COPPER AND MERCURY 



515 



Electrotypes. A copper electrotype of an object like a 

 medal is made by first preparing a cast of the medal in plaster 

 of Paris or wax. The surface of this is rubbed with graphite, 

 to render it a conductor, and the cast is used as the cathode 

 in a cell like that just described. The deposit of copper, when 

 stripped off, is found to show an exact reproduction of the 

 engraving, etc., on the object. 



Book plates are made by taking a cast of each page of type, 

 preparing the copper electrotype, and then strengthening and 

 thickening it by filling the back with melted lead. The printing 

 is then done from the electrotype. For newspapers this process 

 is too slow, and the plate is made from the cast by means of melted 

 stereotype-metal (lead, antimony, tin; 82 : 15 : 3). 



Copper Refining. The copper, as obtained from the ores by 

 the treatment already described (p. 510), contains a certain 

 amount of silver, gold, and baser metals. The former pay for the 

 cost of refining, and the simultaneous _ + 



removal of the latter gives pure copper 

 suitable for electrical purposes. 



The principle is the same as that used 

 in electroplating. The heavy plates of 

 poled copper (p. 511) are hung at intervals 

 in large, lead-lined vats of copper sulphate 

 solution and form the anodes (Fig. 118, 

 diagrammatic, view from above). The 

 metal is deposited on thin sheets of copper, 

 which are coated with graphite to permit 

 the deposit to be easily stripped off. These 

 sheets hang in the vat between the anodes, 

 and are connected with the negative wire. The copper, along 

 with such traces of more active metals, like zinc, as are present, is 

 ionized and goes into solution, until the anode is reduced to a 

 skeleton and is exchanged for a fresh one. The less active metals, 



FIG. 118 



