Bronze

BRONZE (Heb. נְחֹ֫שֶׁת, H5733; Gr. χαλκός, G5910) an alloy formed chiefly or wholly of copper and tin (q.v.) in variable proportions. It is harder and less malleable than copper. It is also more suited for casting than copper because of its greater fusibility. The greater the proportion of tin, the lower the melting point: with 98-90% copper the melting point is c. 1080-1005oC, with 80-70% copper the melting point is c. 800-750oC and with the copper: tin ratio about 60:40, the melting point is 630oC. The arrangement of the atoms in bronze is like that in copper, with the atoms lying on the points of a face-centered cubic lattice. This structure is made up of repeated cubic units, each unit having atoms at the corners of a cube and at the centers of the six faces of each cube. The relative proportions of copper and tin atoms control the physical properties of the particular type of bronze.

As bronze is relatively easy to fabricate, has a pleasant brownish color and resists corrosion, this alloy has been used from prehistoric times for ornaments, statues, coinage and utilitarian articles. The use of copper, from c. 6000-5000 b.c., and then of bronze, from c. 3700 b.c. are among man’s early metallurgical achievements and the term “Bronze Age” (q.v.) denotes a cultural stage during which tools and weapons were fabricated from bronze (and copper) rather than from stone, bone and wood previously, and iron subsequently.

The discovery of the relationship of copper-bearing rock to the metallic metal, which could be produced by the action of fire and charcoal, took place c. 4500 b.c. with copper workings in northern Iraq, Persia and Turkestan in existence before 4000 b.c. The subsequent discovery that the addition of tin hardened the metal prob. resulted from the accidental use of campfire stones composed of ores of copper and of tin, as natural alloys of copper and tin are very rare. Bronze was produced by the ancient Sumer. civilization in the Tigris-Euphrates Valley, e.g. at Ur c. 3500 b.c. Animal drawn plows and chariots with bronze parts were in use in Iraq before 3000 b.c. From there the use of bronze spread to Persia and about 2000 b.c. bronze was first used in Egypt.

The molten metal was made by smelting the ores, with charcoal, using a forced draught created by bellows or by blowing with the lungs. The draught was through a non-inflammable clay nozzle with the molten metal collected in a crucible of the same substance. The extracted metal was then cast into ingots, or cakes, or poured directly into molds. The earliest castings were in sand, but later clay and stone were used with instruments like knives and axes cast in shallow recesses in stone or clay.

Copper was being mined on the Sinai Peninsula before the discovery of bronze and about 3000 b.c. Cyprus was a major producer. However, the tin deposits discovered in the 3rd millennium b.c. were small and not numberous. Only late in this millennium were larger deposits located. The tin deposits of Cornwall, England, were much used in the 2nd millennium b.c., and possibly even earlier, with the Phoenicians trading in tin from Cornwall and from Spain. They prob. played an important role in spreading the early bronze culture with much of the bronze of the middle part of the 2nd millennium b.c., and later, being a combination of tin from western Europe and copper from the eastern Mediterranean and Middle E.



From the end of the 2nd millennium b.c. iron progressively replaced bronze, at least in its more utilitarian uses, with the Bronze Age coming to an end and the Iron Age beginning. See Brass.

Bibliography

J. R. Partington, A Textbook of Inorganic Chemistry, 6th ed. (1950), 718, 722; T. P. Hoar, “Bronze,” Chambers’ Encyclopaedia, II (1950), 603; S. Piggott, “Bronze Age,” Chambers’ Encyclopaedia, II (1950), 603-605.

See also

  • Minerals