178 ELECTROLYSIS. 



It is, however, useful, and particularly to the large illus- 

 trated papers which require prompt illustrations of the most 

 recent events, to obtain electrotypes in less than twelve hours, 

 and good proofs have been obtained in eight, six, and even four 

 hours ; but this should not serve as a basis for a normal pro- 

 duction in an industrial workshop, as the electros become more 

 expensive and are not so regular. 



The best means for operating with the greatest possible 

 speed is to coat the mould, before its immersion in the galvanic 

 bath, with a concentrated solution of sulphate of copper, and 

 to spread over it some iron filings, perfectly pure and without 

 grease. The filings and the sulphate are mixed with a camel's 

 hair brush. The iron absorbs the acid, and the copper is 

 uniformly deposited on the mould. The mould is then washed 

 with a jet of water and immersed in the bath. 



Adams recommended spreading fine tin powder on the wax 

 when still hot; but notwithstanding its increased conductivity, 

 the mould could not receive too great a number of amperes per 

 square metre, and the operation was not much quickened. 



FINISHING OF THE ELECTROTYPES. When the operation is 

 terminated, the frame is removed, the electrotype washed in 

 running water, and the edges which could prevent the deposit 

 being removed are chipped. The frame is heated over a gas 

 jet so as to detach the wax and liberate the electrotype. The 

 latter is then placed on an iron slab, brushed externally 

 with hydrochloric acid saturated with zinc (spirits of salts for 

 soldering), and sprinkled with small pieces of solder. 



The slab is placed in a cell filled with molten stereotyping 

 metal, until the solder melts and it can be spread with a rag or 

 some oakum. This operation is called the tinning of the electro. 

 Finally the slab is taken away and a frame is formed round it 

 with some iron bars, and a sufficient quantity of metal is run 

 into the frame. This metal is generally composed of 91 parts 

 of lead, 5 of antimony, and 4 of tin. 



The thickness of the metal backing the electro varies be- 

 tween 4 and 10 millimetres according to the size of the 

 engravings. 



MOUNTING. There only remains to mount the plates on 



