Alloys: Strong Combinations

The properties of these different metals can be combined by mixing two or more of them together. The resulting substance is called an alloy. Some of our most useful building materials are actually alloys. Steel, for example, is a mixture of iron and small amounts of carbon and other elements; a combination that is both strong and easy to use. (Add chromium and you get stainless steel. Check your kitchen pots and pans to see how many are made from stainless steel!)

Other alloys like brass (copper and zinc) and bronze (copper and tin) are easy to shape and beautiful to look at. Bronze is also used frequently in ship-building because it is resistant to corrosion from sea water. Titanium is much lighter and less dense than steel, but as strong; and although heavier than aluminum, it's also twice as strong. It's also very resistant to corrosion. All these factors make it an excellent alloy material. Titanium alloys are used in aircraft, ships, and spacecraft, as well as paints, bicycles, and even laptop computers!

Gold, as a pure metal, is so soft that it is always mixed with another metal (usually silver, copper, or zinc) when it's made into jewelry. The purity of gold is measured in karats. The purest you can get in jewelry is 24 karat, which is about 99.7% pure gold. Gold can also be mixed with other metals to change its color; white gold, which is popular for jewelry, is an alloy of gold and platinum or palladium.

Metal from Ore

Ores are rocks or minerals from which a valuable substance - usually metal - can be extracted. Some common ores include galena (lead ore), bornite and malachite (copper), cinnabar (mercury), and bauxite (aluminum). The most common iron ores are magnetite and hematite (a rusty-colored mineral formed by iron and oxygen), which both contain about 70% iron.

There are several processes for refining iron from ore. The older process is to burn iron ore with charcoal (carbon) and oxygen provided by bellows. The carbon and oxygen, including the oxygen in the ore, combine and leave the iron. However, the iron does not get hot enough to melt completely and it contains silicates left over from the ore. It can be heated and hammered out to form wrought iron.

The more modern process uses a blast furnace to heat iron ore, limestone, and coke (a coal product, not the soft drink). The resulting reactions separate out the iron from the oxygen in the ore. This "pig iron" needs to be further mixed to create wrought iron. It can also be used for another important purpose: when heated with carbon and other elements, it becomes a stronger metal called steel.

Considering the process involved, it's not surprising that iron was not used until around 1500 BC. But some pure metals - gold, silver, and copper - were used before then, and the alloy bronze is thought to have been discovered by the Sumerians around 3500 BC. But aluminum, one of the most essential metals in modern use, wasn't discovered until AD 1825, and wasn't commonly used until the 20th century!