§ 133 Cold Silvering.
A thin but brilliant coat of silver may be readily applied to small articles of brass or copper in the following way. A saturated solution of sodium sulphite (neutral) is prepared, and into this a 10 per cent solution of nitrate of silver is poured so long as the precipitate formed is redissolved. A good deal of silver may be got into solution in this way. Articles to be silvered need only to be cleaned, brushed, and dipped in this solution till a coat of the required thickness is obtained.
I must admit, however, that the coating thus laid on does not appear to be so permanent as one deposited by simple immersion from the cyanide solution, even though it is thicker. The cyanide plating solution will itself give a good coat of silver if it is used boiling, and if a little potassium cyanide be added.
For purposes of instrument construction, however, a thin coat of silver is seldom to be recommended, on account of its liability to tarnish and its rapid destruction when any attempt is made to repolish it. For these reasons, nickel or gold plating is much to be preferred.
This art deserves to be much more widely practised than is usual in laboratories. Regarded as a means of preserving brass, copper, or steel, it is not appreciably more “time robbing” than lacquering, and gives infinitely better results. Moreover, it is not much more expensive. Strange as it may seem, the costliness of gilding seldom lies in the value of the gold deposited; the chief cost is in the chemicals employed to clean the work, and in interest on the not inconsiderable outlay on the solution and anode.
The easiest metal to gild is silver, and it is not unusual to give base metals a thin coating of silver or copper, or both, one after the other, before gilding, in order to secure uniformity. To illustrate the virtue of a thin layer of gold, I will mention the following experiment. About three years ago I learned for the first time that to “clean” the silver used in a small household required at least an hour’s labour per diem. I further ascertained that most of this time is spent on the polishing part of the process.
As this seemed a waste of labour, I decided to try the effect of gilding. In order to give the proposal a fair trial I gilt the following articles: half a dozen table spoons and forks, a dozen dessert forks and spoons, and a dozen tea spoons. These were all common electroplated ware. They were weighed before and after gilding, and it was with difficulty that the increase of weight was detected, even though a fine bullion balance was employed. On calculating back to money, it appeared that the value of the gold deposited was about threepence. Assuming that an equal weight of silver had been accidentally dissolved by the free cyanide during the plating — which is unlikely — the total amount of gold deposited would be worth, say, sixpence.
After three years’ continuous use the gilding is still perfect, except at the points on which the spoons and forks rest, where it is certainly rather shabby. Meanwhile the “gold” plate only requires to be washed with hot water and soap to keep it in perfect order, a much more cleanly and expeditious process than that of silver cleaning.