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Bronze and brass are two alloys of copper and other materials (we all know they aren't elements, yes?) which have seen significant use throughout human history. We even have a period of human culture referred to as The Bronze Age (4,000-3,000BC). I know that I can't tell these materials apart just by looking (unless they're tarnished, I think), so what makes these two materials different? How were they employed by our ancestors? Where did they get the stuff? Why should you care?
In a simple material sense, bronze is an alloy of primarily copper and tin, but potentially includes arsenic, phosphorus, aluminum, manganese and silicon. Brass is primarily an alloy of copper and zinc. Apparently, this differentiation is becoming outmoded, as museums and the science of archaeology have begun using the catch-all term "copper alloy" to represent both. However, for the writer, the terms "bronze" and "brass" are still useful, allowing the author to subtly differentiate without getting into the chemistry of it all.
When you realize that the period before the Bronze Age was the Stone Age, you get an inkling of what a step forward this was in human development. Bronze was used for tools, weapons, armor, and decorative pieces (it's pretty, you know this). The earliest bronzes on record are copper-arsenic alloys, which occur together naturally, though you can guess why using (or manufacturing) this material frequently may have been a teensy problem. Anyway, tin bronze turns out to be a much easier material to alloy, is stronger and easier to cast. The trick is that these two materials are rarely found together, requiring long-distance trade and a significant amount of knowledge (these ain't no backwards hillbillys, though those guys can be pretty ingenious when it comes down to it).
It is not known when brass was understood to be a zinc-copper alloy (since it does occur naturally). However, brass also has a long storied history. Produced in Western Asia and the Eastern Mediterranean as early as the 3rd Century BC, "golden copper" may have been valued almost as highly as gold. The Romans may have been responsible for its geographical spread across Europe, as apart from various decorative items, Roman coins (sesterces) were struck from brass. Brass was made by heating ground smithsonite ore in a crucible with copper. The zinc from the ore would permeate the copper, producing brass.
Obviously both materials were used for decorative goods. It is hardly uncommon for one to be mistaken for the other (which does make me feel a little better), so if you had to choose a material for a project, why select one over the other?
Brass is a relatively softer metal than bronze and has a higher malleability (with lower melting point) and better acoustic properties (you've heard of the brass section of an orchestra?). Brass is also used for low-friction applications, like locks, gears, and doorknobs, due to its ease of machining and corrosion resistance (also making it good for maritime use). It seems that brass also saw use in the wool trade, being used in combs for wool harvesting, prior to the widespread production of shears. Pins were widely produced from brass because the material could be cold worked to size.
Bronze is a heavier and stiffer material than brass. It only corrodes superficially, protecting the material underneath (unless copper chlorides are formed, causing "bronze disease") and is a good conductor of heat (pots and pans) and electricity (hey, it's your fantasy world). When casting, bronze will expand slightly just before it sets, making it wonderful to catch sculptural detail. Bronze also has low metal-on-metal friction, making it a good material for firing cannon balls out of. Because of its resistance to corrosion and relative stiffness, brass is still used for ship propellers and underwater fittings.
References
Bronze Age defined - http://www.merriam-webster.com/dictionary/bronze%20age
Brass Wiki - https://en.wikipedia.org/wiki/Brass
Brass History - http://www.copper.org/publications/newsletters/innovations/2000/01/history_brass.html
Bronze Wiki - https://en.wikipedia.org/wiki/Bronze
Bronze bells - http://www.verdin.com/bells/bell-foundry.php
Bronze casting - http://www.modernsculpture.com/bronze.htm
Color/composition chart - http://www.metalreference.com/Forms_Copper_Alloy.html
Care - https://www.thehenryford.org/research/caring/brass.aspx
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