(a) 2 dr. tartar, 2 dr. common salt, 1/2 dr. alum, and 20 gr. silver, precipitated from the nitrous acid by copper. Make into a paste with a little water. This is to be rubbed on the surface to be silvered with a cork.
(c) A stoneware or glass vessel is about three parts filled with liquid soda bisulphite; a solution of silver nitrate in distilled water, of medium concentration, is gradually added while the bath is continually stirred with a glass rod; a white flocculent precipitate of silver sulphite is produced by stirring; this is dissolved by the soda bisulphite. The silver solution is added so long as the precipitate readily disappears, and stopped when it becomes slow to dissolve. This bath is always ready to work, and instantaneously produces a magnificent silvering upon copper, bronze, or brass articles which have been thoroughly cleansed and passed through a weak solution of nitrate of binoxide of mercury, although this last operation is not absolutely necessary. The loss of silver is made good by additions of silver nitrate. When the proportion of the bisulphite is not sufficient to dissolve the metallic salt, add some soda bisulphite to restore the bath to its primitive state. Silver is slowly deposited upon the sides of the vessel: this may be dissolved in nitric acid for future uses.
As the bath is cold it is always ready for use, and the deposit is finer and more unalterable, because only chemically pure silver is deposited, without any mixture of subsalts. Bisulphites of potash, ammonia, and other alkalies may be substituted for the soda bisulphite, but the latter is to be preferred because it is cheaper.
(d) By rubbing, with the thumb, a cork, or a brush. The results are better than those by the whitening process, but not very durable; the method is useful to repair slight defects upon more durable silverings, and to produce mixtures of gold and silver, or gold, upon slightly gilt objects, thus avoiding the use of resist varnishes. Make a paste by thoroughly grinding in a porcelain mortar or with a muller, and, as far as practicable, not in the light: - (1) Water, 3 1/2-5 oz.; white fused silver nitrate, or, preferably, chloride, 7 oz.; potash binoxalate, 10 1/2 oz.; potash bitartrate, 10 1/2 oz.; common salt, 15 oz.; sal ammoniac, 2 3/4 oz. (2) Silver chloride, 3 1/2 oz.; potash bitartrate, 7 oz.; common salt, 10 1/2 oz. When finely pulverised in a porcelain mortar, triturate it under a muller upon a plate of ground glass until there is no granular feeling. Keep the paste in a porcelain pot, or in a black glass vessel, to preserve it from the light, which decomposes it rapidly. When about to use it, add a little water so as to form a thin paste, which is applied with a brush or pencil upon the cleansed articles of copper, or upon those gilt by dipping, or even upon those gilt by the battery, pro-Tided that the coating is thin enough to allow the copper to decompose the silver paste through the coat of gold; allow the paste to dry naturally, or with the aid of a gentle heat.
The chemical reaction is more or less complete, according to the thickness of the gold deposit, and the dry paste is of a pink shade, or entirely green. The salts are removed by a thorough rinsing in cold water, and the silver appears with a fine frosted appearance, the brightness of which may be increased by a few seconds' immersion in a very dilute solution of sulphuric acid, or potassium cyanide. This silvering bears the action of the wire brush and of the burnishing tool very well; and it may also be oxidised. Should a first silvering not be found sufficiently durable, after scratch-brushing, apply a second or a third coat. This silvering is not so adhering or white on pure copper as upon a gilt surface. For the reflectors of lanterns the paste is rubbed upon the reflector with a fine linen pad; then, with another rag, a thin paste of Spanish white, or similar substance, is spread over the reflector and allowed to dry. Rubbing with a fine and clean linen rag will restore the lustre and whiteness of the plated silver.
Put into a tall vessel of glass or porcelain, water, 10 pints; crystallised soda carbonate, 10 lb. Pour a little mercury into the bottom of the vessel, so that the glass tube carrying sulphurous acid gas, which has to be placed in it, may not be stopped by the crystals formed during the operation. Arrange an apparatus for the production of sulphurous acid gas, and let the washed gas pass through the vessel holding the soda carbonate. Part of the soda is transformed into sulphite, which dissolves, and a part falls to the bottom as bicarbonate. The latter is, however, transformed into sulphite by a continuous production of sulphurous acid, and the carbonic acid escapes. When all has dissolved, continue the passage of sulphur-ous acid until the liquid slightly reddens blue litmus paper, and then put the whole aside for 24 hours. After that time some crystals are found upon the mercury, and the liquid above, more or less coloured, is the soda bisulphite for silvering. The crystals are separated from the mercury, drained, and kept for gilding baths. They are not suitable for silvering. The liquid soda bisulphite should be stirred with a glass rod, to throw off the carbonic acid which may still remain. The liquor should then be again tried with blue litmus paper.
If it turns a deep red, add a little soda carbonate for neutralising the excess of sulphurous acid; if red litmus paper becomes blue, there is too much alkali, and more sulphurous acid gas should be passed through the liquid, which is in the best condition when litmus paper becomes violet or slightly red. This solution marks 22°-26° B. and must not come in contact with iron, zinc, tin, or lead.