars in the furnace, the cement is laid one inch thick at the bottom, and half an inch thick between each layer of iron. Our author affirms, that the process would succeed equally well, if the thickness were a little more than a quarter of an inch. The thickness of the bars of iron is indifferent, but there ought not to be a great difference in this respect between bars cemented at the same time. The common thick ness is little more than half an inch. It is not advisable that they should be very broad in proportion to the thickness, as this figure is found to produce flaws and cracks in the direction of the length the bar. The bars may be square, or their breadth may conveniently be some what more than twice their thickness. The fire for cementation must be of con siderable intensity, and kept up until the conversion has perfectly taken place, which is ascertained by proof bars, so disposed as to be taken out from time to time. The cementation is finished on the sixth clay; that is to say, it commonly lasts five times four-and-twenty hours. And, accordingly, the workmen take one of the proofs out on the fifth day, which is forged, hardened,and examined by the fracture. If it break short, and shew no indications of iron, the fire of the furnace is suffemd to go out. But if it contain iron, the fire is kept up for twelve or twenty-four hours, accordingly as the quantity of fibrous iron may have proved greater or less in the first proof. A se cond proof bar, taken out at the proper time, serves to direct them in the same manner with regard to their operations. By this management the cementation is continued somewhat beyond the time re quisite for the entire conversion. For there is less inconvenience attending a slight degree of excess in the cementa tion, than would result from a portion of iron remaining in the steel. The char coal after cementation is as black, and apparently in the same state, as it was before. M. Duhamel moistened it, and applied it to the same use a second time: it answered the purpose, but so much more slowly, that he objects to the use of it in manufactories. From this, as well as other circumstances• attending the steel-making process, it seems advantage ous, at least with regard to expedition, that the coals should contain volatile mat-, ter. And hence, the superior advantag es of animal coal, such as the coal of lea ther, or the hoofs and horns of animals, imperfectly burned, which are used in case-hardening, though they may be less applicable to the longer process of steel making, for various reasons.