PROGRESS IjV THE MANUFACTURE OF STEEL. 811 



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 heat- 

 ing 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, decarburiza- 

 tion 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 prac- 

 ticed 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 num- 

 ber 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 car- 

 bon, 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 sev- 

 eral 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, how- 

 ever, to have applied it to the manufacture of a metal low in carbon, 

 and therefore intermediate between iron and steel, jjartaking in a cer- 

 tain 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 be- 

 came pasty, and on examination was found to j^ossess 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 Reaumur, 

 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, be- 

 sides, they contain cinder, as the metal has never been fused. The 



