104 ELECTRO-PLATING. 



When the precipitate of cyanide of silver has settled, the clear 

 solution is carefully decanted, and the vessel filled up with water, 

 which is again decanted as soon as the precipitate has settled. This 

 process is to be repeated three or four times, so as effectually to wash 

 out the soluble salts. When properly washed, a solution of cyanide 

 of potassium is added to the precipitate, until it is all dissolved. The 

 resulting solution constitutes the cyanide of potassium and silver, and 

 forms the plating solution. It ought to be filtered previous to using, 

 as there is always formed a black sediment, composed of iron, silver, 

 and cyanogen, which, if left in the solution, would fall upon the sur- 

 face of the article receiving the deposit, and make it rough. The 

 sediment, however, must not be thrown away, as it contains silver. 

 The cyanide of potassium, used to dissolve the cyanide of silver, may 

 be so diluted that the plating solution, when formed, shall contain 

 one ounce of silver in the gallon : of course, the proportion of silver 

 may be larger or smaller, but that given is what we consider best 

 for plating. 



In dissolving 100 ounces of silver, the following proportions of 

 each ingredient are those which we have found in practice to be the 

 best. Take 7 pounds of the best nitric acid 1 and 61 ounces of cyanide 

 of potassium, of the average quality described at page 92 : this 

 quantity will precipitate the 100 ounces of silver dissolved in the 

 acid solution. After this is washed, take 62 ounces more of cyanide 

 of potassium, the solution of which will dissolve the precipitate : this 

 being done, the plating solution is then formed. Of course, these 

 proportions will vary according to the difference in the quality of 

 the materials ; but they will serve to give an idea of the cost of the 

 silver solution prepared in this manner. .- 



Cyanide of Silver dissolved in Yellow Prassiate of Potash. We 



have occasionally dissolved the cyanide of silver by yellow prussiate 

 of potash, three pounds of which are required to dissolve one ounce 

 of silver. This forms an excellent plating solution, and yields a 

 beautiful surface of silver. It must have a weak battery power, and 

 consequently the silver is very soft. The positive electrode does not 

 dissolve in this solution : there is formed upon its surface a white 

 scaly crust, which drops off and falls to the bottom : and the solution 

 soon becomes exhausted of silver. 



Solution made with Oxide of silver. It has been recommended to 

 dissolve the oxide of silver in cyanide of potassium, which forms a 

 solution of cyanide of potassium and silver : but this preparation 

 is less economical, because the materials used in converting the 

 silver into an oxide are lost: it requires the same amount of 



1 The nitric acid must be free from hydrochloric (muriatic) acid : to a small quan- 

 tity of the acid add a few drops of a solution of nitrate of silver ; if it gives a milky 

 white precipitate, it contains muriatic acid, and should be rejected. 



