The silver found in the trade, even under the name of virgin silver, retains traces of copper; Silver is purified by several methods: - (a) The impure metal is dissolved by nitric acid, and the solution being largely diluted with water, add to it an excess of a filtered solution of common salt. An abundant white precipitate of silver chloride is produced, which rapidly settles to the bottom of the vessel. AH the silver salt is decomposed when the clear liquid is not rendered turbid by a further addition of salt. The silver chloride is collected, and washed several times, until the liquors are no longer coloured brown by yellow prussiate of potash. This is the proof that all the copper has been washed out. The washed silver chloride is mixed with two or three times its own weight of soda carbonate, dried, and melted in a crucible. After cooling, the metal is found in a conical button at the bottom of the crucible. To granulate it, the molten silver is poured from a height of about 3 ft., into a large volume of water.
(6) The alloy of copper and silver is dissolved in nitric acid, and the solution is evaporated until the salts fuse. After cooling, the fused mass is gradually thrown into a red-hot crucible, when the nitric acid escapes leaving behind the silver in the metallic state, and the cop-per as oxide. The separation of the two takes place naturally, and is aided by the addition of dry borax, which dissolves the copper oxide. Silver is easily dissolved in pure nitric acid, but not so rapidly in one contaminated by chlorine or hydrochloric acid, which produces a coat of silver chloride around the metal, and therefore forms an obstacle to its solution. Sulphuric acid also combines with silver, and the resulting salt is but slightly soluble. Pure silver is employed for the preparation of the nitrate and other silver salts, and for soluble anode in silver baths.
(a) Add silver to nitric acid, previously diluted with twice its weight of water, in a flask, and apply a gentle heat until the metal is dissolved.
The clear liquor is then separated from any black powder which may be present, evaporated and crystallised. The crystals are dried by exposure to the air, taking care that they do not come in contact with any organic substance.
(6) Dissolve the silver in pure nitric acid, and evaporate. The nitrate is yielded in square anhydrous tables. Dissolve this in distilled water, filter, and evaporate again, and the nitrate is obtained pure.
(6) Silver nitrate, 1 part; potassium cyanide, 3. Both are applied by wetting with a little water and rubbing on the article to be plated, which must be quite clean. Plating done by the above will be very thin, but it will be silver.
(c) Get a glazed earthen vessel, put in 1 oz. nitric acid; place it on a slow fire; it will boil instantly. Throw in some pieces of real silver; this will be dissolved at once. As soon as dissolved, throw in a good handful of salt to kill the acid, then make into a paste with common whiting. The article required to be silvered to be cleaned from grease and dirt, and the paste to be applied with a little water and wash leather. This will keep for years.
(d) Silver nitrate, 55 parts; solution of ammonia, 60; soda hyposulphite, 100; precipitated chalk, 100; distilled water, 1000.
(e) Dissolve 2 oz. silver with 3 gr. corrosive sublimate; add tartaric acid, 4 lb.; salt, 8 qt.
(f) 1 oz. silver nitrate is dissolved in 1 qt. rain or distilled water, and a few crystals of soda hyposulphite are added, which form a brown precipitate soluble in a slight excess of hyposulphite. Small articles of steel, brass, or German silver may be silvered by dipping a sponge in the solution and rubbing it over the surface of the article to be coated. A more concentrated solution may be used for coating parts of articles which have stripped or blistered, by applying it with a camel-hair pencil to the part, and touching the spot at the same time with a thin clean strip of zinc.
The bath is a cylindrical stoneware, glass, or porcelain vessel. After cleansing and amalgamation, the articles are attached by clean copper wires to the circumference of a brass ring, supported upon the top of the apparatus by 3 or 4 soldered .cross wires. The ring is connected with the negative pole of the battery, and the positive pole with a platinum anode, or a cylinder formed of a sheet of silver rolled round, which dips into the middle of the apparatus. The articles must be now and then turned upside down, and sideways, so that each face of the object will be, in turn, directly opposite the silver anode, and thus also the points of contact with the suspending wires receive their quota of metallic deposit. Points, edges, corners and all raised parts, offer a more easy passage to the electric current, and therefore become more coated with metal. As the wear of tablespoons and forks is greater on their convex sides, those parts should face the silver anode longer than the concave portions.