442 



♦ KNOWLEDGE ♦ 



[Nov. 28, 1884. 



E L E C T K - P L A T I N G. 



By W. Slingo. 

 XIV.— PEEI'AEING THE OBJECTS. 



IN the jiractice of electro-plating, one of the first 

 necessities is to ensure that the surface of the object 

 to be plated is clean, and this not in the ordinary accep- 

 tation of the term, but as a chemist understands it — that 

 is to say, the surface must be free from particles of foreign 

 matter of all kinds. The smallest particle of greixse or dirt 

 adhering to the object is quite sufficient to spoil the deposit, 

 for wherever that particle happens to be the deposit will be 

 little or nothing. 



There is, perhaps, no species of work which calls so im- 

 peratively for absolute cleanliness. The touch of a finger, 

 tmless the skin be covered with some preventible material, 

 is sure to leave a very distinct impression ; and, even when 

 the surface of the object is chemically pure, there is some 

 danger of the deposit only partially adhering in conse- 

 quence of the intervention of a film of air between the 

 object and the solution. The film of air which associates 

 itself, more or less, with every object exposed to it, must, 

 therefore, be carefully and completely removed ; but of this 

 a word will have to be said presently. Then, again, there 

 are some metals which are so easily oxidised that a brief 

 exposure to the air of the clean surface is sufficient to cause 

 it to be coated with a thin film of oxide, and oxides are more 

 or less non-c >nductor3 of electricity. 



The metals which are most likely to be called into 

 requisition for planting purposes are iron, lead, tin, copper, 

 brass, Britannia metal, and German silver. 



Iron is a metal which has generally a somewhat exten- 

 sive supply of superficial impurities, to remove which it 

 )nay be placed in a weak acid solution consisting of 1 oz. 

 'if hydrochloric acid, and 4 oz. of sulphuric acid to the 

 i:>llon of water. After remaining in this solution for some 

 little time it may be removed and the surface rubbed with 

 a wet brush and a little sand. If this fails to cleanse the 

 surface, the iron must be re-immersed, and the rubbing 

 operation repeated until a clean metallic surface is pro- 

 duced. 



Sometimes emery-cloth will be sufficient to remove the 

 dirt ; but the wet process is, perhaps, preferable, and has 

 the advantage of leaving a smoother surface, unless, indeed, 

 the degree of rust or oxidation is very variable — deep in 

 some places and very thin in others — when special means 

 must be resorted to in order to procure a level surface. It 

 must be borne in mind that the ultimate coating will pre- 

 sent a surface the very counterpart of that upon which it 

 is deposited, and that, therefore, it is necessary that the 

 surface of the object should be freed from all " foreign" or 

 undes'irable irregularities, including file-raarks, hammer- 

 marks, itc. The acid solution is very efficacious for the 

 removal of oxides of the metals, but must not be relied 

 upon for the removal of grease-spots, unless they are very 

 small, and likely to be so far under-eaten as to ensure their 

 being removed by the sand and brush. When a quantity 

 of grease is present, it may be removed by immei-sing the 

 object in a solution of caustic soda, which will effectually 

 dissolve away the troublesome fats. This should be done 

 prior to immersing the iron in the acid solution. The object' 

 having been suVjected to these cleansing processes, is then 

 well washed with clean water, and immersed in the electro- 

 lytic bath. 



It is to be noticed, however, that a silver coating de- 

 posited direct upon iron is generally very unsatisfactory, 

 and likely to strip off. To prevent tkis, a thin layer of 

 copper is interposed. As, however, iron would be dissolved 



in a sulphate of copper solution, the copper is best deposited 

 in a cjanide of copper solution of which there are several, 

 one of them being mentioned in apre-\-ious article. Another 

 one is very easily prepared by adding to a solution of 

 sulphate of copper a solution of cyanide of potassium, 

 adding it gradually until the whole of the copper is thrown 

 down as a greenish precipitate of cyanide of copper. After 

 allowing this precipitate to settle, the supernatant sulphate 

 of potassium solution is carefully poured off, and the cyanide 

 of copper washed several times with clean water, thus effec- 

 tually removing any trace of the acid solution that might 

 otherwise have clung to the precipitated cyanide, and sub- 

 sequeutly caused some little trouble by dissolving a portion 

 of the iron. The precipitate lias then poured over it a 

 solution of cyanide of potassium, until the whole of it is 

 again dissolved. In this, as it is not a solvent for iron, the 

 ferruginous object may be placed and connected with the 

 negative pole of a battery, the positive pole of which is 

 connected with a sheet of copper placed in the same bath 

 as the iron. In a short time a sufficiently thick coating of 

 copper will be deposited, when the object may be removed, 

 and, after being well rinsed, placed in the silver-plating 

 bath. 



Zinc may be treated in a similar way, except that the 

 hydrocholoric acid may be omitted from the acid solution, 

 or the action will probably be too energetic for the 

 purpose. 



Tin, lead, and Britannia metal should not be placed in 

 the acid solution, but should be immersed in the caustic- 

 soda solution, by which process oxides of the metals, as 

 well as superficial grease spots, ic, are removed. 



On being withdrawn from the caustic-soda solution, such 

 articles should be transferred to the depositing-bath without 

 being subjected to the washing process. When very dirty, 

 it is advisable to rub them first with sUver-sand and water, 

 applied with a tolerably stiff brush. The deposition of a 

 preliminary or intermediate coating of copper is almost as 

 advantageous in the case of lead or tin objects as it is in 

 the case of iron. When this method is adopted, it is 

 necessary to prepare the surface of the copper-film by 

 slightly burnishing it, and then well rinsing it in 

 clean water. A high degree of finish to the burnishing 

 is, however, rather advantageous than otherwise, as 

 it is likely to render the deposit of silver more or less 

 non-adhereut. The necessary condition of the surface is 

 best produced by means of a " scratch-brush," which 

 consists of a number of bundles of fine brass wire fixed 

 round the edge of a flat wooden disc. This brush is 

 placed as a chuck on a lathe and made to revolve 

 rapidly, the metal to be brushed being pres-'cd against the 

 wires, while a small barrel or other vessel containing 

 a quantity of stale beer is fixed above the brush and 

 provided with a tap through which the beer is allowed to 

 run on to the object. It is not, however, necessary that a 

 lathe should be called into requisition for this purpose 

 There are any number of simple devices possible for attain- 

 ing the object quite as efficiently as could be done with a 

 lathe. Perhaps the simplest method is to attach a small 

 pulley-wheel to the side of the scratch-brush, and then, mount 

 ing them on a smooth metal spindle, connect the pulley 

 wheel by means of a gut or other band with a considerably 

 larger wheel, similarly mounted, at a short distance from, but 

 in the same plane as the smaller wheel. Very good results 

 may be obtained with apparatus of this kind. The scratch- 

 brush is a tool which is best purchased, and which is not 

 vei'y expensive. 



German silver, if dirty or greasy, is first washed in a 

 solution of caustic soda, and then well rinsed. After this it 

 is treated with powdered bath-brick applied with a hard 



