Iron unites with carbon, sulphur and phosphorus. Of the sulphurets, there are two kinds, the protosulphuret and persulphuret. The former is the magnetic pyrites, and the latter, cubic pyrites, from both of which, green vitriol is obtained by decomposition. Pyrites, we may observe, was the original fire-stone, or the feuer-stein of the Germans, which was used in the place of flint. See Beckman's History of Invention. Iron also unites with some of the metals, forming alloys. The white iron of the French, (Fer blanc,) or tin plate of the English, is found to be any alloy of tin with iron, as well as a covering of tin on iron.
Sheet tin, or tinplate which is necessary in the construction of the apparatus for some fire-works, for canister shot, &c. is made by immersing sheets of iron, previously freed from rust, into melted tin. The number of dippings it undergoes, determines, in some measure, its quality and character.
The union of carbon and iron, forming very important modifications of this metal, is not only interesting in the military art, as concerns the metal for cannon, small arms, and fire-works, but also in relation to the many and highly useful compounds which result.
All the varieties of iron, which are distinguished by artists, under particular names, we may consider under the following heads: namely; cast iron, wrought or soft iron, and steel.
Cast or pig iron is the name of this metal, when first obtained from the ore. The ores of iron are various, and contain a greater or less quantity of iron, which is either combined with oxygen, or found with clay, giving rise to two important classes of iron ore, the calciform and the argillaceous. The reduction of the ore merely requires the presence of charcoal, and occasionally some addition, as limestone, when the clay iron ores are to be reduced. On the application of heat in furnaces, constructed for the purpose, the charcoal unites with the oxygen of the oxide, reducing it to the metallic state, and escapes in the form of carbonic acid; and the lime, if the ore be argillaceous, unites with the clay, forming a kind of glass, which floats on the melted metal. When the iron is suffered to run into moulds, prepared for its reception, it usually takes the name of pig iron.
Manufacturers distinguish cast iron by its colour and other qualities. The white cast iron is hard and brittle, and can neither be filed, bored, nor bent. Gray mottled iron, so called from its colour, is of a granulated texture, softer, and may be cut, bored and turned on the lathe. Cannon are made of this iron. Black cast iron is the most unequal in its texture, but the most fusible.
Cast iron melts at 130° of Wedgwood. Its specific gravity varies from 7.2 to 7.6. It is converted into malleable, usually called soft iron, by a process called refinement. Several modes have been adopted for this purpose. It was formerly done by keeping it in fusion in a bed of charcoal and ashes, and afterwards forging it. The hammering makes the particles of iron approach each other, and expels some impurities.
Among the various improvements for expeditiously and effectually converting crude into malleable iron, the process of Mr. Cort seems to possess advantages. The cast iron is melted in a reverberatory furnace, and the flame of the combustible is made to act upon the melted matter. It is stirred during this operation, by which means, every part is exposed to the air. A lambent blue flame begins to appear in about an hour, and the mass swells. The heat is continued about an hour longer; and, by this time, the iron acquires more consistency, and finally congeals. While still hot, it is next hammered by powerful tilt-hammers. This is called the puddling process.
Iron, obtained in this way, is not however pure; for it contains either some of the other metals, or oxygen, carbon, silicon, or phosphorus.
When small pieces of iron are stratified in a crucible with charcoal powder, and exposed to a strong heat for eight or ten hours, they are converted into steel. Steel is brittle, resists the file, cuts glass, and affords sparks with flint. It loses its hardness by ignition and cooling. It is malleable at a red heat. It melts at 130 degrees of Wedgwood. By being repeatedly ignited in an open vessel, it becomes, by hammering, wrought iron.