154 ELECTROLYSIS. 



judge of the effects which he desires to obtain. This done, the 

 piece is protected in all its parts which are not covered by the 

 pattern, by means of a varnish capable of resisting the action of 

 acids and alkalies, and then placed as an anode in a weak solution 

 of sulphuric acid. The salts of lead, of which the colours are 

 composed, are dissolved and the metal is attacked. When the 

 depth of the cavities so obtained is judged to be sufficient, the 

 piece is rinsed and put in a galvanic bath of silver or gold, of 

 low density and acting on the cold process. The deposition of 

 the precious metal takes place and perfectly adheres in the 

 cells which have been scoured by the action of the acid. When 

 the cavities are full, the operation is stopped, the varnish re- 

 moved, and the piece submitted to a hand polishing which 

 reduces the excess of metal until the surfaces are level. 



WEAKENED SOLUTIONS. Gold solutions which have become 

 altered by usage must be treated like silver solutions, by 

 adding acid or evaporating to dryness ; but the cyanide 

 obtained and which probably contains some other metals, 

 must be dried, mixed with a weight of litharge equal to its own 

 weight, and melted. The residuum after lixiviation is placed 

 in an excess of nitric acid, which dissolves the lead and leaves 

 the pure gold. 



GILDING BY SIMPLE IMMERSION. This system is even to 

 this day the most generally in use for imitation jewellery, 

 cutlery, optical instruments, &c. It is effected in baths con- 

 taining gold in a state of double salt of gold monoxide. 



According to Eoseleur the best solution is the following : 



Potassic pyrophosphate 800 grammes. 



Hydrocyanic acid one to eight of water 8 



Crystallised auric perchloride .. .. 20 



Distilled water 10 litres. 



The pieces for gilding must be prepared as we have in- 

 dicated for the galvanic gilding, and constantly stirred in the 

 bath, in which they remain only a few seconds. 



