Bronzing, the process of covering articles of wood, clay, plaster, metals, ivory, etc, with compositions which give to them the appearance of bronze. These compositions vary in their ingredients, and the process also, with the articles to be coated. An application is first made of size or oil varnish, into which when nearly dry a metallic powder is rubbed, or this may be previously mixed with the varnish. This powder is most commonly a preparation called gold powder, prepared as follows: Gold leaf is ground together with honey upon a stone. When thoroughly mixed, and the particles of gold completely reduced, the preparation is stirred up in water, and washed until the honey is entirely removed. The gold which settles is then collected upon filtering paper and dried. Another variety of powder, called aurura mosaicum or mimvum, is prepared in the following manner: A pound of tin, melted in a crucible, is amalgamated with half its weight of pure mercury. When the amalgam is cold, it is reduced to powder, and ground with 1/2 lb. of sal ammoniac and 7 oz. of sulphur. On subliming this mixture in a matrass, the tin remains at the bottom of the vessel in a flaky golden powder, which is the aurum mosaicum. A shade of red is given to this when desired, by adding a small portion of red lead.
Copper powder is obtained for the same purpose by the precipitation of the metal from its solution in nitric or sulphuric acid, by means of pieces of metallic iron. The copper deposits itself upon these, from which it may be brushed oft' in powder, care being taken to exclude it from the action of the air as it is washed in water, or better in alcohol. It is used either alone or mixed with pulverized bone ash. The preparation called gold size is also used in bronzing. It is made by boiling 4 oz. of powdered gum anime and a pound of linseed oil, the gum being gradually added, and stirred into the oil while this is heated. The boiling is continued till the mixture becomes thicker than tar. This is then to be strained through a coarse cloth. When applied, vermilion is added to render it opaque, and a convenient consistency is given to it with oil of turpentine. After being applied, it is allowed to dry very nearly, and when it has become sufficiently hard the powder selected is rubbed over the work with a piece of soft leather wrapped round the finger; or the application is better made with a soft camel's hair pencil, with which, when quite dry, the loose powder is brushed away.
If gold size is not to be used, the powders may be mixed in gum water and laid on with a brush. - Vinegar is often applied to brass castings to give them the green bronze color, sometimes in combination with sal ammoniac and sometimes with common salt. Coins and medals are sometimes bronzed with a solution of verdigris and sal ammoniac in vinegar, which is afterward diluted with water and boiled, and applied while hot. It is said that the Chinese bronze copper vessels by taking 2 parts of verdigris, 2 of cinnabar, 5 of sal ammoniac, and 5 of alum in powder, making a paste with vinegar, and laying this on the brightened metal, which is then held over a fire, and afterward cooled, washed, and dried; the operation being repeated till the desired color appears. The cinnabar produces a thin coat of sulphuret of copper on the surface of the metal. New bronze articles may be made to assume an antique appearance by the repeated application of a solution of 4 parts of sal ammoniac, 1 of binoxalate of potash, and 50 of vinegar; the application should be made with a soft rag, and the article rubbed with it till dry.
The best bronzing liquid to apply to copper, brass, new bronze, or iron is a solution of chloride of platinum; almost any bronze tint may be produced with it, according to the number of applications or the strength of the solution. - Mr. Hiram Tucker of New York has invented a process for bronzing cast iron, which consists in covering the cleaned or polished surface of the iron with a vegetable oil, and subjecting the article to a degree of heat which will decompose the oil without charring it. By this means the surface of the iron becomes oxidized in such a manner as to cause it to have a close resemblance to real bronze. It is said to possess considerable durability, and as cast iron has the quality of filling the mould completely .and making a fine casting, the invention will probably prove of much value in the arts. Bronzing and browning gun barrels, and other articles of iron, is effected by first thoroughly rusting the surface by an application of chloride of antimony, mixed with olive oil, and rubbed upon the iron slightly heated. The operation is hastened by subsequent rubbing with dilute nitric acid. This, or dilute muriatic acid, is sometimes used instead of the chloride of antimony.
The barrel is then well cleaned, washed with water, dried, and finally polished with a steel burnisher, or rubbed with wax, or varnished with a very weak solution of shellac and alcohol.