Natural steel is that which is formed, by converting the ore first into cast-iron, and exposing it to the action of a strong heat, while the melted scoriæ float on its surface. This steel is inferior to the others. Steel of cementation is formed, on a large scale, by stratifying bars of iron with charcoal, in large earthen troughs or crucibles, the mouths of which are closed with clay. These troughs are put in furnaces, and, in eight or ten days, the process is finished. This is also called blistered steel, on account of the appearance of its surface. The tilted steel is that which is beaten out into small bars by the hammer. When broken, and the pieces again united by welding in a furnace, and made into bars, it is then called German or shear steel.
Cast steel is considered the most valuable of all the varieties; and is used for the manufacture of razors, surgeons' instruments, &c. It is, besides, more fusible than common steel, and for that reason, cannot be welded with iron. It is made by melting the blistered steel, in a close crucible, along with pounded glass, and charcoal powder. It may also be formed by melting together 30 parts of iron, 1 part of charcoal, and 1 part of glass. Equal parts of chalk and clay, put with iron in a crucible, will also produce it.
The Celtiberians in Spain had a singular mode of preparing steel. Diodorus and Plutarch both say, that the iron was buried in the earth, and left in that situation, till the greater part of it was converted into rust. What remained, without being oxidized, was afterwards forged and made into weapons, and particularly swords, with which they could cut asunder bones, shields, and helmets. This process is used in Japan, however improbable it may seem; and Swedenbourg, among the different methods of making steel, has introduced it. Bishop Watson, (Chemical Essays 8vo. i, p. 220,) speaks of the same process. The fact has been verified at Gottingen; for an anvil, which had been buried in the ground for many years, was found to be extremely soft; and a part of it, which appeared in steel-like grains, possessed the properties of steel.
The sabres made in Japan, according to Thunberg, are incomparable. Without hurting the edge, they can be made to cut through a nail at one blow.
The art of hardening steel by immersion in cold water is very old. Homer (Odyssia ix, 301,) says, that, when Ulysses bored out the eye of Polyphemus with a burning stake, it hissed in the same manner as water, when the smith immerses in it a piece of red-hot iron, in order to harden it. Sophocles, Salmasius, Pliny, Justin and others mention the use of water in hardening iron; but the most delicate articles of that metal were not quenched in water, but in oil. As to the opinion of the peculiar virtue of any particular water, for the purpose of hardening iron, which many have believed, it is altogether fallacious, although Vasari asserts, that the archduke Cosmo, in 1555, discovered a water, that would harden instruments, to cut, like the ancient tools, the hardest porphyry. The art of working porphyry, however, was known in every age. Beckman assures us, when treating of the processes of making steel, that the invention and art of converting bar iron into steel, by dipping it into other fused iron, and suffering it to remain there several hours, although ascribed to Reaumur, (Art de Convertir le Fer en Acier, p. 145), are mentioned by Agricola, Imperati, and others, as a thing well known and practised in their time.
Pliny, Diamachus, and other ancient writers mention various countries and places, which, in their time, produced excellent steel. The ferrum Indicum and Sericum were the dearest kinds. The former is the same as the ferrum candidum, a hundred talents of which were given, as a present, to Alexander in India.
Beckman thinks, that the ancient ferrum candidum is the same kind of steel still common in India, and known under the name of wootz; some pieces of which were sent from Bombay in 1795 to the Royal Society. Its silver coloured appearance, when polished, he thinks, may have given rise to the epithet of candidum.
Mr. Faraday of the Royal Institution has lately examined wootz, and imitated it very accurately. The experiments may be seen in Ure's Chemical Dictionary, article Iron. It appears that the presence of silex and alumina distinguishes this kind of steel from the English. Four hundred and sixty grains of wootz gave 0.3 of a grain of silex, and 0.6 of a grain of alumina. It is highly probable, that the much admired sabres of Damascus, are made from this steel.
A small portion of silver, melted with steel, improves the latter very considerably. One part of silver and five hundred parts of steel were melted together, and every part of the alloy formed, when tested, indicated silver. The alloy forged remarkably well, although very hard, and was pronounced to be superior to the very best steel. This excellence is undoubtedly owing to its combination with the silver, however small. The alloy has been repeatedly made, and with the same success. Various cutting tools have been made from it of the best quality. The silver is found to give a mechanical toughness to the steel.