164 PRACTICAL LESSONS IN SCIENCE. 



pasty mass, when it is carried to a squeezer which drives out the 

 slag, welding the iron into a compact body called wrought iron. 

 Wrought iron may be obtained from some of the purer ores by 

 heating them with charcoal in an open forge. It is such ores that 

 are worked by the natives of Africa and Southern Asia. 



The pig iron or cast iron is capable of bearing great pressure, 

 and is useful in the manufacture of articles which are not sub- 

 jected to heavy tension. Wrought iron is tough, and when 

 heated slightly may be drawn into wire or rolled into bars or 

 plates, and is extensively used, especially where toughness is a 

 quality desired. Steel, in composition, is intermediate between 

 cast iron and wrought iron. They are each compounds of iron 

 and carbon, the wrought iron containing very little carbon, one 

 or two parts in a thousand only, while steel contains from three 

 to ten parts and cast iron from fifteen to twenty-five parts. In 

 neither case does there seem to be a definite chemical combination. 



Steel is made by heating wrought iron bars with charcoal to a 

 temperature of about 2,000 for several days, during which time 

 carbon penetrates the iron and the fibrous structure changes to 

 a fine granular form. This process is called cementation. These 

 bars broken up, heated and hammered become more uniform in 

 texture, more tenacious and ductile, forming what is often called 

 shear steel. Steel from the cementation furnace is melted in cru- 

 cibles and molded, forming cast steel, which is more dense, uniform 

 and hard than the shear steel. 



Cast iron is converted into steel by melting it in a large vessel 

 called a converter, and then oxidizing the mass by currents of air 

 forced through it. When the oxidation is nearly complete cast 

 iron enough is added to furnish carbon for converting the whole 

 mass to steel, when it is cast into ingots. This is called the Bes- 

 semer process. Steel becomes very hard and brittle when heated 

 and suddenly cooled, but when allowed to cool slowly it can be 

 given almost any degree of hardness, with some degree of elas- 

 ticity. Pure iron is almost unknown, and of no value in the arts. 

 With all the care exercised in working iron and steel it is almost 

 impossible to secure uniform product. 



