§ 139. The process of copper-plating from sulphate or nitrate may, according to Mr. Swan (Journal of the Royal Institution, 1892, p. 630), be considerably accelerated by the addition of a trace of gelatine to the solution. As success appears to depend upon hitting the exact percentage amount of the gelatine, which must in any case be but a fraction of one per cent, and as Mr. Swan refrains from stating what the amount is, I am unable to give more precise instructions. A few experiments made on the subject failed, doubtless through the gelatine content not having been rightly adjusted. Mr. Swan claims to be able to get a hard deposit of copper with a current density of 1000 amperes per square foot, but seems to recommend about one-tenth of that amount for general use.
The solution employed is a mixture of nitrate of copper and ammonium chloride — proportions not stated. Electrolytic copper, as generally prepared, is very pure, but this is a mere accident depending on the impurities which, as a rule, have to be got rid of. Electrolysis seems to have no effect in purifying from arsenic, for instance.
Roughly speaking, about 11 grms. of copper are deposited per ampere hour from cupric salt solutions. When the current density is too high the anode suffers by oxidation, and this introduces a large and very variable resistance into the circuit.
[§ 140. Alkaline Coppering Solution —]
Coppering Base Metals. — It is often desirable to coat lead, zinc, pewter, iron, etc., with a firm and uniform layer of copper preparatory to gilding or silvering. If copper or brass articles are soldered with soft solder it is found that the solder does not become silvered or gilt along with the rest of the material, but remains uncoated and of an ugly dark colour. This defect is got over by giving a preliminary coating of copper.
This is done in an alkaline solution, generally containing cyanogen and ammonia. The following method has succeeded remarkably well with me. The receipt was taken originally from Gore's Electro-metallurgy, p. 208. A solution is made of 50 grms. of potassium cyanide (ordinary commercial, say, 75 per cent) and 30 grms. of sodium bisulphite in I.5 litres of water. Thirty-five grammes of cupric acetate are dissolved in a litre of water, and 20 cubic centimetres of the strongest liquid ammonia are added. The precipitate formed must be more or less dissolved to a strong blue solution. The cyanide and bisulphite solution is then added with warming till the blue colour is destroyed. This usually requires the exact amount of cyanide and bisulphite mentioned, but I have not found it essential to entirely destroy the colour.
The solution contains cuprocyanide of sodium and ammonium (?), which is not very soluble, and this salt tends to be deposited in granular crystalline masses on standing. However, at a temperature of 50° C. the above receipt gives an excellent coppering liquid, which will coat zinc with a fine reguline deposit. Brass or copper partly smeared with solder will receive a deposit of copper on the latter as well as on the former, and, moreover, a deposit which appears to be perfectly uniform.
In using the bath the anode tends, as a rule, to become incrusted, and this rapidly increases the resistance of the cell, so that the current falls off quickly. The articles should be scratch-brushed and plated for about two minutes with a current density of about ten ampères per square foot.
As soon as the deposit begins to look red the articles are to be removed and rebrushed, after which the process may be continued. About five minutes' plating will give a copper deposit quite thick enough after scratch-brushing to allow of a very even gilding or silvering.
Aluminium appears to be fairly coated, but, as usual, the copper strips after soldering. Iron receives an excellent and adherent coat.