COATING OF IRON WITH COPPER. 95 



cyanide of potassium crystallizes in the solution, and may be col- 

 lected and preserved for use again. If the solution of cyanide 

 of potassium used to dissolve the precipitate is dilute, it will be 

 necessary to condense the liquor by evaporation, to obtain the 

 yellow prussiate in crystals: the remaining solution is the coppering 

 solution. Should it not be convenient to separate the yellow prus- 

 siate by crystallization, the presence of that salt in the solution does 

 not deteriorate it, nor interfere with its power of depositing copper. 



Peculiarities in working Cyanide of Copper Solution. The true 



composition of the salts thus formed by copper and cyanide of potas- 

 sium has not yet been determined, but their relations to the battery 

 and electrolyzation are peculiar. The solution must be worked at a 

 heat of not less than from 150 to 200 Fahrenheit. All other 

 solutions we have tried follow the laws laid down by Spencer and 

 Smee, namely, that if the electricity is so strong as to cause gas to 

 be evolved at the electrode, the metal will be deposited in a sandy 

 or powdered state ; but the solution of cyanide of copper and potas- 

 sium is an exception to these laws, as there is no reguline deposit 

 obtained unless gas is freely evolved from the surface of the article 

 upon which the deposit is taking place. This necessitates the use of 

 batteries of several pairs intensity, varying from five to nine pairs 

 of Wollaston's battery, according to the heat and the state of the 

 solution. 



As this solution is used hot, a considerable evaporation takes 

 place, which requires that additions be made to the solution from 

 time to time. If water alone is used for this purpose, it will pre- 

 cipitate a great quantity of copper as a white powder, but this is 

 prevented by dissolving a little cyanide of potassium in the water at 

 the rate of about four ounces to the gallon. The vessels used in 

 factories for this solution are generally of copper, which are heated 

 over a flue, or in a sand-bath the vessel itself serving as the 

 positive electrode of the battery; but any vessel will suit if a copper 

 electrode is employed, when the vessel is not of copper. 



Preparation of Iron for coating with Copper. When it is 



required to cover an iron article with copper, it is first steeped in 

 hot caustic potash or soda, to remove any grease or oil. Being 

 washed from that, it is placed for a short time in dilute sulphuric 

 acid, consisting of about one part of acid to sixteen parts of water, 

 which removes any oxide that may exist. It is then washed in 

 water, and scoured with sand till the surface is perfectly clean, and 

 finally attached to the battery, and immersed in the cyanide solu- 

 tion. All this must be done with despatch, so as to prevent the iron 

 from combining with oxygen. An immersion of five minutes' dura- 

 tion in the cyanide solution is sufficient to deposit upon the iron a 

 film of copper. But it is necessary to the complete protection of 



