This operation is still employed for whitening small wares for which durability is of secondary importance, and which simply require the whiteness of silver; such as hooks and eyes, or buttons. This whitening is made as follows: - (a) Dissolve a certain quantity of pure granulated silver in double its weight of pure nitric acid. The solution is largely diluted with water, and the metal is precipitated in heavy white clods by common salt or hydrochloric acid. All the silver nitrate has been decomposed when a further addition of hvdrochloric acid or common salt to the clear supernatant liquid does not produce any turbidity. The clear liquors are then thrown away, and the silver chloride obtained is washed several times, to deprive it of all free acid. If this precipitate is to be kept some time before use, it should be removed from the sunlight, which blackens it rapidly. The silver chloride, with a little water, is thoroughly mixed with at least 80 times its weight, of finely powdered potash bitartrate, and kept in a stoneware pot.
(6) Pure silver for making the chloride, 1 part; powdered cream tartar, salt, 83 parts each; a few spoonfuls of the paste thrown in, and dissolved in boiling water contained in a pure copper kettle. The articles are dipped into this bath by a hook, or in a basket of wire gauze. Or have another basin of copper, shallow and perforated with holes, which rests against the upper sides of the kettle. By means of handles, this basin can be removed at once with its contents. Stir the articles with a wooden spatula; and at each operation add a quantity of paste, proportioned to the surfaces to be whitened. These baths do not work well when freshly made, but improve as they are more used. They acquire a dark green tint, due to the copper which is dissolved, and which takes the place of the deposited silver. Varnishing, colouring, and cleansing may be done in aquafortis; but these cleansing methods are inferior to those employed for gilding; in general, use the worn-out acids of gilders. Brighten the articles by friction with saw-dust. The smallest particle of iron, zinc, or tin introduced into the whitening bath imparts a red colour to the brass or copper articles in the liquor.
The iron is separated by a magnet; the zinc is dissolved in pickles of hydrochloric or sulphuric acid, which, when cold, do not sensibly corrode the copper articles; tin or lead must be picked out by hand. If the operation has not succeeded, the articles are plunged for a few seconds into a boiling solution of water, 2 1/5 gal.; silver nitrate, 3 1/2 oz.; ordinary potassium cyanide, 21 oz. This bath retains its strength for a long time, and increases the brightness and whiteness of the deposit. The process of silvering by dipping has nearly superseded this method.