(1) Gold alloys, particularly those containing copper, acquire, through repeated heatings during their manufacture, an unseemly brown or brownish-black colour, caused by the oxide of copper, to remove which they are boiled or pickled in very dilute sulphuric or hydrochloric acid, according to the colour they are to have. If we have an alloy containing only gold and copper, either sulphuric or hydrochloric acid is employed, for gold is not attacked by either of them, while the oxide of copper dissolves so easily that after the pickling the articles have the colour of pure gold, for the surface is covered with a thin film of gold. If the alloy consists solely of gold and silver, the liquid employed is nitric acid, and the articles are left in it a very short time; the acid dissolves a small quantity of silver, and the articles acquire the colour of gold. If the alloy contains both copper and silver, besides the gold, the method of pickling can be varied to suit the colour it is desired to give to it. If, for instance, it is put in sulphuric acid, the copper alone is dissolved, and the colour obtained is that of an alloy of gold and silver, for the surface consists of the two.
If nitric acid were used, both copper and silver would be dissolved, and in this case the colour obtained would be that of pure gold. The articles are gently heated and allowed to cool again before boiling. The object of the heating is to destroy any grease or dust that adheres to them. If they are soldered with soft solder, they cannot of course be heated, and must be cleansed from grease and dust by first putting them in a strong lye, then washing with water and putting them in the acid. The acids are used dilute, usually in the proportion of 1 part concentrated acid to 40 of water. The articles are laid aide by side in a porcelain or earthenware dish, and the dilute acid poured over them. From time to time one is taken out to see if it is yellow enough. When the proper colour has been reached, they are washed in clean water and dried. While this pickling is merely to bring out the colour of the gold, the colouring of gold has for its object the imparting to inferior goods of the appearance of fine gold. Different mixtures can be employed for colouring gold, two of which are given below as affording very good results. Mix together 2 parts saltpetre, 1 of table salt, and 6 of alum, with 6} of water, and warm the mixture in a porcelain vessel.
As soon as it begins to rise, add 1 part hydrochloric acid, and bring the contents of the vessel to a boil, stirring in the meantime with a glass rod. The articles to be coloured, suspended on hooks made of strong platinum wire, or of glass, are first dipped in sulphuric acid and then put in the slowly cooking solution last described, and moved to and fro in it. In about 3 minutes they are taken out and dipped into a large vessel of water so as to see what colour they are. If the desired shade is not yet attained, they are dipped in again as often as necessary until they have it. In the subsequent dippings they are only left in the liquid for 1 minute. Articles coloured in this way have a light yellow colour, but matted appearance. They are repeatedly washed in water to remove the last trace of the liquid, and then dried in soft sawdust that has been warmed.
Instead of drying in sawdust, they can be dipped in hot water the last time and left in there a few seconds, and when taken out the water that hangs on them will evaporate almost instantly. The second method of colouring gold alloys is by means of a mixture of 115 parts white table-salt and 230 of nitric acid, with enough water added to dissolve the salt. This is boiled down to a dry mass of salt. The salt is put in a porcelain dish, and 172 parts fuming hydrochloric acid poured over it and heated to boiling. As soon as the suffocating odour of chlorine is perceived, the articles to be coloured are dipped in, and the first time they are left 8 minutes in the liquid. In other respects the treatment is the same as above described. Articles polished previously do not require polishing again. Care must be taken not to inhale the dangerous gas; the operation must be conducted under a draught or out of doors. (Schlosser.)
(2) Place 4 oz. saltpetre, 2 oz. common salt, and 2 oz. alum in a plumbago crucible. Add sufficient water to cover the mixed salts. Now place the crucible on the fire and allow the mixture to boil. When this takes place, place the article to be coloured in the mixture, taking care that it is suspended by a hair. It may be left in the crucible for about 15 minutes, when it should be withdrawn, washed in warm water, well brushed with beer and a fine scratch-brush, and re-dipped if the colour is not intense enough.
