STEEL MANUFACTURE. 



159 



requisite quantity of carbon by the process known as cemen- 

 tation, in which the bars of wrought-iron are heated in a 

 closed iron case with powdered charcoal for several days. 

 The iron under this treatment was converted into steel. The 

 process was so costly and tedious that steel commanded a very 

 high price and was only used for special purposes. 

 Analyses of pig-iron, wrought-iron, and steel 



Graphite . . . 



Combined carbon 



Silicon 



Phosphorus 



Sulphur . . . 



Manganese 



Copper 



Iron 



99-989 100-000 100-000 



In 1856 Bessemer introduced his process for making steel 

 directly from pig-iron. This consists in running the molten 

 pig-iron into an egg-shaped vessel mounted on trunnions and 

 constructed of boiler plates lined with a particularly infusible 

 fire-clay known as ganister and consisting of almost pure 

 silica. At the bottom of this vessel, known as a converter, are 

 holes through which air can be blown by means of a pipe 

 passing through one of the trunnions. The blast of air pass- 

 ing through the molten iron oxidises the carbon, silicon, 

 sulphur, and part of the iron, producing thereby a higher 

 temperature, so that although pure iron has a much higher 

 melting point than cast-iron, the contents of the converter do 

 not solidify. The carbon by its oxidation produces carbon 

 monoxide, w r hich burns at the mouth of the converter, and the 

 end of the operation can be detected by the sudden dying 

 down of this flame ; the contents of the converter then consist 

 of practically molten wrought-iron. It is converted into steel 

 by the addition of a suitable quantity of a particular variety of 

 cast-iron known as "spiegeleisen," which contains a high percent- 

 age of carbon. The metal is then poured out of the converter. 



