In order to finish silver plated articles a bright vat is employed, which needs a somewhat stronger current than that used for the ordinary plating, the operation proceeding somewhat slowly, and brightening being observed to proceed from the bottom of the articles upwards, while the entire operation should be completed in about 20 minutes. A bright solution may be prepared by adding to each gallon of the usual solution of silver in cyanide of potassium 6 oz. of bisulphide of carbon, the mixture being put into a stoppered bottle, shaken, and set aside for 24 hours, and then adding to every 20 gal. of the ordinary plating solutiou in the vat 2 oz. of the bright solution, and the whole stirred together ; this proportion must be added every day on account of the loss by evaporation, but when the mixture has been made several days, less than this may be used at the time. This operation gives a bright deposit, but by adding a larger amount a dead surface may be obtained very different from the ordinary dead surface. It is as well to remove the articles from the bright vat at once into boiling water, for unless this is done they are very liable to blacken rapidly.
The brightening solution should always be added with care, for an excess is apt to spoil the bath, and its management is always a matter requiring attention, but it is very serviceable in plating a surface which cannot easily be scratch-brushed. Another method of preparing a bright solution is to add to the preparation we have detailed above, 4 oz. of liquor ammonia, and 2 oz. of ether.
For a silvering solution which requires no mercury dip or quicking in preparing the work, for each gallon of solution dissolve 1 oz. of fine silver in the ordinary way, that is in the liquid of one part nitric acid to two parts water, evaporate and crystallize. Then add 3 pints of water until the nitrate is dissolved, take 2 oz. of iodide of potassium, dissolve in pint of water and add this to the nitrate solution, when a yellow flocculent precipitate of iodide of silver will be formed, until, on the addition of a drop of the solution of iodide of potassium, no precipitate appears. The precipitated iodide of silver is repeatedly washed, and then dissolved in strong solution of cyanide of potassium in hot water, which is added gradually with stirring ; when nearly all the iodide is dissolved, the vessel is allowed to rest for a few hours, the clear solution poured off, and any iodide remaining undissolved is again treated with the solution of cyanide, care being taken to avoid an excess of the latter ; water is then added to make up about 1 gal., which is set on one aide for at least a day, and the clear solution poured off. It is advisable that it should not be freely worked until several days after this preparation ; the work is passed through the potash, scoured, washed, and immediately placed in the depositing bath ; this is suitable for German silver, brass, and copper goods, and although it will not afford a dead white deposit, still that given is tenacious, and obtained without the use of mercury ; it may be frequently used to advantage to give a preliminary coating to articles, which may then be transferred to the ordinary cyanide bath to receive a full deposit.
Silvered articles may be coloured by various processes ; the simplest and moat effectual is that in which the articles are dipped in a saturated solution of borax, and allowed to dry ; a film of the borax is left on the surface after the evaporation of the water. They are immersed a
second or third time, until complete film of borax covers every part of the article ; when large articles are experimented on, the solution may be applied by means of a brush. The articles thus treated must be exposed to heat, until the borax is perfectly fused ; for this purpose, t4 heat nearly approaching to redness is required. After cooling, they should be immersed in dilute sul phuric acid, by which the borax is removed ; they should be finally dried, by being shaken in heated saw-dust, and warmed, in order more effectually to drive off all moisture. This process possesses the admirable property of preventing the rapid tarnishing of the plated articles. It was observed by those who first practised electroplating, that the silver reduced from its salts by electrical agency tarnished more rapidly than that manufactured in the ordinary way. This was ascribed to the purity of the silver, which was supposed to be favourable to its combination with the sulphurous and other vapours, by which it became tarnished. By the above process, however, the tarnishing of the voltaic silver is prevented ; and it is now scarcely, if at all, more readily tarnished than that reduced in the common way. It appears that the discoloration was not pro duced by any external action, but that there remained in the pores of the reduced metal a small quantity of undecomposed salt, which suffers slow decomposition, and thus injures the colour of the surface. The treatment with borax completely removes this saline matter, and renders the metal unalterable from any internal action.
Solutions for gilding may be either formed by the battery or by chemical means, the best of these being that formed with the cyanides of gold and potassium. Various solutions may be used for producing different coloured gilding, either by a separate current or by simple immersion, and they are employed both hot and cold, being also varied when used for iron, steel, and baser metals. A good simple immersion solution may be made by dissolving 14 oz. of pyrophosphate of soda in 9 pints of distilled water, and adding oz. hydrocyanic acid. The first two ingredients are heated together, ands oz. of soluble dry chloride of gold is added when the solution is coo], and afterwards the hydrocyanic acid, drop by drop, until the liquor becomes colourless ; an excess of the acid renders the gilding difficult, but this may be corrected by adding a small proportion of chloride of gold. It is used hot, the articles to be gilded upon must be previously passed through a very diluted solution of nitrate of mercury, and must be continuously shaken. It is best to employ three baths, the first containing au exhausted solution of the same kind, the second one that has been but partially used, and the third a freshly prepared bath which will produce the required shade ; the gilding is done in a few seconds. Green and white gilding may be obtained from a similar liquid by adding, drop by drop, a solution of nitrate of silver until the desired shade is arrived at ; before gilding green or white, gild the objects yellow in the ordinary bath, pass them rapidly through the nitrate of mercury solution, and then into the gold bath having the nitrate of silver.