An alloy is a combination of two or more metals in which the atoms are thoroughly mixed together like two liquids stirred together. Not all metals will form alloys, just as not all liquids will mix.
Humans have been forming alloys almost since the earliest days of metal technology. (Note that the Bronze Age is named for the alloy bronze.) Typically, one metal dominates the alloy, representing at least 85% of the total. This metal, the base, will be melted in a furnace, and other ingredients are then added. Other methods of forming alloys are possible, however – see the entry on brass formed with zinc.
The purpose of alloys is to create metals that have better properties or more suitable ones than the base alone. For example, copper is plentiful and easy to extract from its ore, but too soft to make useful tools. Alloying fixes this limitation.
An alloy is usually not just crafted as a single ratio of the components, but can be a range of percentages depending on the specific need. For example, stainless steel resists corrosion, but the amount of stainless alloyed into the steel depends on the amount of corrosion it will be exposed to.
Some important alloys are discussed below. Also see steel alloys and superalloys. In truth, though, almost all modern metals are alloys. Large items made of aluminum, nickel, or any other major metal likely includes small amounts of other substances to improve strength, machinability, and other properties.
Bronze is the earliest popular alloy, one based on copper. Bronze is copper with about 12% tin. It is the earliest hard metal, capable of forming swords and armor and giving a name to the earliest age of metal smithing.
Copper and tin deposits are seldom found together, and so the need for bronze also encourages trade.
Brass is another early alloy, of copper with zinc. Brass can be hard or soft, and it can be modified in other ways with the addition of another metal. A small amount of lead, for example, will improve the machinability, and aluminum will resist corrosion.
Brass has many uses. It is shiny and decorative. It can be cast or machined into many different shapes for small parts. It has low friction, which is important for hinges and bearings. Musical instruments are brass. Brass does not throw sparks, which can be important around gasoline vapors.
Curiously, brass was made by Roman smiths long before zinc was isolated or recognized as a distinct metal. Calomine (a kind of zinc ore) was heated with copper in a closed furnace. The zinc would vaporize and soak into the copper, making an imprecise alloy.
Pewter is tin alloyed with lead, copper, or perhaps other metals to make it usefully hard. Pewter, like brass, can be cast into a variety of useful shapes.
Simple iron is not the best or only magnetic metal. Alnico is made from iron with small amounts of aluminum, nickle, and cobalt. Alnico magnets can generate a magnetic field at least twice as strong as what a steel magnet is capable of. Rare earth magnetics are stronger yet. The best practical ones are made of an alloy of neodymium and iron (below). Rare earth magnetics have wide commercial applications, including in electric motors and computer hard disk drives. Other magnets are found under superconductor.
Gold and silver are prized for their beauty, but both metals are too soft to form large objects or items that will be handled frequently. Alloys mitigate this problem. Sterling silver is probably the best-known example, an alloy of silver with copper that has enough durability to use in tableware while retaining the color and shine of natural silver.
Gold used in circulating coinage has historically been alloyed with copper for hardness and sometimes with a small amount of silver. The United Kingdom standard for such coins is called crown gold; it is 22 karat gold with copper being the remaining 2 parts in 24. The South African kruggerand and some other foreign coins have also been minted to the crown gold standard.
Gold is also alloyed with other metals for decorative purposes, to change its color. The best-known examples of this are rose gold (with copper), white gold (usually with nickel or platinum), and electrum (a natural or artificial mix of gold and silver).