Page 20 - Curriculum Visions Dynamic Book

Copper alloys: bronze
Bronze is an alloy of copper that is significantly different from brass. Bronze is a copper alloy with tin as its major secondary constituent (brass is an alloy with zinc as described on page 18).
Bronze has been used since ancient times
for decorative metal objects and also for coins.
It was one of the earliest metal alloys used, giving rise to the first metal-working age, known as the Bronze Age, over 3000 years ago. Bronze Age people, however, did not know about alloying (mixing) metals, but used copper ores that naturally contained tin impurities.
 Bronze is usually made from about
78% copper and 12% tin. The less tin, the softer the metal will be. Usually, no more than 25% tin is added.
Adding zinc and lead to the bronze alloy produces a material that is much more suitable for casting.
An alloy with about nine-tenths copper and equal proportions of the other metals is called gunmetal. It was commonly used
in cannons, not only for its corrosion resistance but also for its machinability. Above is a cannon that was used at Gettysburg during the American Civil War.
Tin
Copper
Wrought bronze
Wrought bronze, also known as phosphor bronze, has a low tin content (perhaps 8% or less) and 0.3% phosphorus. It is commonly used for bearings in machines or engines where shafts continually rotate. It is also used for coins.
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 A bronze coin with a portrait of the Roman emperor Hadrian, who reigned between 117 and
138 A.D. This particular coin was minted sometime between 126 and 138 A.D. Despite being in damp soil for more than 1850 years the coin still shows a lot of detail from the original casting.
Gastight or porous bronzes
If lead is used in bronze alloys, the alloy will run into any pores created as the molten alloy sets. Leaded bronze is thus gastight and can be used for pressure vessels.
On the other hand, bronze can
be made intentionally porous, so
that it can be used as a filter. For
this purpose tin and copper powders are mixed, then heated in the absence of air. The result is a porous bronze.