ELECTROPLATING AND ELECTROTYPING. 175 



may be covered during the operation, so as to avoid breathing 

 the plumbago dust. 



The brushes are made of goat's hair of the finest quality. 



The plumbago must be very pure and well crushed, and, 

 before being used, passed through a very fine sieve. It is 

 essential to take great precautions in the preparation and use 

 of the plumbago, for almost all the imperfections of the 

 electros arise from a neglect in the metalisation of the surfaces 

 which are to receive the galvanic copper deposition. 



The machine gives a regular work and avoids the numerous 

 spots which are often to be seen on the electros the moulds of 

 which have been plumbago coated by hand. 



BATHS. Simple baths, that is to say, electric cells in which 

 the moulds are immersed, are still in extensive use, but the 

 really industrial process consists in using depositing baths and 

 sending through them the current produced by a dynamo- 

 electric machine. In the first instance the liquid used in the 

 cell is a nearly saturated solution of cupric sulphate, with 

 120 grammes of sulphuric acid for each 10 litres of liquid. To 

 obtain a better deposition, 4 grammes of arsenious acid are 

 added, or, instead, a small quantity of stannic chloride. 



The cupric sulphate must be dissolved in distilled boiling 

 water ; it is left to cool, and the sulphuric acid is then added. 

 The cupric sulphate must be very pure. 



In the second case, when the electric current is produced 

 outside the bath, the liquid used must be composed as follows : 



Sulphate of copper 825 grammes. 



Sulphuric acid 825 



Distilled water 10 litres. 



The water is first put in the vat and the acid is added by 

 small quantities at a time, stirring constantly ; then a quantity 

 of sulphate of copper as great as the acidulated water will 

 dissolve cold is put into the solution, which is maintained in a 

 state of agitation. 



The saturated bath must be at 25 Baume ; it is used cold 

 and must be maintained saturated by the addition of crystals. 



The sizes of the vats are naturally in proportion to the 



