TAKING SILVER FROM COPPER, &C. 113 



and besides this, it happens that the parts of the " replate" which 

 are sound, the silver not being acted upon by the acid, but rather 

 protected by the galvanic action, are not in a fit state to receive and 

 maintain a perfect adhesion of the deposit, and therefore the risk is 

 great that the new coating will separate from the old, or, in technical 

 language, that the part will strip. Under these circumstances, 

 experience has taught that the best way to proceed is to take 

 all the old silver off the article, and deposit an entirely new 

 coating. 



There are two methods of taking off the silver : 

 Taking Silver from Copper, &c. First, stripping or dissolving it 

 off: this is done by putting into a stoneware or copper pan some 

 strong sulphuric acid (vitriol), to which a little nitrate of potash is 

 added : the article is laid into this solution, which will dissolve the 

 silver without materially affecting the copper ; saltpetre is added 

 by degrees, as occasion requires ; and if the action is slow a little 

 heat is applied to the vessel. The silver being removed, the article 

 is washed well, and then passed through the potash solution, and 

 finished for plating. When the sulphuric acid becomes saturated 

 with silver it is diluted, and the silver is precipitated by a solution 

 of common salt : the chloride of silver formed is collected and fused 

 in a crucible with carbonate of potash, when the silver is obtained 

 in a metallic state. 



The article thus stripped by acid often snows a little roughness, 

 not from the effects of the acid, but because the copper under the 

 silver had not been polished : it is therefore a common practice in 

 the plating factories to polish the articles before plating. This is 

 done by means of a circular brush, more or less hard as required, 

 fixed upon a lathe, and a thin paste made of oil and pumice-stone 

 ground as fine as flour. By this process the surface of any article 

 can be smoothened and polished ; but a little experience is required 

 to ensure success, and enable the operator to polish the surface 

 equally without leaving brush marks. We need scarcely say, 

 that after this process the article must be cleaned before it is 

 plated. 



Second method. Instead of stripping off the silver by means of 

 acid, it is a more common and preferable mode to brush off the 

 silver by the operation just described. In this case the brushings 

 must be collected, dried, and burned, and the remainder fused with 

 carbonate of soda and potash, when the silver is obtained, in com- 

 bination with a little copper. 



Cyanide of Silver and Potassium, its Decomposition during the 



Plating Process. The silver salt in the plating solution is a true 

 double salt, being, as already described, a compound of one equiva- 

 lent of cyanide of silver, and one of cyanide of potassium two 



