STEEL, (from the Saxon stal,) is made of the purest malleable iron by an operation called cementation, by which it acquires a small addition to its weight, amounting to about the hundred and fiftieth, or two hundredth part. In this state it is much more brittle and fusible than before. It may be welded like bar-iron, if it has not been fused or over cemented : but its most useful and advantageous property is, that of becoming extremely hard when heated and plunged into cold water; the hardness which it thus acquires is greater, as the steel is hotter and the water colder. The sign that should direct the mechanic in the tempering of steel, is the variation of colour which appears on its surface. If the steel be slowly heated, it exhibits a yellowish white, yellow, or gold colour, purple, violet, or deep blue. if the steel be too hard, it will not be proper for tools intended to have a fine edge, as it will be so brittle that the edge will soon become notched : and if too soft the edge will soon turn aside, even by very slight use. Some artists heat their tools, plunge them into cold water, and then brighten the surface of the steel upon a stone ; the steel being then laid upon hot charcoal, or upon the surface of incited lead, or on a bar or piece of hot iron, it gradually acquires the desired colour, and must at that instant be plunged iuto water. If a hard temper be required, as soon as the yellow tinge appears, the piece is dipped again, and stirred about in the cold water.
In tempering tools for working upon metals, it will be proper to bring them to a purple tinge before the dipping. Springs are tempered by bringing the surface to a blue tinge. This temperature is also desirable for tools employed in cut ting soft substances, such as cork, leather, and the like ; but it' the steel be plunged into water when its surface has acquired a deep blue, its hardness will scarcely exceed the temperature of iron. When soft steel is heated to any one of these colours, and then plunged into water, it does not acquire "so great a degree of hardness as if previously made quite hard. The degree of heat required to harden steel varies in the different kinds. The best require only a low red
heat ; the harder the steel, the more coarse and granulated will its fracture be. Hardened steel has less specific gravity than the soft. The texture of steel is rendered more uniform by fusing it before it. is made into bars, and in this state it is called cast steel, which is wrought with more difficulty than common steel, because it is more fusible, and will disperse under the hammer if heated to a white heat.
Every species of' iron is convertible into steel by cemen tation ; but the best steel can be made only from iron of the best quality, which possesses stillness and hardness as well as malleability. Swedish iron has been long remarked as the best for this purpose.
The east steel of England is made as follows: a crucible, about ten inches high, and seven in diameter, is filled with ends and fragments of the crude steel of the manufitetories and the filings and fragments of steel works ; they add a flux, the component parts of which are usually concealed. It is probable, however, that the success does not much depend upon the flux. This crucible is placed in a wind furnace, like that of the founders, but smaller, being intended to contain only one pot, and surmounted by a cover and chimney to in crease the draught of air ; the furnace is then entirely filled with coke, or charred pit-coal. Five hours are required for the perfect fusion of the steel. It is then poured into long, square, or octagonal moulds, each composed of two pieces of cast-iron fitted together. The ingots, when taken out of the mould, have the appearance of east iron. It is then forged in the same manner as other steel, but with less heat and more precaution. Cast steel is almost twice as dear as other good steel ; it is excellent for razors, knives, joiners' chisels, and all kinds of small work requiring an exquisite polish : its texture is more uniform than that of common steel, which is an invaluable advantage. It is daily more and more used in Etgland, but it cannot be employed in works of great mag nitude, on account of the facility with which it is degraded in the fire, and the difficulty of welding it.