July 9, 1886

Hall–Héroult process

Charles Martin Hall files his first U.S. patent application for electrolytic aluminum production

Hall's claim to priority rested on two postmarked letters to his brother and testimony from the family members who had watched him work.

The filing

On 9 July 1886, Charles Martin Hall filed a United States patent application covering an electrolytic process for producing aluminium from alumina dissolved in a fused fluoride bath.

It followed his successful experiment of 23 February in Oberlin, Ohio, in which an electric current passed through alumina dissolved in molten cryolite produced small aluminium globules. His former professor Frank Fanning Jewett confirmed that the silvery globules were aluminium.

A race he had to prove he had won

The application ran into Paul Héroult's comparable French patent, No. 175,711, filed on 23 April, and Héroult's own American application in May. Hall therefore had to establish that he had reduced his invention to practice first.

He prevailed in the United States by producing witness testimony — his sister Julia Brainerd Hall had watched the experiments — and postmarked letters showing successful work before Héroult's filing date.

A filing, not a discovery or a factory

No patent issued that day. The application matured into U.S. Patent 400,664, granted on 2 April 1889.

Nor did it make aluminium cheap immediately: commercial scale-up, improved cells, reliable dynamos, investment and later hydroelectric power produced the price decline, with pilot commercial production beginning in 1888.

The American Chemical Society reports that Hall's improvised laboratory consumed about a pound of zinc electrodes to obtain one ounce of aluminium.

Sources

Researched 24 Aug 2026 7 sources not yet audited

How this was checked

Researched from the web into a fact sheet and rewritten from that sheet. This article has not yet had an independent model audit. How the pipeline works →

What the sources leave uncertain

The July 9 filing date is independently supported by the patent record and the American Chemical Society.
“Hall–Héroult process” is a retrospective joint name recognizing independent work. Hall’s and Héroult’s original apparatus and patent claims were comparable but not identical, and both evolved during commercialization.
The event is patent filing, not discovery or industrial production. Hall’s breakthrough was February 23; pilot commercial production began in 1888.