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Gold Baths

Both warm and cold baths are used, the former being preferable, as they yield denser depositions, require less strength of current, and need not be so rich in gold as cold baths. Many platers prepare gold baths by dissolving 3 1/2 ounces of chemically pure cyanide of potassium in a quart of distilled water, and connecting two cells of Bunsen battery with two gold anodes, and working it until the solution contains 32 grains of gold to the quart of water, which can be told by weighing the anodes from time to time. It does not give as bright a color, however, as dissolving an equivalent quantity of chloride of gold in the cyanide, owing to the presence of silver and copper in the gold anodes, and consequently in the bath. In purchasing your chloride of gold, where possible get the brown neutral variety, as it is preferable to others, as it contains less acid.

A good warm bath is prepared as follows: Neutral chloride of gold, 7 pennyweights; 99 per cent potassium cyanide, 33 grains; water, 1 quart. Dissolve the potassium cyanide in one-half of the water and the chloride in the other half, mix both solutions, and boil for half an hour, replacing the water lost by evaporation. The solution will last indefinitely if the anode used is large enough to make up for the gold used. When the anode is covered with a slight coating in the bath, it shows a want of cyanide of potassium, which is best added by dissolving an ounce of chemically pure cyanide in a pint of water, and adding a little at a time, until the bath works all right again. Too much free cyanide in the bath gives an ugly, pale color and irregular deposit.

A cold solution will give a pale color; increasing the temperature to 140 deg. gives a reddish color; 120 deg. to 130 deg. gives a fine yellow. The reason of this is that heat hastens the chemical reactions in the bath, and the gold deposits much more rapidly, so the operator should look out not to deposit more gold than he wants, and thus strip his solution. The anode surface should also be less with the hot solution.

Changes of color can also be made by increasing the current and anode surface in a cold solution, by putting in some sort of resistance in the circuit for the paler colors, and withdrawing it for the redder ones. As anodes, it is best to use sheets of fine gold, which gradually dissolve, and thus convey fresh metal to the bath. The current must not be so strong that a formation of bubbles is perceptible; it is best to use a current of such strength only that deposition takes place slowly, a coating of the greatest density being thus obtained. Avoid using cheap and inferior chemicals, as the difference in price is more than offset by the time and damage that often results from inferior grades. To obtain good results, always use as pure water as possible, filtered rain water being the most desirable. The best temperature for cold baths is 66 deg. F. Care should also be taken to see that the baths are covered with cloths to exclude dust, and where it does penetrate, the baths should be skimmed off.

Only copper, brass and bronze can be directly gilded; other metals must first be coppered or brassed; this applies to good work. In gilding parts of watches, gold is seldom directly applied upon the brass; there is generally a preliminary operation called graining, by which a slightly dead appearance is given to the articles. They are thoroughly finished, all grease removed as described above, threaded upon a brass wire, cleansed in the compound acids for a bright luster, and dried in sawdust. The pieces are fastened upon the flat side of a piece of cork by means of brass pins, and the parts are thoroughly rubbed over with a clean brush dipped in a paste composed of fine pumice stone powder and water. The brush is moved in circles, in order to rub evenly. Thoroughly rinse in clean water in order to remove every particle of pumice stone, both from the article and the cork. Place the whole in a weak mercurial solution, composed of nitrate of mercury, 4 grains; water, 9 quarts; sulphuric acid, I dram; which will slightly whiten the brass. Pass quickly through this solution and then rinse. After the parts are grained in the manner described, they may be gilded the same as ordinary work.