152 ELECTROLYSIS. 



Iron and steel are coppered or nickeled before being gilded. 

 Massive silver and silver-plated pieces are gilded directly ; the 

 coppering preceding the gilding, however, consolidates the 

 latter and renders it more durable, especially when the coating 

 is thin. The angles rapidly become white when a layer of 

 copper does not intervene between the two precious metals. 



CONDUCT OF THE OPEKATION. Gold is possessed of very 

 developed coating properties, so that a very fine pellicle of that 

 metal has an appearance and gives a protection superior to 

 that of any other metal having the same thickness. The 

 pieces are sufficiently gilt after remaining in the bath a few 

 minutes. The great rapidity with which the gilding is effected 

 constitutes a great practical difficulty, as minute precautions 

 must be taken to insure immediate success : the electromotive 

 force must not exceed 1 volt, for the baths have a very little 

 resistance, and the intensity per square metre of surface of 

 cathodes must not exceed 10 amperes. 



Small objects can be gilded in bulk, a stoneware sieve being 

 used, and care being taken to constantly shake them so as to 

 always expose new surfaces to the action of the current. 



If any difficulty is experienced in obtaining the deposit 

 in the cavities, the pieces must be thoroughly cleaned again 

 and scratch-brushed, some cyanide must be added, and a more 

 energetic current used until the deposit begins to be formed in 

 the cavities. The smaller articles, like brooches, earrings, &c., 

 must be scratch-brushed as completely as possible and stirred 

 in the bath until the coating is uniform. 



When copper and silver articles are to be gilded, they must 

 be treated separately, beginning with the silver articles. 



EAPIDITY OF THE DEPOSITION. A bath containing 1 gramme 

 of gold per litre, can deposit about 25 centigrammes per hour 

 and per square decimetre. 



COLORATION OF THE GILT ARTICLES. In order to give 

 a good appearance to gilt articles, a paste must be prepared as 



follows : 



Alum ., 3 parts by weight. 



Potassic nitrate 6 



Sulphate of zinc 3 



Sodic chloride . . 3 



