Aluminium

Category: post-transition metal Atomic number: 13 Colour: silvery grey Melting point: 660°C (1,220°F) Boiling point: 2,519°C (4,566°F) First identified: third century or 1827

Aluminium is a hugely useful element that is used in everything from cans, foil, kitchen equipment and household items to aeroplanes, cars and power cables. It is a light metal that is soft and malleable, while being nontoxic, non-magnetic and an electrical conductor. Crucially, while iron rusts when it oxidizes, aluminium forms an extremely thin but tough layer of aluminium oxide, which only strengthens the metal. It is also the most abundant metal (by mass) in the Earth’s crust, so is used extensively, in its pure form and in a variety of alloys; for instance, with magnesium, silicon, manganese and copper. These alloys are often used in aeroplanes, bicycles and cars that need to be lightweight.

Aluminium may have been refined as long ago as the third century, and the tomb of the Chinese military leader Chou-Chu contained a metal ornament that was 85 per cent aluminium. However, if the Chinese did have a method for partially refining the metal, it was lost for centuries. Eighteenth-century chemists had worked out that aluminium oxide must contain a metal, but it wasn’t until 1827 that the German chemist Friedrich Wöhler perfected a method previously attempted by his Danish counterpart, Hans Christian Ørsted: aluminium chloride was heated with potassium, and this eventually produced pure aluminium.

Humphry Davy had come close to producing it twenty years later and had named the metal he was trying to refine ‘aluminum’ (with only one ‘i’), from one of its compounds, the bitter salt alum. This led on to the current difference between American English and British English – over the years, the International Union of Pure and Applied Chemistry (or IUPAC) ruled that, as a metal, it should take the ‘-ium’ ending, but the American Chemical Society chose to return to the original spelling, and now both countries think the other one is spelling it and pronouncing it ‘wrongly’.

The Hall–Héroult Process

A cheap production method for aluminium was discovered independently by Charles Martin Hall, a twenty-two-year-old American amateur who carried out his ingenious experiments with his sister in a woodshed, and, on the other side of the Atlantic, Paul-Louis-Toussaint Héroult, a French chemist of the same age. The method, which was named jointly after them, was to dissolve aluminium oxide in a vat of molten sodium hexafluoroaluminate (better known as ‘cryolite’), then separate the aluminium and oxygen with an electrical current. This is still the method used today for large-scale production.

Aluminium is now relatively easy to produce, but before modern methods were developed it was regarded as a luxury metal – in the 1860s, the court of Napoleon III of France is said to have served visiting kings and queens food on aluminium plates, while lesser nobles were only given gold plates to eat off.