Dissolve in 20 parts by weight of ordinary strong vinegar 3 parts of carbonate or hydrochlorate of ammonia, and 1 each of common salt, cream of tartar, and copper acetate, and add some water. When an intimate mixture has been obtained, smear the object with it, and let it dry at the ordinary temperature for nearly 48 hours. After that time the object is entirely covered with verdigris of various- tinges. Then brush the whole, and especially the reliefs, with the waxed brush. If necessary, the raised parts are set off with chrome yellow, or other suitable colours. Light touches with ammonia give a blue shade to the green portions, and ammonia carbonate deepens the colour of the parts on which it is laid.
A steel bronze is obtained by wetting the articles with a dilute solution of platinum chloride, and slightly heating. This bronze will sometimes scale off by friction. ' It may also be obtained by dipping the cleansed copper into a weak warm solution of antimony chloride in hydrochloric acid. But sometimes the colour is violet instead of black.
(a) Cleaned up brasswork, if left in damp sand, acquires a fine brown colour, which, when polished with a dry brush, remains permanent and requires little cleaning. A green and light coating of Verdigris on the surface of the brass may be made by means of dilute acids, allowed to dry spontaneously. The processes are, however, slow, and quicker means have to be adopted. Before bronzing, the brass is annealed, pickled in old or dilute nitric acid till the scales can be removed from the' surface, scoured with sand and water, and dried. Bronzing is then performed according to the colour desired. A steel-grey colour is deposited on brass from a dilute boiling solution of arsenic chloride; and a blue by careful treatment with strong soda hydrosulphate. The Japanese are said to bronze their brass by boiling it in a solution of copper sulphate, alum, and verdigris. Success in bronzing greatly depends on the temperature of the brass or of the solution, the proportions of the metals used in forming the alloy, and the quality of the materials; the moment at which to withdraw the goods and the drying require attention.
(6) The articles, which must be free from grease, and polished, are first immersed, for 1/2 minute, in a cold solution of 10 grm. potassium permanganate, 50 grm. iron sulphate (ferrous), and 5 grm. hydrochloric acid in 1 litre of water. They are then washed off and dried in fine, soft sawdust.
(c) If the colour has become too dark, or if a more reddish-brown colour is required, the objects are immersed, immediately after they have been taken out of the liquid, for about 1 minute in a warm (140° F.) solution of 10 grm. chromic acid, 10 grm. chloric acid, 10 grm. potassium permanganate, and 50 grm. iron sulphate in 1 litre of water, and treated as above. By using the second liquid alone, a brighter, dark-yellow, or red-brown colour is produced. The objects may afterwards be heated in a drying oven, whereby the colour fa considerably improved.
First make a sieve by glueing a piece of fine muslin or butter-cloth all round a frame, made of deal, 4 in. square, l 1/2 in. deep, to put the bronze powder in, next make some glue-water rather thinner than cream, and strain through a fine cloth or muslin; put the glue-water in a basin, and place it where the lid should be on the teakettle; place the kettle on. the fire to heat the glue (hotter the better). Have your cardboard quite flat; put on the glue-water very quickly and evenly with a 2 in. flat camel-hair brush; go over the card with the sieve in your left-hand, tapping the sieve gently with the fingers of your right-hand, it will be ready for dusting in an hour's time; use a piece of card about 14 in. by 12 in. for a trial.
Bronzing copper by the well-known method of heating it over a fire is a tedious and not altogether satisfactory process. It involves the exercise of some skill, and a considerable amount of labour must be expended in the preliminary processes of cleansing and polishing; and very often the whole operation has to be performed over again, owing to some accidental blemish imparted to the surface of the article in some subsequent process - e.g. brazing or soldering. The polishing powders principally employed are crocus and plumbago, the latter giving a deeper and more permanent colour to the finished article than the former, while shades between can be obtained by mixtures of the two powders. There are several secret processes employed by the principal workers in the art, the substances used in which are known, but the exact methods are undescribed. Potassium sulphide and ammonia hydro-sulphate are both capable of imparting to the surface of clean copper an appearance of antique bronze. The solution is brushed on carefully and allowed to dry, the metal being previously heated to about 70° F. A solution of verdigris and sal-ammoniac in vinegar, diluted with water, boiled and filtered, is used as a sort of pickling bath for brass and copper articles it is desired to bronze.
The bath must be kept at the boiling point, and care must be taken that the articles are removed as soon as the desired effect is produced. A bronze, said to be used by the Chinese, is made, like the last, of sal-ammoniac and verdigris, with the addition of alum and vermilion (i.e., the pure article prepared from mercury and sublimed sulphur). These ingredients, when reduced to a fine powder and made into a paste with vinegar, are spread over the surface of the article to be bronzed, which is then placed in an oven, where it heats slowly but uniformly. When thoroughly warm, the paste is carefully washed off in hot water, and the article is rapidly dried, with the assistance of hot box sawdust, if of special value. If the bronzing is not of a sufficiently deep tone, the process is repeated immediately after the washing. The common bronzing solution for metallic statuettes is made by dissolving about 1 part potassium binoxalate and 3 of sal-ammoniac in strong vinegar, or preferably in a vinegar made by adding pure acetic acid to distilled water. The articles to be bronzed are placed in a warm but moist chamber, and are repeatedly painted over with the solution, a soft brush or mop being used for the purpose.