Aluminium

Aluminium

The first person to produce aluminium, in 1825, was Hans Christian Oersted (1777–1851) at Copenhagen, and he did this by heating aluminium chloride with potassium metal. In fact it was better to use sodium rather than potassium as German chemist Friedrich Wöhler (1800–1882) found and this also gave a purer metal. Either way, it was expensive to produce and consequently aluminium was for many years treated as a novelty of the rich and powerful. In the 1860s the Emperor Napoleon III of France impressed visiting heads of state with special cutlery made of aluminium.

In February 1886, a 21 year old US student, Charles M. Hall (1863–1914) of Ohio, developed a method for extracting aluminium from molten aluminium salts using an electric current, his researches being carried out in the family woodshed and using a homemade battery. On 9 July 1886 he was granted a patent for the method.

Earlier that year, a young Frenchman had been granted a patent for the same process. He was 23-year old student Paul-Louis-Toussaint Héroult (1863–1914). Both patents were contested and lengthy litigation followed, until it was agreed that Hall had the American rights and Héroult the European rights.

Both men had discovered that in order to reduce aluminium oxide to aluminium electrolytically it had to be dissolved in molten cryolite, in which it was quite soluble, and that carbon electrodes had to be used. Cryolite is sodium aluminium fluoride, Na₃AlF₆. It occurs naturally in Greenland from where it was exported. Cryolite's remarkable property is that it melts easily on heating to give a liquid which will dissolve aluminium oxide.

As a result of the Hall–Héroult process, the price of aluminium fell until it cost a mere fraction of the price when Napoleon III had purchased his precious spoons.

Of course aluminium in other forms had been known for millennia. As long ago as the first century AD, the Roman army doctor Dioscorides wrote the medical book De materia medica, in which he recommended alum to stop bleeding. (Alum is potassium aluminium sulfate, basic formula KAl(SO₄)₂.) The ancient world got its alum from naturally occurring deposits in Greece and Turkey. Then in the Middle Ages it was discovered that aluminium could also be made from clay and sulfuric acid. The discovery of alunite, a potassium aluminium sulfate rock, at Tolfa in territories controlled by the Pope, led to a Papal monopoly on alum in Europe from the 1460s onwards. The industry employed 8,000 men, produced 1,500 tonnes of alum a year, and generated a large revenue for Rome.

This state of affairs continued until English alum production started in the early 1600s in north Yorkshire where a large deposit of alum shale was discovered. The price of alum fell dramatically. Production continued in Yorkshire for more than 250 years.

In the succeeding centuries, alum found use in two main areas. Added to paper it acted as a preservative, and added to cloth it acted as a mordant prior to dyeing. (The aluminium ions, Al³⁺, bond to the fibres of the cloth and the dye bonds to the aluminium ions.) Meanwhile alum can still be used as a styptic pencil to stop bleeding if you nick your face when shaving.