STEEL. This most useful and curious substance is a compound of iron and car bon : their relative proportions vary in steel of different qualities ; but in that used for ordinary purposes, the carbon rarely exceeds 2 per cent., and is gene rally below it. Certain kinds of iron are preferred to others in this manufacture ; but this relates entirely to its purity, which is the essential requisite. Steel is made by a process called cementation, which consists in filling a proper furnace with alternate strata of bars of the purest malleable iron and powdered charcoal : atmospheric air is carefully excluded from the boxes containing the bars, and the whole kept for several days at a red heat. By this process carbon, probably in the state of vapor, penetrates, and combines in the above small relative pro portion with the iron, the texture of which, originally fibrous, becomes gran ular, and its surface acquires a blistered character. The malleability of steel fills far short of that of iron; but it is harder, and more sonorous and elastic, and sus ceptible of a higher polish, and has less tendency to rust. At a red heat it ad mits of hammering into various forms, and of being welded or united by the blows of the hammer to another piece of steel or iron. Blistered steel, rolled or beaten down into bars, forms shear steel; and if melted, cast into ingots, and again rolled out into bars, it forms cast steel, which, when well prepared, has the great recommendation of perfect unifor mity of texture, and a finer and closer grain. The peculiarity of steel, upon which its high value in the arts in great measure depends, is its property of be coming, by sudden quenching in water, when at a bright red heat, extremely hard, and of being again softened down to any requisite degree by the application of a certain temperature, which may be indicated by a thermometer, commencing at about 300°, and terminating at a dull red heat. This process is often called tempering ; and the workman is some times guided in the extent to which it is carried by the color of the polished sur face of the heated steel, which is at first rendered by a pale straw tint, then yel low, brownish, purple, and blue, as the temperature rises from one extreme to the other. The latter color indicates ex
treme softness and elasticity, such as be longs to watch-springs, some sword blades, &c. ; pale straw indicates great hardness, as for razor blades ; yellow is somewhat softer, and shows a fit temper for penknives ; and the incipient blues announce the temper that belongs to coarser cutting instruments, and to table knives, any of which, made of hard steel, would soon get spoiled and notch ed, but the edges of which, when duly tempered, resist breaking on the one hand, and bending on the other. When a large mass of steel is hardened by quenching in water, it undergoes a cer tain degree of expansion, so that the spe cific gravity of hard steel is somewhat less than that of soft. It has been at tempted to improve the quality of steel for certain purposes by adding to it a mall portion of other metals : hence the ,erm silver steel &c. ; hut none of these 'Boys have on the whole proved superior o well-made common steel. There is a rind of steel imported from India, known order the name of ?roots, the cutting in struments of which are celebrated for the oughness and durability of their edge. [t appears probable that itspeculiarities lepend upon the presence of a little alu ninum. When the surface of some duds of steel is washed over with a ffeak acid, it acquires a peculiar mottled damasked appearance, as if its texture consisted of an intimate mixture of two different kinds of steel, or of fine fibres of steel and iron. Steel, alloyed with a little nickel, often puts on this appear ance ; but these and some other imita tions of the celebrated Damascus sword blades have not led to any important im provements in the manufacture of our cutting instruments.