SMELTING. 



397 



C is the throat of the furnace ; D the chimney. The lower part, B, is called 

 the boshes. As soon as the ore in the furnace has become ignited the 

 carbon and oxygen unite and form carbonic acid, which escapes, and the 

 metal fuses at last and runs away. The coal and ore are continually added 

 year after year. The glassy scum called " slag " protects the molten iron 

 from oxidation. 



The metal drawn from the blast furnace is " pig iron," or " cast " iron, 

 and contains carbon. This kind of iron is used for casting operations, and 

 runs into sand-moulds. It contracts very little when cooling. It is hard 

 and brittle. 





Fig. 403. General foundry, Woolwich Arsenal. 



Bar Iron is the almost pure metal. It is remarkably tenacious, and 

 may be drawn into very fine wire or rolled. But it is not hard enough for 



tools. It is difficult to fuse, and 

 must be welded by hammering at a 

 red heat Wire-drawing is performed 

 by taking the metal as a bar, and 

 passing it between rollers (fig. 404), 

 which flattens it, and then between 

 a new set, which form cutting edges 

 on the rolled plate (fig. 405), the pro- 

 jections of one set fitting into the 

 hollows of the other closely as in the 

 illustration. The strips of metal 

 come out at the aperture seen at A 

 in the next illustration. These rods arc drawn 

 through a series of diminishing holes in a steel 

 plate, occasionally being heated to keep it soft and 

 ductile. When the wire has got to a certain 

 to a cylinder and drawn away, at the same 



|J -J ,a j J 



, ^| 



F,g. 405. (Jutting 

 ech 



Jges. 



Fig. 404. Wire rollers. 



fineness it is attached 



