On this page
Aluminium is in foil, cans, window frames and aircraft. In the mid-1800s it was a luxury metal. What changed was a method, worked out in 1886 by two young men working independently on two continents. This post tells their story from reliable sources, works out with Faraday's laws how much aluminium a small early cell could make, and ends with what you can learn from the way they worked.
Why was aluminium so expensive?
Aluminium is common in rocks but holds on tightly to oxygen, so it is hard to extract. In a short history for The Electrochemical Society, Theodore Beck (2014) notes that the earlier method, sodium reduction of aluminium chloride by Henri Sainte-Claire Deville, left aluminium "as expensive as silver and used only for luxury items and jewelry."
Electrolysis of a water solution does not give the metal: water is reduced at the cathode more easily than aluminium ions, so hydrogen comes off instead. The answer was to dissolve aluminium oxide in a molten salt.
How did Charles Martin Hall do it?
Beck records that Hall was born on 6 December 1863 in Thompson, Ohio, and entered Oberlin College at 16. His professor, Frank Fanning Jewett, told him that a fortune awaited whoever found a cheap way to make aluminium from its oxide, and Hall told a fellow student he meant to be that person.
After graduating he carried on in a woodshed at home, helped by his sister Julia, who had also studied chemistry. His current came from Bunsen-Grove cells. Experiments in water failed, so he turned to molten fluorides: he melted cryolite, Na₃AlF₆, in a clay-lined iron tube heated by a gasoline stove and dissolved alumina in it. At first he got only a grey deposit. With a graphite lining and aluminium fluoride added to lower the melting point, he found "several small silvery globules", which Jewett confirmed were aluminium.
Hall applied for a US patent on 9 July 1886. His patent US 400,766, Process of reducing aluminium by electrolysis, granted on 2 April 1889, describes dissolving alumina in a fused bath of the fluorides of aluminium and sodium and passing a current through it.
And Paul Héroult?
Beck notes that Héroult was born on 10 April 1863 in France. He was dismissed from the École des Mines after his first year because he spent his time thinking about aluminium instead of his set subjects. Back at his late father's tannery, his mother gave him her last 50,000 francs to buy a 400-ampere, 30-volt dynamo. In April 1886 he made small amounts of aluminium from alumina dissolved in cryolite, and his French patent dates from 23 April 1886. Hall had to use family letters and Jewett's evidence to establish his own priority in the United States. The two men later became good friends, and both died in 1914.
A worked example: what could Héroult's dynamo make?
The NCERT Class 12 chapter Electrochemistry notes that "aluminium is produced by electrolysis of aluminium oxide in presence of cryolite," and states Faraday's laws. At the cathode:
Al³⁺ + 3e⁻ → Al
Suppose, as an upper limit, that Héroult's 400 A dynamo ran for one hour and every electron deposited aluminium.
Charge: Q = It = 400 A × 3,600 s = 1.44 × 10⁶ C.
Moles of electrons: 1.44 × 10⁶ ÷ 96,485 = 14.9 mol.
Moles of aluminium: 14.9 ÷ 3 = 4.97 mol.
Mass: 4.97 × 26.98 g/mol = 134 g.
So even a full-power hour gives at most about 134 g. Beck gives the overall cell reaction with a carbon anode as Al₂O₃ + 3/2 C → 2Al + 3/2 CO₂. That means 0.75 mol of carbon is burned per mole of aluminium: 4.97 × 0.75 × 12.01 = 44.8 g of anode carbon in that hour.
Common mistakes:
Using 2 or 1 electrons instead of 3. Aluminium is +3.
Leaving time in minutes or hours. Q = It needs seconds.
Forgetting that real cells are not 100% efficient. This calculation is a ceiling, not a prediction.
For more practice of this kind, see our post on electrolysis and Faraday's laws. If you keep getting the electron count wrong, the Fermi Problem Library has a step-by-step solver that explains the reasoning at each stage.
What happened next?
Beck records that the Pittsburgh Reduction Company was started on 18 September 1888, with Hall and its first employee building the first commercial pilot plant; it later became the Aluminum Company of America (Alcoa). World production was about 4,682 tonnes in 1900, and 49.7 million tonnes in 2013. Hall left most of his estate to charity, including five million dollars to Oberlin College.
On the syllabus: the separate metallurgy chapter is not in the current rationalised NCERT Class 12 book, so the process now appears through that one line in Electrochemistry. Check the current year's syllabus for your exam.
What can a student take from Hall and Héroult?
Change the conditions when the first route fails. Water did not work, so Hall moved to molten salts.
Fix one problem at a time. A graphite lining and added aluminium fluoride turned a grey smear into metal.
Keep dated records. Hall's postmarked letters decided a patent dispute.
Work out tonight how long a 400 A current would take to deposit 1 kg of aluminium, and check your answer is about 7.5 hours.
