Bronze is usually defined as an alloy of copper and tin. Neither ancient nor modern bronzes consist always of only these two metals. Zinc, lead, and even silver are often present in Greek and Roman bronze objects, although the general result of analysis of these shows a composition of from 80 to 90 parts copper to 10 or 12 of tin. Some of the fine French bronze statues of the time of the Renaissance are made of an alloy of nearly this composition—copper 92, tin 2, zinc 5, lead 1. Much of our modern statuary consists of bronze varying from 10 parts copper to 1 of tin—a proportion recommended by Sir H. Davy—to 8 parts copper plus 2 of tin. Some zinc or lead, or both, as well as other metals, are occasionally substituted for part of the tin.
Mr W. Graham has classified the alloys of copper and tin in this way: 12 to 20 parts copper with 1 of tin yield red-coloured alloys; 5 to 10 parts copper with one of tin yield alloys of strength; 2 to 4 parts copper with 1 of tin yield alloys of sound (bell-metal); to 1 part copper with 1 of tin yield alloys of reflection (speculum metal). The alloys of strength here referred to include the bronze used for statues, and also that, sometimes called gun-metal, employed for bearings and other parts of machinery, as well as the alloys of which cannon are, or perhaps we should rather say were, made. A good deal of the bronze for all three purposes is very similar in composition.
A bulky and valuable volume of reports on the strength and other properties of metals for cannon, by officers of the Ordnance Department of the United States Army, published in 1856 (H. C. Baird, Philadelphia), contains a section on the manufacture of bronze guns. The bronze used in casting the various guns tested was of the same composition—namely, copper 8 parts, tin 1 part. It had a mean density of 8.519, and an average tenacity of 27,793 lb. per square inch. It is clearly seen by the results of these experiments, that bronze made of the same mixture may vary considerably in its quality. The report states that 'the same kind of copper, and of tin, mixed in the same proportions, and melted in like manner, was used in all the castings; yet the table exhibits a remarkable diversity in the results.'
General Uchatius (1811-81) of Vienna devised a plan of hardening Austrian bronze guns by forcing hardened steel pistons into them. The metal is then called steel bronze.
Alloys of copper, tin, and zinc, employed for parts of engines and other machinery, are frequently called brass, as well as gun-metal, and are referred to under the head BRASS; and see ALUMINIUM.
History.—The brass referred to in the Bible was no doubt bronze (see BRASS). The Egyptians, Greeks, Etruscans, Romans, and other ancient nations, employed bronze not only for art objects pure and simple, but for a great number of useful articles, very many of these, however, being artistically decorated. The magnificent collection of bronzes in the National Museum at Naples shows, almost at a glance, how extensive was the use of this alloy for articles of daily and domestic use. It is believed that both the Greeks and Romans, for such objects as swords, spears, saws, razors, and other tools, were able to make an alloy of copper and tin which surpassed modern bronze in some properties, and that they possessed some method of tempering bronze, the secret of which has been lost. See BED, LAMP, MIRROR, SCULPTURE, SWORD, and VASE.
Reference is made in the next article to the use of bronze in northern countries in prehistoric times during the Bronze Age. Bronze has been used from early times for statues and statuettes, chiefly representations of deities, as well as for ornamental purposes, in India and China. The Japanese, who are very clever workers in inlaid metals, make bronze of several colours, such as black, blue, purple, and green. These are all different mixtures.
Phosphor-bronze.—It is difficult to make ordinary bronze a quite homogeneous compound. An alloy more perfect in this respect is formed when copper is mixed with tin phosphide instead of metallic tin. The phosphorus, which is added to the extent of from 0.25 to 2.5 per cent., makes the bronze harder, more elastic, and of greater tenacity. This alloy suits best for machinery purposes, for which it is now largely employed, when the proportion of tin is from 7 to 8 per cent. Phosphor-bronze is used for bearings of shafts, piston-rings, pump-rods, propellers, boiler-fittings, and other parts of engineering work. Ornamental castings are also made of it. See CAST, FOUNDING, and BRASS.