(3) For small gold articles, such as a keeper or plain ring, etc., a very good plan is to place them on a lump of charcoal and make them red-hot under the blow-pipe flame, and then to throw them into a pickle composed of about 35 drops strong sulphuric acid to 1 oz. water, allowing the articles to remain therein until the colour is sufficiently enhanced. Washing the article in warm water in which a little potash has been dissolved, using a brush, and finally rinsing and drying in boxwood sawdust, completes the operation.
(4) Another colouring mixture, which has been greatly recommended, consists of a mixture of 20 gr. sulphate of copper, 40 gr. French verdigris, 40 gr. sal-ammoniac, and 40 gr. saltpetre, dissolved in 1 oz. glacial acetic acid. The articles, suspended by a horse-hair as before, are to be immersed in this mixture, withdrawn, and heated on a piece of copper until black. They are then to be placed in a pickle of equal parts oil of vitriol and water, which removes the black coating and brings up the colour. Washing in weak potash-water, rinsing and drying as before, terminates the treatment.
(5) An Indian Native Method
Clean the article thoroughly by washing in hot soap and water, taking care to get rid of all greasiness and of all the soap. The natives use tamarind-water, and also the soap-nut. Prepare a paste of the consistence of soft butter by mixing the following ingredients with quantum suf. of pure water - viz. 1 oz. saltpetre, 1 oz. crude sal-ammoniac, 2 oz. sulphate of copper; grind each separately to a fine powder, then mix with water and form the paste. Apply this paste pretty thickly and evenly over the article to be coloured, and place while wet on ignited charcoal; warm till it dries and smokes; then immediately dip into cold water, and clean by using tamarind-water and a soft brush. If the colour is not deep enough, repeat the process. Plunge the article, whether of pure gold, alloyed gold, or gilt, into the following solution, and afterwards clean thoroughly with a soft brush and soap and water. To increase the depth of colour, plunge 4 or 5 times, cleaning after each plunge: - Powder finely 2 oz. alum, 2 oz. saltpetre, 1/2 oz. sal-enixum (the refuse from aquafortis), put all into an earthenware pipkin with 5 oz. water, warm over the fire, add 1 oz. gilders' wax, and gently simmer for a short time.
To be used when nearly cold.
(6) This amplifies (2). Jewellery to be coloured should be at least 15 carats fine, and the solder should be only a shade under that. There must be no pewter or silver solder on it. It should just be annealed, and pickled in water to which sufficient sulphuric acid has been added to render it sharply acid to the taste. The best vessel to use for colouring is an ordinary clay crucible; the colouring mixture is composed of 2 oz. best saltpetre, 1 oz. alum, and 1 oz. common salt. These are placed in the crucible with sufficient water to moisten them, and when they are melted place the gold articles in the mixture. The jewellery must be strung on a piece of wire. It is better to keep a piece of platinum for this purpose, which should be annealed each time before use; failing that, a piece of gold wire (15 carat). You may use silver wire, but nothing baser. The colouring composition dissolves the silver, so you will require a fresh piece occasionally. You must move them about at intervals in the crucible, and as the composition gets thick add a very little hot water from time to time. It must not be made too thin, but just sufficiently liquid to boil. The goods require to be in almost continual motion, or they will stick to the bottom of the crucible.
Should this untoward (and in the hands of a novice far from unlikely) event happen, don't attempt to pull them out by force, but boil them out with hot water. After they have been in the crucible for a few minutes, take them out and examine them; but whenever they are taken out they must be plunged at once into boiling water, or the composition will dry on them, and you will have some difficulty in removing it. They should now be scratch-brushed and returned to the crucible. From 10 to 20 minutes will be sufficient. When they are coloured, take them out, scratch-brush, wash in clean hot water, and dry in boxwood sawdust. This process acts by dissolving away the alloy, and leaving only the pure gold on the surface. If the goods are anything less than 15-carat gold, they must be electro-gilt.