98 



COATING OF IRON WITH ZINC. 



would have anticipated upon the whole had the batteries been 

 properly connected. 



Non-adherence of Deposit. Objections have been made to cover- 

 ing iron with copper for its protection, from an impression that the 

 copper will not adhere to the iron ; but if the operation is care- 

 fully performed the copper will adhere : when it does not, it will 

 generally be found that -it is the copper deposited from the sulphate 

 which loosens from the copper deposited from the cyanide occa- 

 sioned, no doubt, by the article not having been sufficiently washed 

 from the cyanide solution, and thus having a thin film of cyanide of 

 copper precipitated upon the surface, which prevents the adhesion 

 of the after deposit. Or, as it happens sometimes, on putting the 

 article into water, the cyanide of copper is precipitated upon the 

 surface. If a little cyanide of potassium is dissolved in the first 

 water used for washing out the depositing solution, this will be 

 prevented. 



We have repeatedly deposited copper upon iron wire, and after- 

 wards had it drawn out to twice its original length without the 

 copper stripping off; but, as the copper becomes hard and brittle, 

 it is liable to break if the wire is much bent. And if it be made 

 red hot, to anneal or soften it, the copper will oxidise, and if the 

 coating is thin, the iron will be left bare in some places. We have 

 seen iron bolts, covered with copper, driven through 17 inch wood, 

 and nails of all sizes subjected to rough work, without the deposit 

 being injured. These remarks are also applicable to iron covered 

 with zinc. 



COATING OF IRON WITH ZINC. 



In covering iron with zinc, the precautions necessary for copper 

 are not required: zinc being the positive metal, acids have a 

 stronger affinity for it than for iron, and therefore an acid solution 

 may be used. The one generally used is the sulphate. 



sulphate of Zinc. Zinc dissolves easily in sulphuric acid, and the 

 solution by evaporation yields crystals of sulphate of zinc ; but as 

 the salt is very cheap and abundant in the market, it is more con- 

 venient and economical to buy than to make it. The solution for 

 depositing is made by dissolving 21bs. of the crystallized salt in one 

 gallon of water. The single cell process cannot be used advan- 

 tageously with this solution. A separate battery is necessary, and 

 a zinc positive electrode. The metal is very easily deposited one 

 or two pairs of Wallaston's battery being sufficient for coating small 

 articles. 



Zinc may be deposited upon black-leaded surfaces in the same 

 manner as copper ; but, unless more than ordinary precautions are 

 observed, an article formed in this manner is so brittle that it can 



