Page:Popular Science Monthly Volume 19.djvu/831

Rh spathic ore, crushed fine; the mixture, to which a little flux has been added, if necessary, is then fused in clay crucibles. If very soft steel is required, some wrought-iron scrap is added.

Lastly, in this category we have a process which consists in heating cast-iron, but not so as to soften it, in oxide of iron, in the form of ore or iron-scale. In this way partial, or even total, decarburization of the metal can be produced at will.

So far the difference between iron and steel has seemed to be merely one of degree, depending on the amount of carburization. The methods we have considered are, in fact, only modifications of those practiced for the production of malleable iron. We will now consider the different processes that have for their object to impart a certain amount of carbon to malleable iron. The Hindoos have practiced one of them from time immemorial. They place in unbaked-clay crucibles, of the capacity of a pint, a piece of malleable iron, some chopped wood, and a few leaves of certain plants; the top of the crucible is then closed with clay, and the whole well dried near a fire. A number of these crucibles are then strongly heated for about four hours in a cavity in the ground, by means of charcoal and a blast of air forced in by a bellows. There is some reason to believe that an excess of carbon, over that required to produce the hardest steel, has to be added, in order to fuse the metal at the temperature which can be commanded in these furnaces. Before being drawn out into bars, the cakes of metal obtained in this way are exposed in a charcoal-fire during several hours to a temperature a little below their melting-point, the blast of air playing upon them during the time. The object of this is, doubtless, to remove the excess of carbon.

In 1800 a patent was taken out by David Mushet for a process in every respect analogous to that just referred to. He appears, however, to have applied it to the manufacture of a metal low in carbon, and therefore intermediate between iron and steel, partaking in a certain degree of the properties of both.

In another method referred to by Biringuccio, in 1540, steel was produced by keeping malleable iron in molten cast-iron until it became pasty, and on examination was found to possess the properties of steel. In connection with the theory of steel manufacture this process is of great interest. It shows that iron in a strongly heated condition is capable of absorbing carbon by direct contact, unless we suppose that the carburization is effected by dissolved gases, which is possible.

In the cementation process, which was well described by Réaumur, in 1722, bars of iron are kept at a glowing red heat, surrounded with charcoal in boxes, into which the air is prevented from entering. The operation lasts from seven to ten days, according to the quality of steel required. These bars are never uniformly carburized, and, besides, they contain cinder, as the metal has never been fused. The