Bronze— The HistoryThe oldest alloy known to human beings was a bronze made of copper and arsenic. People learned to make it about 3500 B.C. Gradually, people replaced the arsenic with tin. The period in history between the Stone Age and the Iron Age became known as the Bronze Age because bronze was commonly used to cast containers such as cups, urns, and vases. People also shaped bronze into battle-axes, helmets, knives, shields, and swords. They also made it into ornaments, and sometimes even into primitive stoves. Bronze Age (3,500 to 0 BC or later, depending on location)Bronze was developed about 3500 BC by the ancient Sumerians in the Tigris-Euphrates Valley. Historians are unsure how this alloy was discovered, but believe that bronze may have first been made accidentally when rocks rich in copper and tin were used to build campfire rings. As fire heated these stones, the metals may have melted out and mixed, forming bronze. This theory is supported by the fact that bronze was not developed in North America, where natural tin and copper ores are rarely found in the same rocks. Around 3000 BC, bronze-making spread to Persia, where bronze objects such as ornaments, weapons, and chariot fittings have been found. Bronzes appeared in both Egypt and China around 2000 BC. The earliest bronze castings (objects made by pouring liquid metal into molds) were made in sand, and this method is sometimes still used, even for casting bells. However, clay and stone molds were developed later on, and clay is usually used nowadays for bells. Zinc, lead, and silver were added to bronze alloys by Greek and Roman metalworkers for use in tools, weapons, coins, and art objects. At first, copper-arsenic alloys were used, but only for a short while. One disadvantage of this alloy was that the fumes emitted by arsenic during smelting tended to kill the metalsmiths! Eventually, tin was found to be an ideal alloying agent for copper. The optimum ratio is about 10 to 20% tin to copper. The melting point of bronze is 950 deg. C, compared to 1084 deg. C for pure copper. The bronze melt flows freely into molds without formation of gas bubbles, which is a problem with copper, and bronze is hard immediately after casting and cooling— it does not need to be tempered. Bronze is harder and less brittle than copper-arsenic alloys, and it can be hardened even further by hammering Modern bronzesAfter its discovery (for the Middle East, around 700 BC) iron, and later steel, quickly replaced bronze in the ancient world as the metal of choice for weaponry and industry because it is harder and more durable. Today, bronze is used for making products ranging from household items such as doorknobs, drawer handles, clocks, and sculpture, but it is also still used in some industrial applications, such as engine parts, bearings, and wire. And of course, it is used in bells.
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