Bronze, an alloy consisting of proportions of copper and tin which vary according to the purpose desired, and to which lead, zinc, and silver are sometimes added for the purpose of giving greater brilliancy to the compound, or rendering it more fusible, the zinc being introduced in the form of brass. In some of the modern bronzes brass is used instead of tin; these are then nothing more than brass consisting of very large proportions of copper. The principal varieties of bronze are speculum metal, bell metal, gun metal, and statuary bronze. Speculum metal of reflecting telescopes is the whitest, hardest, most brilliant, and most brittle of all the bronze alloys; it consists of 100 parts of tin and 215 of copper. Bell metal is usually composed of 78 parts of copper and 22 of tin. This is also the composition of the Chinese gongs, which are forged under the hammer, the alloy being rendered malleable after casting by plunging it at a cherry-red heat into cold water; the plate is kept in shape by confining it between two disks of iron. Cannon metal consists of 90 to 91 parts in 100 of copper, and the rest of tin; the strength of this compound is stated by Dr. Thomson to be one half that of malleable iron. Antique bronze consisted of about 87 parts of copper to 13 of tin.
Most of the modern French bronzes are composed of about 91 per cent, of copper, 2 of tin, 6 of zinc, and 1 of lead. The following table gives the composition of several ancient and modern bronzes from various authorities:
Coin of Alexander the Great, 835 B.C......
PtolemvIX., 70 B.C.
Iron, a trace.
Old Attic coin...
Roman coin. 500 B. C .
Lead, 29 32.
Lead, 1.18; zinc, 1.44.
Old bell at Rouen......
Zinc, 1.8; iron. 1.2.
The qualities of bronze of the same composition depend much upon the temperature of the alloy when poured, and on the rapidity of the cooling. - In making bronze, the metals are melted separately, and the tin is added and the mixture stirred until it is homogeneous. It is then turned into the mould as quickly as possible, and when the exterior is sufficiently solidified the casting is uncovered, in order, by hastening the setting of the interior, to prevent as much as possible the formation of strata of unequal composition, which is liable to occur from the great difference in the fusibility of the metals, that of copper being above 2,200° F., while tin melts at 442°. In casting cannon, where there is a great mass of metal, such stratification is liable to occur, the parts which solidify first being richer in copper. This difficulty is largely overcome by the addition of about one tenth per cent, of phosphorus, by which the grain of the bronze is also improved. After cooling, if it is again raised to a temperature of about 1,000° and allowed to cool slowly, its toughness of texture will be improved.
The dark olive hue which bronze acquires by exposure is hastened by the application of oxidizing washes, and different shades may be given according to the chemical qualities of the wash employed. Some extract the tin from the surface, and leave the copper in excess, and others remove the copper and leave the tin most prominent. - Among the ancient Egyptians, Greeks, and Romans, the manufacture of bronze articles was very extensively carried on. In Greece especially the taste for statuary in this material was cultivated to an unparalleled extent. The wealth of some cities was estimated by the number of their statues. In Athens alone no fewer than 3,000 statues have been found, and in Rhodes, at Olympia, and in Delphi many more. The famous colossuses were cast of this alloy. The names of many of the ancient artists are still celebrated, and their groups of statuary continue to be our models. The alloy was employed by them for purposes to which we apply the harder metals, as in some periods for their arms and armor, medals, and even their surgical instruments, a set of which was discovered at Pompeii. By them it was regarded as a sacred metal, and endowed with a mysterious power of driving away evil spirits.
The laws were inscribed on tables of bronze, and upon bronze coins alone were placed the words moneta sacra. The Phoenicians were the first known workers of it; they made it into plates, which were nailed together; and they also cast it solid, and cored. The Athenian sculptor Myron employed it of a pale color and unknown composition, in the 5th century B. 0. The Corinthian bronze is supposed to have been suggested by the accidental fusing of metals at the burning of Corinth, 146 B. C.; it was of three colors, white, yellow, and the last not known. The antique liver-colored cinque-cents, and the Florentine bronze, are of the same shade, approaching a dull reddish brown. - Aluminum bronze is composed of 90 parts of copper and 10 of aluminum. It resembles gold, and is used in ornaments.