Electrotyping 131 



passes from the copper to the mould, depositing copper upon 

 the blackleaded surface near the wire, and gradually spreading 

 to the whole mould. It should be noted that the amount of 

 copper deposited is proportional to the amount of current, not 

 to pressure, hence no advantage accrues through the use of 

 more than one cell, the resistance of copper sulphate solution 

 being small; while the condition of the deposited copper is 

 perfectly under control by the amount of zinc allowed to act 

 in the cell. If the zinc be fully let down, the amount of 

 current (proportional to surface acting) will be large and the 

 deposition rapid, in which case the copper is often deposited 

 in a flocculent condition, in which state it is useless, as it does 

 not adhere to the blacklead. On the other hand, too slow a 

 deposition will probably produce a crystalline coating, so much 

 time being taken in the process as to make it a nuisance. 

 About J" of zinc let down into the solution is generally suffi- 

 cient, and, in this condition the process may continue over- 

 night, when the coating of copper should be complete, and of 

 a bright red colour. The resistance of the liquid may be 

 increased and the rate of deposition decreased by increasing 

 the distance between the plate and mould, the copper coming 

 down brighter and more closely adherent at the greater dis- 

 tance. In any case the plate and mould should be kept 

 parallel, or one portion of the deposited copper will be thicker 

 than the other. 



The deposition having been completed, the mould may be 

 removed from the cell, the copper portion dried, filled up with 

 solder, with zinc chloride as flux, and cleaned up with benzene 

 the gutta-percha will soften and peel off under the influence 

 of the heated solder. The copy may be polished by rubbing 

 with (a) benzene, (b) powdered bathbrick, (c) precipitated 

 chalk, (<f) rouge, and trimmed and mounted as desired. 



Carbon-metal junctions are easily and satisfactorily made 

 by depositing copper upon the carbon in the manner indicated 

 above, the carbon being attached to the negative pole of the 

 cell, and immersed in the plating solution to the depth to 

 which the copper is required. The carbon is removed after 

 deposition, dried, and the coppered portion soldered to a 



