The first and most important operation in all branches of the deposition of one metal upon another, is to effect a thorough and chemical cleansing of the surface of the metal upon which the coating is to be deposited.
This is done in six operations. (1) Cleansing by fire or by alkalies. (2) Dipping. (3) Dipping in old aquafortis.
(4) Dipping in new aquafortis and soot.
(5) Dipping in compound acids for a bright or dead lustre. (6) Dipping in. nitrate of binoxide of mercury.
This is to remove any foreign substances, especially those of a fatty nature, which are destroyed by heating the pieces in every direction over a gentle fire of charcoal, breeze, or spent tan. A muffle furnace, heated up to a dull red heat, is preferred; but small articles may be cleansed in a hot revolving cylinder. This operation is not adapted to very delicate articles, or for table-forks and spoons, which must keep their toughness, or to those pieces in which the different parts are united by soft solders. Boil such articles in a solution of potash or soda, which renders the fatty substances soluble in water. This is done in a cast-iron kettle, provided with a coyer, containing a boiling concentrated solution of carbonate of potash or soda, or of caustic alkali. The caustic potash or soda must be dissolved in ten times its weight of water. This solution lasts a long time; when it has lost part of its power, it may be revived by a few fragments of caustic alkali. At the boiling point it will cleanse copper in a few seconds.
If the articles to be scoured are joined with tin solder, they must not be allowed to remain too long in the caustic liquor, which would dissolve the solder and blacken the copper.
The pieces are then dipped in a mixture of 5-20 parts in weight of sulphuric acid at 66° B. for 100 parts of water. Most of the pieces to be cleansed may be dipped hot in this mixture; but certain alloys, in which tin, zinc, or antimony predominates, such as cast bronze, must not be so treated, as the sudden cooling will occasion cracks and flaws. Copper articles may remain any length of time in the dipping bath; they should not be removed before the black coat of copper binoxide, caused by the heating, is entirely dissolved. The remaining coat of red copper protoxide is unacted upon by the sulphuric acid. Articles having parts made of iron or zinc must not be submitted to the action of dilute sulphuric acid, or they will be entirely dissolved; therefore avoid the use of implements or wires of iron, zinc, or steel. A dipping bath which contains copper in solution from previous operations will not suit for articles which may contain iron, tin, tin solder, antimony, bismuth, or lead. In such a case, use a newly-made dipping bath and a small proportion of acid. Articles which have been cleansed by alkalies must be washed before being put into the dipping bath, or pickle.
Thorough and rapid rinsing in fresh water, before and after each of the following operations, must be strictly attended to. The various manipulations which complete the cleansing succeed each other without interruption; and the articles must be stirred as well as possible in the acid baths, and in the rinsing water. After dipping and rinsing, the various pieces are fixed to a brass wire, or hooked upon brass, or copper hooks. Small articles of jewellery are suspended to a stout copper wire. These hooks are better if made of pure copper than of brass, and it is still better to use glass hooks, which are cheap and are not corroded by the acids. Such hooks or supports can be made by bending glass rods, by the heat of a charcoal fire, or of a gas burner, to the desired shape. Those objects which cannot be suspended or attached to hooks, are put into perforated ladles of porcelain or stoneware. It is less economical, but sometimes absolutely necessary, to use baskets of brass or copper wire cloth. Those who frequently have to cleanse very small articles will find it advantageous to employ a basket of platinum wire cloth, which, although expensive in the first cost, will be found cheaper in the end, as it is almost indestructible.
If you have any aquafortis (nitric acid) already weakened by preceding dippings, plunge into it the articles which have passed through the sulphuric acid pickle bath, and have been rinsed. They may remain there until the red coat of copper protoxide has entirely disappeared, leaving, after rinsing, a uniform metallic lustre. The dipping in old aquafortis, though not absolutely necessary, is recommended for two reasons; it economises the cost of fresh acids; and, as its action is slow, it prevents the too rapid corrosion of the cleansed copper during the time of the solution of the protoxide.
-After rinsing in fresh water, the articles are well shaken and drained, and then plunged into a bath composed of 100 parts nitric acid at 36° B., 1 common salt, 1 calcined soot. This mixture attacks the metal with the greatest energy, and the pieces should therefore not remain in it more than a few seconds. The volume of acid should be about 30 times that of the articles to be cleaned, in order to prevent too great an elevation of temperature due to the chemical reaction, which would result in the rapid weakening of the acid. After this bath, and rapid rinsing, in order to prevent the production of nitrous vapours, the pieces present a fine red lustre, gold-yellow, or greenish-yellow, according to the alloy employed, and such as to make one believe that they are entirely cleansed of foreign matter; yet if the pieces in this state are plunged into a gilding or silvering bath, they become entirely black, and without any metallic lustre. If the pieces are put aside without rinsing, there rises on their surface a green froth and nitrous vapour, which indicate the decomposition of the acid with which they are contaminated.