A common element that was hard to get
Aluminum is one of the most abundant metals in the Earth’s crust, but it is never found as a metal in nature. It binds tightly to oxygen, mainly in clays and in the ore bauxite. The usual trick of ancient and medieval metalworkers, heating ore with charcoal, does not work, because aluminum holds on to oxygen too strongly for carbon to pull it away at ordinary furnace temperatures. That is why aluminum was unknown as a metal until the nineteenth century, long after iron, copper and tin.
The chemical era
The Danish scientist Hans Christian Ørsted is generally credited with producing a small, impure amount of aluminum in 1825, and Friedrich Wöhler in Germany made it more reliably in 1827. In the 1850s the French chemist Henri Sainte-Claire Deville developed a method that reacted an aluminum compound with metallic sodium, and he made it in small commercial quantities.
It was still expensive. Accounts say that aluminum was displayed as a precious novelty in the 1850s, and a story that Napoleon III served his most honored guests with aluminum tableware is often repeated, though the details are probably embellished. A well-documented use of that era is the small aluminum pyramid placed at the top of the Washington Monument in 1884, at a time when the metal was still a rarity. It weighed on the order of 100 ounces.
Two inventors, one year
In 1886 two young men, working separately and on different continents, found essentially the same method. Charles Martin Hall, in Oberlin, Ohio, and Paul Héroult, in France, discovered that aluminum oxide (alumina) dissolves in molten cryolite, a mineral made of sodium, aluminum and fluorine. When an electric current is passed through the molten mixture, aluminum collects at the bottom. Both men were born in 1863 and both died in 1914, a coincidence that is often mentioned. Because they are credited jointly, the method is named the Hall-Héroult process.
How the process works
A modern smelter uses large steel cells lined with carbon. Inside, alumina is dissolved in molten cryolite at roughly 950 degrees Celsius. Carbon electrodes carry a strong direct current through the bath. Aluminum metal forms at the cathode and sinks to the bottom, where it is periodically tapped. Oxygen released from the alumina reacts with the carbon anodes, which are therefore gradually consumed and need replacement.
The process needs a great deal of electricity, which is why smelters are often located near cheap power, such as hydroelectric dams. The electricity cost is a large part of what aluminum costs to produce, and it is also why recycling scrap aluminum, which uses only a fraction of the energy, is so economical.
Making the raw material
Hall-Héroult smelting needs purified alumina, not raw bauxite. In 1888 the Austrian chemist Karl Josef Bayer patented a process that dissolves alumina from bauxite using hot caustic soda solution, leaving the impurities behind. Bayer’s refining step and the Hall-Héroult smelting step remain the two stages of primary aluminum production. This pairing is a classic piece of industrial chemistry.
From novelty to everyday metal
Hall’s company, which grew into what became Alcoa, and Héroult’s European partners built smelters in the late 1880s and 1890s. As production grew, the price fell dramatically, although exact figures vary by source and period. The metal found uses in cookware, wire and, increasingly, transport. Aluminum is light, conducts electricity reasonably well and forms a thin protective oxide film that resists corrosion. In the twentieth century, aluminum alloys became central to aircraft engines and airframes, and the metal became common in cans, foil, window frames and car parts.
What the story shows
Aluminum’s cheapness did not come from finding more ore or from a better furnace. It came from using electricity to do a job that heat and carbon could not. That makes it a good example of how a new energy source can open the door to a material. Smelting is still energy-intensive today, so ongoing work focuses on cleaner power and on recycling.