Aluminium Imagination Architectural Awards: Strength in lightness

From Babylon to Boeing: the secret of one metal's success.

Aluminium is a subtle metal: it is not what it seems. Every time you see it, whether in jewellery, aircraft wings, or a drinks can, no matter how brightly it shines, you're not seeing the pure metal.

The reason is that it is covered with a microscopically thin layer of alumina - aluminium oxide - a very stable compound which defeated attempts to reduce it to its components for roughly 7,000 years. Produce a piece of aluminium, and as soon as it is exposed to air it forms an atom-thin crust. Though invisible to the eye, that crust is the metal's saving grace: it is what makes aluminium resistant to corrosion.

Babylonian potters knew about alumina: they used its amazing heat-resistant properties for clay firing processes. Indeed, aluminium is the third most abundant element in the Earth's crust, making up roughly 8 per cent of its weight. (Only oxygen and silicon are more common.)

In 1807 the British chemist Sir Humphrey Davy named aluminium, but couldn't produce any. In 1825 the Danish scientist Hans Oersted produced a tiny lump. In 1827 a German chemist, Friedrich Wohler, managed to produce aluminium particles. By 1845 Wohler was able to produce some larger samples (enough to determine the metal's basic properties).

That problem was only cracked in 1886, when two scientists working independently, one in France and one in the United States, discovered a workable production method within a month of each other. Named after them, it became the Hall- Heroult process, which is still used today.

The key is a mineral called cryolite, a compound of sodium, aluminium and fluorine, which is used to dissolve refined alumina. A direct current is then run through the mixture. Though the voltage is tiny, at about five volts, the current is enormous, usually over 300,000 amps. Only in these conditions can the oxygen which binds so tightly to the aluminium atom be tempted away, reacting with the carbon to form carbon dioxide gas. Pure molten aluminium is left in the bottom of the reactor vessel, from which it can be siphoned off.

The amount of energy that it takes to produce aluminium (the "embodied energy" of the metal) is so great that aluminium production is reckoned to consume 1 per cent of the US's entire electricity output; the smelting plants' location is usually determined by those of power plants. That is why aluminium recycling is one of the easiest ways to be eco-friendly. The energy needed to make one new drinks can will recycle 20 old ones; it takes five tons of bauxite to make about two tons of alumina, which then needs 14,000 to 16,000 kilowatt hours to produce one ton of aluminium, which will make 30,000 drinks cans. But 95 per cent of the energy required in primary smelting can be saved by recycling.

Once the refining process was introduced, the possibilities seemed boundless: aluminium could replace iron because it was both lighter and resistant to corrosion. Its real strength emerges when it is alloyed with other metals. Aircraft manufacture has led to a galaxy of alloys: adding between one and two per cent of copper increases the strength and hardness; magnesium increases tensile strength, resistance to marine corrosion, weldability and hardness; manganese increases resistance to corrosion; silicon lowers the melting point and improves castability; zinc improves strength.

Once aluminium became available in large commercial amounts, its potential for use in buildings - either inside or outside - was quickly recognised. It is light; it won't tarnish; it looks bright, even in dark surroundings; it can be rolled extremely thin; and it can be welded like other metals.

One of its first main uses came in the 1890s, when it could be found in lift construction in the United States and in the dome cladding of San Gioacchino Church in Rome. That became more common after the Second World War, when it began appearing in facades and roofs.

As a cladding, it gave skyscrapers the appearance of disappearing into the skies (by reflecting the sky around them). True, its cost per unit made it too expensive to replace iron in the framework of new buildings. But that didn't matter, because the new metal could stand on the outside.

The fact is that after 7,000 years spent stolidly refusing to help humans, aluminium - once tamed - has really changed the world. Simply by making jet travel economical it has shrunk the globe.

Nowadays airframes are a major use, with an estimated 300,000 tons used to build civil and military aircraft each year. Other favourites are alloy wheels, engine blocks and heads, ships' hulls, and of course drinks cans. In fact, it is so useful that it has gone extraterrestrial: the Lunar Rover of the Apollo missions was made of aluminium. It's a long way from a Babylonian pot.