Water, 2 1/5 gal.; potassium cyanide, pure, 17 1/2 oz.; pure silver for cyanide, 8 3/4 oz. The composition of commercial potassium cyanide is exceedingly irregular. The pure, or No. 1, contains 90-100 per cent. real cyanide, and is especially employed for gilding and silvering baths. No. 2 contains 60-70 per cent. real cyanide; it is the article prepared by Liebig's method, and is used for electro-baths of copper and brass. No. 3, which marks 55-60 per cent., is for scouring and preparing baths, and for photographic operations, (a) Put in a porcelain dish, holding a quart, pure granulated silver, 8 3/4 oz.; pure nitric acid at 40° B., 17 1/2 oz. Heat by charcoal or gas. The dish should be supported by an iron triangle, and not in direct contact with the fire. The acid rapidly attacks and dissolves the silver, with an abundant production of yellow nitrous vapours, which must not be inhaled. When the vapours have disappeared, there remains a liquid more or less colourless, according to the proportion of copper held by the* commercial silver, which is seldom entirely pure. The heat is then increased in order to evaporate the excess of acid, which escapes in white fumes. The material in the dish swells up and dries, and, with a further increase of heat, melts like wax.
The dish is then removed from the fire, and being held with a cloth, the molten mass is made to flow upon the sides, where it soon solidifies; the fused silver 'nitrate (lunar caustic) is more or less white, or grey, according to the purity of the silver employed. When perfectly cold, turn the dish upside down, and by a gentle tap on the sides, the mass is detached.
(6) Dissolve the silver nitrate in 10-15 times its weight of distilled water; hydrocyanic acid poured into this solution immediately produces an abundant white precipitate of silver cyanide. A sufficient quantity of prussic acid has been employed when, by adding a few drops of it to the clear liquid, no precipitate or turbidity appears. Throw the liquid upon a filter of calico stretched on a wooden frame: the silver cyanide remains on the cloth, the solution with: the nitric acid and excess of prussic acid passes through.' Wash the precipitate ' left upon the filter two or three times with pure water.
(c) The silver cyanide is put into the vessel intended for the bath, and stirred with the 2 1/5 gal. water. The potassium cyauide is then added, dissolves it, and also dissolves the 'silver cyanide, thus giving a solution, of a double potassium and silver cyanide. Those who employ small baths, often renovated, may substitute for the silver cyanide the chloride or the nitrate of this metal. In the latter case, the quantity of potassium cyanide should b increased. Such baths will be prepared as follows: -
(d) The silver nitrate is prepared in the manner indicated above, and 5 1/4 oz. of it, nearly equal to 3 1/2 oz. pure silver, are dissolved in 2 1/5 gal. water.
(e) The potassium cyanide No. 1, about 8f oz., is then added. Stir to facilitate the solution, filter the liquor, to separate the iron contained in the cyanide. This operation may in some cases be dispensed with, because the iron rapidly falls to the bottom of the bath, and the solution becomes limpid. The proportion of potassium cyanide employed is more than is required for dissolving the silver, as 1 1/2 part good cyanide is sufficient for 1 part silver; but unless there is an excess of potassium cyanide, the liquors do not conduct electricity well, and the deposit of silver is granulated and irregular. The silvering is effected with a battery, and with baths either warm or cold. The latter method is generally adopted for articles which require great solidity. The hot process is used for small articles, and is preferable for steel, iron, zinc, lead and tin which have been previously electro-coppered. The hot baths are generally kept in enamelled cast-iron kettles, and the articles are either suspended, or moved constantly about in them. The preliminary cleansing in acids, and passing through the mercurial solution, are necessary. A somewhat energetic current is needed, especially when the articles are moved about, in order to operate rapidly.
There is too much electricity when the articles connected with the negative pole of the battery become grey or black, and produce many bubbles of gas. A platinum, large wire or thin foil anode, is generally preferred to the soluble anode of silver employed in cold baths, but the solution is rapidly impoverished. In hot silvering baths, the separate battery is often replaced by a zinc wire wrapped around the articles. The points of contact of the two metals are black or grey, but the stain disappears by plunging the object into the liquor for a few moments, after it has been separated from the zinc, and carefully scratch-brushed. Instead of separate batteries, a simple apparatus may be made of a glass, porcelain, or stoneware vessel holding the bath, and in the centre of which is a porous jar filled with a solution of 10 per cent. potassium cyanide or common salt. The zinc cylinder, immersed in this porous jar, carries a larger circle of brass wire, the cross diameters of which are soldered to the zinc. This brass ring projects over the bath, and the articles, suspended to the ring by slinging wires, hang down into the bath.
At the beginning, the operation goes on rapidly, and the de-posi' is good; but, after a time, the solution of zinc traverses the porous cell and impairs the purity of the bath. An impoverished hot bath is reinvigorated by additions of equal parts potassium cyanide and silver salt. It is necessary to replace the water in proportion as it is evaporated. When the silver baths rapidly deposit metal without the aid of electricity, it is a proof that they are too rich in cyanide, or too poor in silver. A deposit effected under such conditions is rarely adhering, especially when upon articles previously coppered, because the excess of cyanide dissolves the deposited copper, and the silver which takes its place may be removed with the finger.