First prepare ingots or pieces of copper or brass, in convenient lengths and sizes. Then cleanse them from impurity, and make their surfaces level. Prepare plates of pure gold, or gold mixed with a portion of alloy, of the same size as the ingots of metal, and of suitable thickness. Having placed a piece of gold upon an ingot intended to be plated, hammer and compress them both, together, so that they may have their surfaces as nearly equal to each other as possible; then bind together with wire, in order to keep them in the same position during the process required to attach them. Afterwards mix silver filings with borax, to assist the fusion of the silver. Lay this mixture upon the edge of the plate, and next to the ingot of metal. Having prepared the two bodies, place them on a fire, where they must remain until the silver and borax placed along the edges of the metals melt, and until the adhesion of the gold with the metal is perfect. Remove the ingot carefully from the stove.
It is not always necessary in electrogilding to use a battery, for a salt or acid liquor is enough to produce electricity; thus it is sufficient to plunge the articles, attached by zinc wires, into gold baths prepared for the use of batteries, to have the operation taking place in the same manner as with a separate battery. Electrogilding in the cold is employed for large pieces, such as clocks; whilst electrogilding by heat is more adapted to the gilding of small articles, such as i forks and spoons. The deposits prodluced by hot gilding are more smooth and clean, the colour is deeper, and the articles when removed from the bath may not require colouring; and with the same quantity of gold, gilding by heat is much more durable than that obtained from cold baths. Steel, tin, or lead can be gilt in hot baths, but not in cold.
(a) Distilled water 2 1/5 gal.; potassium cyanide, ordinary 70 per cent., 10 1/2 oz.; pure geld, 3 1/2 oz.; aqua ammonia, 17 1/2 oz. Heat the gold in a glass flask with 9 oz. pure hydrochloric acid, and 4 1/2 oz. pure nitric acid. When the gold is dissolved, continue the heat in order to expel the acid fumes, and until the colour of the liquid is dark red, nearly black. Remove from the fire, and dissolve the crystalline mass formed in cooling in 3-4 pints water, and pour into a large porcelain dish.
Add the ammonia, which produces an abundant yellow precipitate of gold ammonium; pour upon filtering paper, and the filtered liquid, which still contains traces of gold, is kept with the saved waste. Wash the precipitate remaining upon the filter several times with cold water, until it no longer smells of ammonia. It must not be dried, as it is a fulminating mixture, and consequently very dangerous. Next dissolve in the vessel used as a bath the potassium cyanide in the distilled water. Filter, and add the wet gold ammonium, which rapidly dissolves when stirred, and forms a clear gold bath. But before using it cold, the ammonia should be expelled by boiling for about one hour. For a newly-prepared cold electrogilding bath, the ordinary potassium cyanide is preferable, on account of the potash it contains, which renders the liquor a better conductor of electricity. But for the preservation of the strength, the pure cyanide is better, as it possesses the advantage of a constant composition, and does not load the solution with foreign salts.
The gold solution for maintaining the metallic strength of the bath is prepared as follows: - Transform the gold into precipitate of gold ammonium, as above described, place it in water, 2 pints water to 4 oz. gold, then add potassium cyanide until the liquor is colourless. If there is not sufficient water with the gold ammonium, the liquor will be dark red, and will not be decolourised by cyanide.
(6) Distilled water, 2 1/5 gal.; potassium cyanide, pure, 7 oz.; or ordinary cyanide, according to strength, 10-14 oz.; pure gold, 3 1/2 oz. Make a neutral gold chloride, as in the preceding formula, and, when cold and crystallised, dissolve it in 3 1/2 pints water. Filter if needed. Dissolve the cyanide in 14 pints water; filter, and mix the two solutions, which become colourless. When it is possible to boil this bath for 1/2 hour before using it, it becomes a better conductor of electricity, and the gilding is more uniform. Its strength is maintained by additions of neutral gold chloride and pure potassium cyanide, 1-1 1/2 pure cyanide to 1 of gold. Both the above baths may be diluted with once or twice their volume of water; the gilding will remain tine, but the proportion of gold deposited will be less in a given length of time.
(c) Yellow prussiate of potash, 7 oz.; pure potash carbonate, 5 oz.; sal ammoniac, 1 oz.; pure gold transformed into chloride, 1/2 oz.; water, 2 1/5 gal. Boil all the salts together, less the gold chloride, separate by filtration the precipitate of iron carbonate, then add the gold chloride dissolved in a little water, and allow the bath to cool off. Any kind of gold salt, and the oxide, or even finely-powdered metal, may take the place of the gold chloride; but the latter is preferred on account of the facility of its preparation, and of its solubility. Any kind of gold salt will be transformed into cyanide by the potassium cyanide. The small proportion of the potassium chloride resulting from the transformation of the gold chloride into cyanide does not prevent the good working of the baths. The addition of a little prussic acid produces a brighter, but thinner gilding. The indicated cyanides may be replaced. by the cyanides of sodium, calcium, and ammonium.
(d) Cold gilding baths are generally kept in porcelain or stoneware vessels; but for large volumes of liquor, use wooden troughs lined with gutta-percha plates. The sides of the troughs support anodes of laminated gold, which dip entirely into the liquor, and are held by small platinum wires; they are connected with the positive pole of the battery. Suspend the articles by means of metallic slinging wires to a movable frame of clean brass rods connected with the negative pole. The deposit of gold should be pure yellow, but it has sometimes a dull earthy grey colour. In that case scratch-brush it with the greatest care, and then pass it through the ormolu colouring. The gold anode conducts the electricity, and also maintains the metallic strength of the bath up to a certain point; but it is necessary to add now and then either oxides or chloride of gold, and a certain proportion of potassium cyanide, to make up for that transformed into potash carbonate and ammonia cyanide. The proportion of cyanide is about double that of the gold chloride added; this is ascertained by the colour of the bath and the shade of the deposit; if the proportion of the gold chloride is too great, add more cyanide. ' If gold predominates, the deposit is quite black or dark red; when the cyanide is in excess, the gilding is very slow and grey, and it will sometimes happen that pieces already gilt will lose their gold.