(a) Dissolve 1 oz. silver in nitric acid; add a small quantity of salt; then wash it and add sal ammoniac, or 6 oz. salt and white vitriol; also 1/4 oz. corrosive sublimate; rub them together till they form a paste. Rub the piece which is to be silvered with the paste, heat it till the silver runs, after which dip it in a weak vitriol pickle to clean it.
(b) For small articles a bath is made by dissolving in an enamelled cast-iron kettle in 2 gal, water, 17 1/2 oz. ordinary potassium cyanide. Also dissolve 5 1/4 oz. fused silver nitrate in 1 3/4 pint water contained in a glass or porcelain vessel. The second solution is gradually poured into the first. Stir with a glass rod. The white or greyish-white precipitate produced soon dissolves, and the remaining liquor is Altered if a perfectly clear bath is desired. When brought to the boiling-point, it will immediately silver the cleansed copper articles plunged in it. The objects must be quickly withdrawn. The silvering should immediately follow the cleansing, although the rinsings after each operation should be thorough and complete. This bright and light silvering is adapted for set jewellery, which cannot be scratch-brushed without flattening the clasps, and to which a bright lustre is absolutely necessary as a substitute for the foil of burnished silver placed under the precious stones of real jewellery. The employment of the solution of nitrate of binoxide of mercury is useless, and even injurious, for this bath.
It is useless to keep up the strength of the solution by new additions of cyanide and silver salt; thus reinvigo-rated, it gives results far inferior to those of the former solution. The bath should, therefore, be worked out as long as the silvering is satisfactory, and when exhausted, put away with the waste. With this process a battery and a soluble anode may be used to obtain a more durable deposit.
(c) A solution which, when boiling, produces a very fine silver coat, with a dead, or partly dead, lustre, upon cleansed coppers, is made by dissolving with the aid of heat, in a well-scoured copper kettle, distilled water, 9 pints; potassium ferrocyanide, 21 oz.; potash carbonate, 14 oz. When the liquid boils, add the well-washed chloride obtained from 1 oz. pure silver. This should boil for about 1/2 hour, and be filtered before using; part of the silver deposits upon the copper kettle, and should be removed when a new bath is prepared. On account of this inconvenience, the process has been nearly abandoned,, although the products are remarkably fine. All the dipping silvering baths, which contain a comparatively great excess of potassium cyanide to the proportion of the silver salt, will silver well copper articles perfectly cleansed, even in the cold; whereas this property diminishes in proportion to the increase of the amount of silver in the bath,or with the decrease of the amount of cyanide.
(d) For small articles, partly copper and partly iron, such as those used for saddlery and carriage wares, a particular process of silvering is used. The bath is composed of: - Water, 9 pints; caustic potash, 6 oz.; potash bicarbonate, 3} oz.; potassium cyanide, 2 oz.; fused silver nitrate, 2/3 oz. The cyanide, caustic potash, and bicarbonate are dissolved in 7 pints of water in an enamelled cast-iron kettle, then the remaining quart of water, in which the silver nitrate has been separately dissolved, is added to the former solution. For the silvering operations, the articles are cleansed, thoroughly rinsed, and put into a small enamelled kettle. Enough of the silver bath is poured in to cover the articles entirely, and the whole is brought to a boil for a few seconds, and stirred with a wooden spatula. When the silvering appears satisfactory, the liquor employed is put with the saved waste; the same liquid is never used for two batches of articles. This process gives a somewhat durable silvering with a dead lustre, of a greyish-white, which is increased in whiteness and brightness by soap and burnishing. Silvering with Foil. - This method is never practised except upon objects already manufactured, in their definite shape; and is adapted to all kinds of copper, bronze, or brass.
It is, in certain respects, superior to plated silver; but is very difficult of execution, and has less adhesion to the metal underneath. After annealing the articles, they are thrown whilst hot into a bath of sulphuric acid with a small proportion of hydrochloric and nitric acids. They have then a dull and dead lustre, owing to a multitude of small holes, which are so many points of attachment for the silver foil. The objects, thus prepared, are tightly fixed upon an iron rod, which is held in a vice. Their temperature is raised to about 300° F., by means of incandescent charcoal put at the proper place, so as to open the pores of the metal, which, by cooling afterwards, will imprison the silver applied. The silver foils, taken from the book with small tweezers, are cut to the proper size upon a cushion with an ivory or steel knife. After each foil is deposited upon the object, it is made to adhere by a light pressure of a rag pad, and afterwards by the friction of a steel burnishing tool. The parts of the silver foil which do not adhere Are removed with a soft brush. Gold-beaters prepare silver foil either with bright or dead lustre. The latter is made to adhere only by the pressure of the pad, and not by the burnishing tool.
This dead lustre cannot compare in fineness with that obtained by the battery; however, it resists handling and the sulphur gases of the atmosphere better. Articles thus silvered are only burnished after, all the silver foils have been applied; round or cylindrical objects are burnished upon the lathe, other forms by the hand; there are always places and lines showing the vibrations of the burnishing tool. This method of silvering is only employed for very large objects, such as high chandeliers and other church ornaments. Spoons and forks may be covered with silver foil as follows: - First slightly silver with a dead lustre in a silver bath by dipping; heat; and then cover with silver foil, by the pressure of an iron scratch-brush striking vertically, forcing the silver foils into the pores of the metal underneath. Burnish by the usual method; it is impossible to obtain a dead lustre by this method.