SHEET METALS 443 



then the metal is again roasted so as to oxidize completely 

 the iron and other metals still combined with the copper. 

 Finally the copper is toughened by being covered in a fur- 

 nace with charcoal. While in the furnace it is stirred with 

 a pole of birch wood to cause ebullition. The grain gradually 

 becomes finer, the color lighter, and the metal more malleable. 

 Copper obtained in this way, however, is not 100% pure. 



512. Copper Refining by Electrolysis. Pure copper is 

 obtained by means of electrolysis.* Bars of impure copper 

 are melted in an ordinary furnace and are granulated by 

 being placed on a copper tray at the bottom of a tank of cold 

 water. This highly concentrated alloy of copper serves as 

 the anode of a battery. The cathode is an exceedingly pure 

 bar of copper (about 99.93% pure). The electrolyte is a 

 copper sulphate solution containing free sulphuric acid. 

 The principle of the process is that by electrolytic action 

 the metal to be refined is dissolved from the anode by the 

 free acid in the electrolyte. The current of electricity passing 

 through the solution deposits the copper from the electrolyte 

 on the cathode in a pure form. The foreign metals and im- 

 purities remain on the anode or in the anode slime. The 

 cost of the electrolytic process is covered first by the high 

 price of the refined metal and second by the value of the 

 silver and gold that is recovered from the copper. To have 

 high electric conductivity the copper must be free from 

 arsenic and antimony. 



513. Physical Properties of Copper. Copper when pure 

 is of a red color, exceedingly malleable and ductile. When 



*See page 171. 



