Bronze is a golden brown alloy of copper and tin with other elements. It was the hardest metal in common use during the Bronze Age and continues to be an important metal in modern time. Here is a collection of bronze facts, including its composition, properties, and uses.
Bronze Composition
Bronze consists of around 88% copper with about 12% tin and other metals (e.g., aluminum, zinc, nickel, manganese, lead) and sometimes other metalloids or nonmetals (arsenic, silicon, phosphorus).
Difference Between Brass and Bronze
Under modern definitions, bronze is a copper and tin alloy, while brass is a copper and zinc alloy. However, the distinction between the two alloys has not always been so clear. In fact, the word “bronze” comes from the French word bronze, which in turn comes from the Italian word bronzo, meaning “bell metal or brass.” The Italian word traces its roots to an old Persian word for brass. Older objects are best termed “copper alloys” because of their different compositions.
Early History
Bronze replaced brittle stone and soft copper at least as early as the 5th millennium BC. The bronze used in the Bronze Age was arsenic bronze, which people discovered in nature or made by mixing copper and arsenic ores. Tin bronze came into use in the 3rd millennium BC. Tin bronze is superior to arsenic bronze in that its stronger, easier to cast, and not toxic to refine.
Bronze Properties
Bronze properties depend on its composition and processing. However, most bronze shares several common properties:
- Bronze ranges in color from brown to golden.
- It’s usually duller than brass.
- Bronze has a slightly higher melting point than brass.
- Both bronze and brass metal commonly bear faint ring-shaped markings on the metal surface.
- Bronze is a highly ductile metal.
- Bronze exhibits low friction against other metals.
- Striking bronze against a hard surface doesn’t generate sparks. This makes the alloy useful for applications involving explosive or flammable materials.
- Unlike most metals, bronze expands slightly as it solidifies from a melt. This is desirable for casting, as it means the metal fills a mold as it cools.
- Bronze is relatively brittle, but not as much as cast iron.
- The alloy has a lower melting point than iron or steel.
- Bronze conducts electricity and heat better than most steels.
- In air, bronze oxidizes and develops a dull copper patina. But, the patina only affects the surface, protecting the underlying metal. Initially, the patina consists of copper oxide, which eventually changes to copper carbonate.
- While bronze patina protects the alloy from air, bronze corrodes in seawater. Chlorides cause “bronze disease,” where corrosion permeates throughout the metal. But, like copper and brass, bronze generally has good saltwater corrosion resistance.
Uses
Bronze occurs in common, everyday items. Here are some bronze uses:
- Architectural components, such as stair railings, mailboxes, decorative classing, and window frames
- Bearings
- Bells
- Bronze wool, which is an alternative to steel wool that doesn’t shed metal threads that could cause shorts and sparks
- Coins, including older pennies
- Cymbals
- Electrical contacts and connectors
- Industrial castings, such as pumps, valve stems, and automobile transmissions
- Marine architecture, including hulls, pumps, engine parts, ad propellers
- Medals
- Mirrors
- Oil rig components
- Some saxophones
- Sculptures
- Small electric motors
- Guitar and piano strings
- Metal clips
- Safety tools (hammers, mallets, wrenches)
- Screws
- Springs
Bronze Alloys
Metallurgists classify bronze alloys according to their composition. Here are some common alloys:
- Aluminum bronze: Aluminum bronze contains 6% to 12% aluminum, up to 6% iron, and up to 6% nickel. It is a strong alloy with excellent corrosion resistance and wear resistance. Aluminum bronze is the alloy of choice for pumps, valves, or other hardware exposed to corrosive fluids.
- Cupronickel: Cupronickel or copper nickel is a bronze alloy containing 2% to 30% nickel. The alloy displays high thermal stability and corrosion resistance, particularly in moist air or steam. It is also superior to other types of bronze in seawater. Uses of cupronickel include ship hulls, pumps, valves, electronics, and marine equipment.
- Nickel silver: Despite its common name, nickel silver doesn’t contain any silver. It gets its name for its silvery color. Nickel silver contains copper, nickel, and zinc. It is moderately strong and has decent corrosion resistance. Nickel silver finds use in musical instruments, optical equipment, decorations, and dinnerware.
- Phosphor bronze (tin bronze): Phosphor bronze contains 0.5% to 1.0% tin and 0.01% to 0.035% phosphorus. This alloy is tough and strong and has a fine grain, low coefficient of friction, and high fatigue resistance. Phosphor bronze finds use in springs, washer, electrical equipment, and bellows.
- Silicon bronze: Silicon bronze includes both red silicon brass as well as red silicon bronze. Red brass contains about 20% zinc and 6% silicon, while red bronze contains less zinc. Silicon bronze is low in lead and may contain manganese, tin, or iron. Silicon bronze has high corrosion resistance and strength. It is used for pumps and valve stems.
References
- Alavudeen, A.; Venkateshwaran, N.; Winowlin Jappes, J. T. (2006). A Textbook of Engineering Materials and Metallurgy. Firewall Media. ISBN 978-81-7008-957-5.
- Gale, W. F.; Totemeier, T. C. (eds.) (2003). Smithells Metals Reference Book (8th ed.). Butterworth-Heinemann. ISBN 9780750675093.
- Thornton, C.; Lamberg-Karlovsky, C.C.; Liezers, M.; Young, S.M.M. (2002). “On pins and needles: tracing the evolution of copper-based alloying at Tepe Yahya, Iran, via ICP-MS analysis of Common-place items”. Journal of Archaeological Science. 29(12): 1451–60. doi:10.1006/jasc.2002.0809
- Tylecote, R.F. (1992). A History of Metallurgy (2nd ed.). London: Maney Publishing, for the Institute of Materials. ISBN 978-1-902653-79-2.