Aluminum: Common Metal, Uncommon Past

Tom Geller

Aluminum largely remained a curiosity for the next 20 years, in part because the metal produced by the Deville process was notoriously difficult to work with. The typical sample was only about 97% pure, with at least 1% each of iron and silicon introduced by impurities in the apparatus and starting materials. With low demand there was little economic reason to build aluminum plants. Production worldwide in 1869 was only about 2 metric tons. Fifteen years later, when a 6-pound aluminum cap was famously placed on the Washington Monument, world production had increased to only 3.6 metric tons—compared with the 2,834 metric tons of silver that were produced that year. Only 112 pounds of aluminum was produced in the United States, virtually all by a Philadelphia immigrant named William Frishmuth who had studied with Wöhler in Germany. The bulk of the remainder came from France, Germany, and England.

A big hurdle to achieving lower-cost aluminum production was the lack of a good power source. Even if someone developed an advantageous electrochemical reaction, it needed to be sufficiently strong, sustainable, and economical. The growth of reliable, commercial electric dynamos in the last third of the 19th century meant that reliable electrical power would be available wherever mechanical energy existed, and it returned attention to the possibilities of an economical electrolytic process for aluminum. Improvements demonstrated by Zénobe Gramme in 1871 increased the dynamo's voltage and made the current more consistent and predictable.

This was the world in which Charles Hall entered his second year at Oberlin College and Frenchman Paul Louis-Toussaint Héroult started preparatory school before entering mining college. Both were 18 years old in 1881. Although they ultimately shared the same idea, the two couldn't have been more different.

Charles Martin Hall

Charles Martin Hall was born on 6 December 1863, in Thompson, Ohio, where his father was a Congregational minister. When he was nine they moved 75 miles to Oberlin, Ohio, a town renowned for its college, music conservatory, and status as a terminus of the Underground Railroad. His mother and father had graduated from Oberlin College, and in turn he and his six siblings all graduated from it as well.

Hall had taken an early interest in chemistry, spending his teenage years experimenting with minerals and chemicals in his family's house and eventually going to the college to further his studies. His professor of chemistry was Frank Jewett, who as a student in Germany in the early 1870s had gained an interest in aluminum from discussions with Friedrich Wöhler. Legend has it that Jewett, who had been named professor of chemistry and mineralogy at Oberlin in 1880, passed around a lump of aluminum in class, stating that "any person who discovers a process by which aluminum can be made on a commercial scale will bless humanity and make a fortune for himself." Hall, who already had an interest in aluminum before entering college, allegedly told a classmate, "I'm going for that metal."

Hall made good on this promise shortly after graduating, working partly in Jewett's college lab and partly in his family's woodshed. Like many 19th-century scientists, he fabricated much of his own equipment and synthesized some of his own chemicals. When his first attempts at creating an improved chemical process to extract aluminum failed, Hall had to use numerous Bunsen batteries with carbon cathodes to effect electrolysis. But first he had to find appropriate starting materials.

For an aluminum source he precipitated alumina by mixing the common household products alum with washing soda (sodium carbonate, Na2CO3) and drying the filtered results. Finding a solvent that would liquefy the mixture and make it more amenable to electrolysis turned out to be a bit more difficult. Hall tried fluorspar (calcium fluoride), potassium fluoride, sodium fluoride, magnesium fluoride, and aluminum fluoride, all to no avail. Then on 9 February 1886 Hall discovered that cryolite (sodium hexafluoroaluminate, Na3AlF6), once heated beyond its melting point of 1,000ºC with his gasoline-powered furnace, dissolved alumina like sugar in coffee.

From there the experiments were effected at a lightning-quick pace. A week after Hall's first electrolytic attempts failed (probably owing to contamination by silicates in the clay crucible), he produced his first pieces of metallic aluminum on 23 February 1886 and filed for a patent on 9 July, using the following reaction:

2Al2O3 + 3C → 4Al + 3CO2

Where at the cathode we have: Al3+(melt) + 3e- → Al(l)

And at the anode: 2O2-(melt) + C(s) → CO2(g) + 4e-