VII. 



SPECIAL MANUEES 



149 



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 con- 

 sisting 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 passing 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, which 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 percentage of carbon. The metal is then 

 poured out of the converter. 



This process quite revolutionised the iron industry, and steel 

 became so cheap that it almost replaced iron. The Bessemer process, 

 however, could only be applied to pig-iron fairly free from phosphorus, 

 for the process does not remove any phosphorus, and if steel contains 

 much of this element its properties are so altered that it becomes 

 valueless. 



The following table shows the change in the composition of the 

 metal in the " Bessemer converter" (acid lining) : 



It will be noticed that there is no reduction in the amount of phos- 

 phorus, but rather an increase, while the sulphur, carbon and silicon 

 are almost entirely removed. 



About 1878-9, a modification in the method of working the 

 Bessemer process was introduced by Thomas and Gilchrist, by which 

 pig-iron containing high percentages of phosphorus could be success- 

 fully converted into steel of good quality. Their improvement con- 

 sisted in lining the converter with lime, or a mixture of lime and 

 magnesia, and the introduction of freshly burnt lime into the con- 

 verter. They found that, under these circumstances, the phosphorus in 

 the pig-iron (existing in combination with iron as phosphide) was 

 oxidised by the air after the carbon .had been completely removed, and 

 the phosphoric acid so formed united with the lime and magnesia of 

 the basic lining to form a slag which floated on the molten iron. The 



