Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/415

Rh  The alloy known as aluminium bronze is one endowed with great strength, malleability, and ductility. It is formed of 10 parts aluminium and 90 of copper. In the melting of ordinary bronze, reverberatory fur naces have long been used, as rapid fusion is desirable in order to prevent loss of tin, zinc, or lead by oxidation. Bellfoundors often use dome-topped furnaces, as their alloy does not require so intense a heat for fusion ; but there is some waste of material with these. The copper is melted first, and covered with small charcoal or coke ; and the tin is rapidly thrust down to the bottom of the melted mass. After stirring, the alloy is poured into the moulds, in which the cooling should be as rapid as possible. Sometimes pressure is applied during cooling, in order to make the cast free from pores. In the old method of bronze -casting, known as the cire perdue, wax is first used for the thickness of the statue (between core and mould, which are of baked clay), and is melted and run off before the metal is poured in, the core and mould being held apart by stays of iron wire. In the present day large works are never cast in one piece, but in several, which are afterwards united by heating and appli cation of fused metal. A model is made in plaster, and a piece mould of Caen sand, about 1 Statues and various ornamental objects may also be manufactured by the process of electro-deposition from a metallic solution; and some excellent results have been obtained in this way by Oudry, Christofle, Elkington, and others. While the method offers some advantages in regard to cheapness, lightness, &c., of the products, the bronzes thus produced are not so hard and durable as those got by casting, and are thus less suited for exposure.

Bronzing is the process by which a bronze-like surface is produced on objects made of metal, plaster, wood, or other material. It may be done variously. The green bronze colour is sometimes produced on metal with vinegar alone, or dilute nitric acid, or sal-ammoniac. To give an antique appearance to newly made articles of bronze or brass, it is recommended to dissolve three-quarters of an ounce of sal-ammoniac and a drachm and a half of bin- oxalate of potash (salt of sorrel) in a quart of vinegar, moisten a soft rag or brush with the solution, and rub over the clean bright metal till its surface becomes quite diy through the friction. This process should be repeated several times, and the object should be kept a little warm. With a solution of chloride of platinum (which is, however, an expensive liquid) almost any colour can be imparted to copper, brass, iron, or new bronze, according to the degree of dilution and number of applications. The following solution is suitable for bronzing coins and medals : Two parts of verdigris and one part of sal-ammoniac are dissolved in vinegar, the solution is boiled, skimmed., and diluted with water till it has only a weak metallic taste, and on further dilution lets fall no precipitate. This solu tion is made to boil briskly and poured on the objects. These are well washed with clean water and then lacquered. Objects of cast-iron may be made to assume a fine bronze appearance by being coated with a very thin layer of vege table oil, and then placed in a drying oven, the tempera ture being such that oxidation of the iron and decomposi tion of the oil take place simultaneously. Another mode of bronzing iron (lately recommended by Weiskopf) is with a solution of one part sylvate of silver in twenty parts of oil of lavender. The object is lightly coated with this, and warmed rapidly up to 302 C. For bronzing tin or white metal a solution may be used consisting of 1 oz. sal- ammoniac, -J- oz. alum, and ^ oz. arsenic, dissolved in 1 pint of strong vinegar. A good method of bronzing wood, porcelain, stoneware, composition picture and looking-glass frames, &c., is first to coat the article with a thin solution of water glass, using a soft brush, Bronze powder is then dusted on, and any excess not adherent is knocked off by a few gentle taps. The article is next heated to dry the silicate, and the bronze becomes firmly attached. Bronze powders are pre pared of many different shades. In Messrs Brandeis s process the alloy used (generally copper-zinc) is laminated into very fine leaves, which are then ground. The powder is washed out and dried, and by revolving in a box, which contains some mineral varnish, the particles receive a coating of the varnish. Bronze powders are also prepared from leaf gold ground with honey on a stone, mosaic gold ground with bone ashes, compounds of tungsten and soda, and in other ways. As regards cleansing of bronze statues.that have become coated with dirt in large towns, it has been found that a dilute solution of caustic alkalies removes the overlying dirt and allows the green patina to become visible. Where the metal was not originally oxidized, the alkali simply cleanses it and does not promote any formation of green rust. An occasional rubbing with oil (all excess being carefully removed) is also found to preserve a fine bronze surface. The shining brown colour of gun barrels or other arms, is sometimes imparted by first producing a very thin uniform film of oxide or rust on the iron, e.g., with .vapour of muriatic acid, and giving a gloss to the surface by rub bing wax over it, or coating it with a shellac varnish, But the most common material for browning is the butter or chloride of antimony, sometimes called bronzing-salt. It is mixed with olive oil and rubbed on the iron, which is slightly heated. A little aquafortis is then rubbed on to quicken the operation ; the barrel is then cleaned, washed with water, dried and polished, either with a steel burnisher, or by rubbing with white wax, or is varnished with a very weak solution of shellac and spirit of wine. (See Ure s Dictionary of Arts, &c.)

Greek and Roman Bronze.

The bronze (Greek, ^aX/cos ; Latin, ces) of classical anti quity consisted chiefly of copper, with an alloy of one or more of the following metals, zinc, tin, lead, and silver, the quantity and the character of the alloy changing as times changed, or as was required for different purposes. Among existing bronze remains the copper is found to vary from C7 to 95 per cent. At present the only valuable results which we possess are derived from the analysis of coins (Von Bibra, Die Bronzen und Kupferlegirungen der alien und altesten Volker, Erlangen, 1869), from which it appears that for their bronze coins the Greeks adhered to an alloy of copper and tin till 400 B.C., after which time they used also lead with increasing frequency. Silver is rare in their bronze coins. The Romans also used lead as an alloy in their bronze coins, but gradually reduced the 