40 TOWERS AND TANKS FOR WATER-WORKS. 



should retain, as far as possible, the natural qualities of the 

 metal ; and that the bar should be heated only when it is 

 necessary to make a permanent bend." 



Constructive or soft steel is produced, as has been stated, 

 by one of two processes, the Bessemer and the open-hearth, 

 and a technical classification of the product is determined by 

 the character of the lining employed in the furnace, whether 

 acid or basic. An authentic, brief, and comprehensive state- 

 ment descriptive of the two general methods of manufacturing 

 structural steels is copied in full from the work so frequently 

 herein quoted, and is as follows : 



Bessemer Steel. " The acid-Bessemer process consists in 

 blowing air into liquid pig iron for the purpose of burning most 

 of the silicon, manganese, and carbon of the metal, the opera- 

 tion being conducted in an acid-lined vessel, and in such a man- 

 ner that the product is entirely fluid. The way in which the 

 air is introduced is a matter of little importance as far as the 

 character of the product is concerned. . . . The lining is made 

 of either stone or brick, or other refractory material, and is 

 about one foot thick. . . . The blast is kept at a pressure of 

 from 25 to 30 pounds per square inch during the first part of 

 the blow, but, in the case of a very hot charge, or if the slag is 

 sloppy, the pressure must sometimes be reduced to 10 pounds 

 after the flame * breaks through ' (i.e., after the carbon begins 

 to burn), * to prevent the expulsion of the metal from the 

 nose . . . the heats, whether light or heavy, are usually blown 

 in from 7 to 12 minutes/ ' 



After the chemical change has taken place whereby the 

 cast iron has become molten steel, the fluid metal is tapped or 

 drawn off into cast-iron moulds, where the metal solidifies so 

 that it may be handled, when it is then called an ingot, and, 

 as such, reheated in a furnace, passed through trains of rolls, 

 as is the case with wrought iron, and rolled into the desired 

 shape. 



