The Challenge
Hall took his first formal course in chemistry as a junior in college. Earlier, with Jewett's guidance and encouragement, he had worked on aluminum chemistry and other projects in Jewett's laboratory and in his own laboratory at home. He heard Jewett lecture on the chemistry of aluminum, display his sample of the metal, and predict the fortune that awaited the person who devised an economical method for winning aluminum from its oxide ore. To a fellow student, Hall declared that he intended to be that person.

After many unsuccessful experiments with chemical methods of reducing aluminum ores to the metal, Jewett and Hall turned to electric current to provide the powerful reducing conditions that were needed. To obtain electricity in a college town in the 1880s, one had to assemble batteries. Hall and Jewett used Bunsen�Grove cells, which consist of a large zinc metal electrode in a sulfuric acid solution that surrounds a porous ceramic cup containing a carbon rod immersed in concentrated nitric acid. Assembling enough of these cells to provide sufficient electrical energy for aluminum production was a large undertaking. The eventual laboratory process used about one pound of zinc electrodes, hand cast by Hall, to obtain one ounce of aluminum.

Hall did the first experiments with electricity in Jewett's laboratory during his senior year of 1884�85. He prepared aluminum fluoride from hazardous hydrofluoric acid in special lead vessels, and he passed a current through aluminum fluoride dissolved in water. Unfortunately, this system produced only unwanted hydrogen gas and aluminum hydroxide at the negative electrode.

After graduation, Hall continued the work in the woodshed behind his family's house. He experimented with molten fluoride salts as water-free solvents. He knew that the fluoride salts had the advantage over previously studied chloride salts of not absorbing water from the air. Hall was aware of Richard Grïtzel's success in obtaining magnesium metal by using an electric current in a magnesium chloride melt as reported in the Scientific American in 1885.

To work with molten fluoride salts, he needed a furnace capable of producing and sustaining higher temperatures than the coal-fired furnace of his earlier experiments. For this purpose, Hall adapted a second-hand, gasoline-fired stove to heat the interior of a clay-lined iron tube. Despite the higher temperature of this furnace, he was unable to melt calcium, aluminum, or magnesium fluorides. Potassium and sodium fluorides melted but did not dissolve useful amounts of aluminum oxide.

Hall moved on to experiment with cryolite (sodium aluminum fluoride) as a solvent. He made cryolite, found that it would melt in his furnace, and showed that it would dissolve more than 25% by weight of aluminum oxide. The melting point of cryolite is 1000 ¯C, an exceptionally high temperature for electrochemistry. He did this crucial experiment early in February 1886 and repeated it the next day for his sister Julia to witness.

Six days later, Hall first attempted to prepare aluminum metal by passing an electric current through a solution of aluminum oxide in molten cryolite. He immersed graphite rod electrodes into the fiery solution in a clay crucible and let the current run for a while. In Julia's presence, he poured the melt into a frying pan and broke apart the cooled mass but found no aluminum. There was only a grayish deposit on the negative electrode, a deposit that did not have the shiny metallic appearance of aluminum. After repeating this process several times, Hall realized that this deposit was probably silicon from silicates dissolved out of the clay crucible. If he had not been acquainted with the appearance of metallic aluminum from seeing Jewett's sample, Hall might have been slower to interpret this false result.

Success
From a large graphite rod, Hall made a graphite crucible to line the clay crucible. He also lowered the melting point of the cryolite solution by adding aluminum fluoride. The first experiment with this new system was performed on February 23, 1886. The electric current ran for several hours, and once again he cooled the melt and broke it open in the presence of his three sisters and father. This time they found several small silvery globules, which he tested with hydrochloric acid. He took them to Jewett, who confirmed that they were aluminum.

On July 9, 1886, Hall applied for a patent. Meanwhile, Paul L.T. H¸roult was granted a French patent on April 23, 1886, for a comparable process based on cryolite and aluminum oxide; he had also applied for a U.S. patent in May. This meant that Hall had to prove that he had made aluminum by the new method before the date of the French patent to obtain patent protection in the United States. Evidence from his family and Jewett, including two postmarked letters to his brother, George, helped to establish the priority of Hall's discovery in the United States in a ruling made by the Patent Examiner. Hall's patent rights were also upheld in two subsequent legal struggles with the Cowles Electric Smelting Co. of Cleveland, Ohio, which made copper�aluminum alloy.

Simultaneous Discoveries
How could it be that Paul H¸roult in Paris, France, and Charles Hall in Oberlin, Ohio, made nearly simultaneous, yet independent discoveries of the same process of refining aluminum? Many factors seem to have contributed. Finding an economical process for refining aluminum was widely recognized as a prime target for inventors. Electrochemistry had begun to mature as an applied science. Large electricity-generating dynamos were being developed commercially. Interest had been aroused in the chemistry of fluorine-containing substances. Although Hall was working in a small U.S. college town, he had access to the latest in scientific thought with Jewett as his mentor. Proximity to Cleveland and its emerging technical industries, such as Standard Oil for gasoline, Brush Electric for large graphite rods, and Grasselli for chemicals, was also a contributing factor.

Hall, like H¸roult, was a resourceful experimentalist, who not only devised a method of making aluminum metal, but made most of his apparatus and prepared many of his chemicals. Like H¸roult, Hall had a burning desire to be a successful inventor and industrialist. In recognition of the contribution these two young men made to the development of this electrochemical process on both sides of the Atlantic, it is now called the Hall�H¸roult process.