A Dictionary of Arts (supplement): Bronzing.

(A Dictionary of Arts, Manufactures, and Mines; containing A Clear Exposition of Their Principles and Practice)
Recent improvements in
Arts, Manufactures, and Mines:
Being A supplement to his Dictionary
by Andrew Ure, M. D.,
F.R.S. N.G.S. M.A.S. LOND.; M. ACAD. N.S. PHILAD.; S. PH. SOC.N. GERM. HANOV.; MUHL. ETC., ETC.

Illustrated with one hundred and ninety engravings.

New York: D. Appleton & Company, 200 Broadway.  Philadelphia: George S. Appleton, 148 Chestnut St.
MDCCCXLVII
1847
BRONZING (of Objects in Imitation of Metallic Bronze). Plaster of Paris, paper, wood, and pasteboard, may be made to resemble pretty closely the appearance of articles of real bronze, modern or antique. The simplest way of giving a brilliant aspect of this kind is with a varnish made of the waste gold leaf of the beater, ground up on a porphyry slab with honey or gum-water. A coat of drying linseed-oil should be first applied, and then the metallic powder is put on with a linen dossil. Mosaic gold ground up with six parts of bone-ashes has been used in the same way. When it is to be put on paper, it should be ground up alone with white of eggs or spirit varnish, applied with a brush and burnished when dry. When a plate of iron is plunged into a hot solution of sulphate of copper, it throws down fine scales of copper, which being repeatedly washed with water, and ground along with six times its weight  of bone-ashes, forms a tolerable bronzing.

Powdered and sifted tin may be mixed with a clear solution of isinglass, applied with a brush, and burnished or not, according as a bright or dead surface is desired. Gypsum casts are commonly bronzed by rubbing brilliant black-lead, graphite, upon them with a cloth or brush. Real bronze long exposed the air gets covered with a thin film of carbonate of copper, called by virtuosi antique arrugo (patine antique, Fr.) This may be imitated in a certain degree by several applications skilfully made. The new bronze being turned or filed into a bright surface, and rubbed over with dilute aquafortis by a linen rag or brush, will become at first grayish, and afterward take a greenish blue tint; or we may pass repeatedly over the surface a liquor composed of 1 part of sal ammoniac, three parts of carbonate of potash, and 6 of sea salt, dissolved in 12 parts of boiling water, to which 8 parts of nitrate of copper are to be added; the tint thereby produced is at first unequal and crude, but it becomes more uniform and softer by time. A  fine green-blue bronze may be obtained with very strong water of ammonia alone, rubbing it at intervals several times upon the metal.

The base of most of the secret compositions for giving the antique appearance is vinegar with sal ammoniac. Skilful workmen use a solution of 2 ounces of that salt in an English quart of French vinegar. Another compound which gives good results is made with an ounce of sal ammoniac, and a quarter of an ounce of salt of sorrel (binoxalate of potash), dissolved in vinegar. A good result will also be obtained by adding half an ounce of sal ammoniac, instead of the spirits of hartshorn. The piece of metal being well cleaned, is to be rubbed with one of these solutions, and then dried by friction with a fresh brush. If the hue to be found too pale at the end of two or three days, the operation may be repeated. It is found to be more advantageous to operate in the sunshine than in the shade.