IRON AND IRON MOLDING 373 



contained in the ore and flux. Being lighter than the iron, 

 the slag floats on the molten metal and is run off near the 

 bottom of the furnace. The chemical action which takes 

 place in the furnace is as follows : Part of the carbon of the 

 fuel burns to carbon monoxide and combines with the iron 

 oxide, setting free the iron; the remainder combines with the 

 oxygen in the air and with the ore, thus forming carbon mon- 

 oxide (CO) and carbon dioxide (CO 2 ) which pass out 

 through a pipe at the top. The burned and unburned gases 

 are allowed to enter other furnaces where they give off great 

 heat on burning. This heat is used to raise the temperature 

 of the air that enters the tuyeres. 



415. Properties of Pig Iron. During the process of 

 smelting, the liquid iron absorbs and combines with a con- 

 siderable quantity of carbon, sulphur, silicon, phosphorus, 

 and manganese from the ore and coke. Some of the carbon is 

 chemically combined with the iron in the form of iron carbide, 

 while the remainder exists as a form of free carbon called 

 graphite. While the fusibility (ease of melting) of iron de- 

 pends upon the percentage of carbon that it contains, too 

 high a per cent of carbon weakens the iron. The slower a 

 casting cools, the larger the amount of graphite formed and 

 thus the softer the iron. Pig iron is graded according to the 

 appearance of its fracture. 



416. Iron Foundry. The pig iron is shipped to different 

 foundries, where it is melted with scrap iron (old pieces of 

 iron parts) in a furnace called a cupola. When the heating is 

 of long duration, or when dirty or burned (oxidized) iron is 

 used in charging the cupola, it becomes necessary to employ 

 a flux, that is, some mineral substance that is lighter than 



