Definition - What does Bronze mean?
Bronze was one of the earliest metals to be discovered, around 3500 BC (the Bronze Age), and before chemical parameters had been put in place for the alloying of this metal. However, in present times bronze is seen as a copper alloy with specific working properties and alloying elements being well defined. Elements such as lead, manganese, nickel, silicon, zinc and others are added to improve bronze and produce a wide array of bronze grades to choose from.
Bronze's high copper content makes it possible for it to oxidize in air, giving it a distinct mottled patina. This oxidation also prevents bronze from corroding, especially when in a saltwater environment. However, chlorine compounds that react with bronze trigger “bronze disease,” which starts when corrosion breeds more corrosion and slowly destroys the alloy over time.
Some properties of bronze:
- Relative density: Approximately 8.8 g/cm3
- Melting point: Ranges 950°C - 1050°C (1,742°F - 1,922°F), depending on the amount of tin present
- Color: Tends to be a reddish-brown/gold color
- Brittleness: Quite brittle, although less so than cast iron
- Coefficient of friction: Low friction when it is in contact with other metals
- Conductive: A ready conductor of heat and electricity
Corrosionpedia explains Bronze
Bronze's physical and mechanical properties are dependent on the specific composition of the alloy as well as its production processes. Typical characteristics include high ductility, low friction against other metals, brittleness, and expanding a small amount when solidifying from a liquid into a solid.
Bronze oxidizes when exposed to atmospheric air, but only on its outer layer. This layer consists of copper oxide, which eventually becomes copper carbonate. The outer oxidized layer helps to protect the interior metal from further corrosion. However, in the case of bronze disease, corrosion is able to spread throughout the metal and destroy it.
The Chemical Composition of Bronze
Bronze is an alloy that is made from copper and another metal, depending on its intended use. As a result its composition may vary, but a large amount of bronze used nowadays is 88% copper and about 12% tin.
At one time, bronze was thought to be an alloy consisting only of copper and tin. Now there is a blurred line between brass and bronze, due to just how wide the possibilities are for what elements can be alloyed together. Copper alloys are usually called brass, with bronze sometimes considered as a type of brass. To avoid confusion, historical texts use the term "copper alloy." In science and engineering however, bronze and brass can easily be defined according to their elemental composition.
Applications for Bronze
Architecture is a sector where bronze is used largely for structural and design elements. Bronze is also used to produce bearings because of its general lack of friction, and for electrical contacts.
Bronze is a popular choice for ship propellers, fittings and submerged marine parts due to its saltwater resistance, and for sculptures that must resist degradation when displayed outdoors. It has great casting properties and is readily cast into bearings, clips, electrical connections and many other items.
Machine tools and some bearings are typically made from aluminum bronze. Bronze wool is used in place of steel wool in woodworking so as to not discolor oak. Bronze is used to make coins, as most "copper" coins are actually bronze, consisting of copper with 4% tin and 1% zinc.