Aluminum: Common Metal, Uncommon Past

Brooch made of aluminum and gold. Courtesy of the Cooper-Hewitt National Design Museum, Gift of Mrs. Gustav E. Kissell, photo by John Parnell.

By Tom Geller

In the mid-1800s aluminum was more valuable than gold. Napoléon III's most important guests were given aluminum cutlery, while those less worthy dined with mere silver; fashionable and wealthy women wore jewelry crafted of aluminum. Today aluminum is a critical component of modern life, found in airplanes, automobiles, soft drink cans, construction materials, cooking equipment, guardrails, and countless other products. The difference between scarcity and abundance (and between obscurity and ubiquity) of this metal depended solely on scientists' ability to find the way to release it—the third most common element in the earth's crust by weight—from its ore.

The most familiar story of the first extraction of aluminum is that the youthful Ohioan Charles Martin Hall developed aluminum's electrolytic extraction process in his family's woodshed in 1886, patented the invention, helped found the company that would later become Alcoa, and died a rich man. A more complicated version reveals that Paul Héroult developed a similar process in France at the same time. In reality both Héroult and Hall were participants in a much larger program of aluminum research that started in the 1850s and lasted until 1903, when the last major patent dispute was settled. By then Alcoa was the undisputed world leader in aluminum production, and Hall himself was a multimillionaire. But neither Hall nor Héroult operated in a vacuum—their nearly simultaneous discovery of a process for aluminum extraction built on several decades' worth of electrochemistry and, indeed, centuries' worth of knowledge on the nature of metals.

Early History

While aluminum metal is a recent discovery, its compounds were fairly common in various industries throughout history. Alum (aluminum potassium sulfate, KAl(SO₄)₂ ), was best known as a dye fixer (or mordant) first developed in Egypt over 5,000 years ago, and clays containing aluminum silicates appear to have been favored by contemporary Persian potters for their strength. Anhydrous aluminum sulfate
(Al₂(SO₄)₃) was used by the ancient Greeks as an astringent to stanch wounds—a use that continues to this day in styptic pencils.

Electrolysis, a process central to the modern history of aluminum, has its roots in the early 19th century. In 1800 the Italian Alessandro Volta invented the "pile" battery, which provided the source of stored power that pioneering Englishmen William Nicholson and Anthony Carlisle used to break a compound (water) into its constitutive elements through a process known as electrolysis. Generally defined, the process involves applying live electrodes to a liquid containing the compound to be electrolyzed. The negative electrode in electrolysis, the cathode, naturally attracts positive ions, which take on electrons; the positive electrode, the anode, attracts negatively charged ions. When water is subjected to electrolysis, hydrogen gas is produced at the cathode and oxygen is released at the anode.

The remarkable Cornish chemist Humphry Davy also started experiments in electrolysis in 1800. He struggled to isolate metals by putting a current through solutions of their alkali salts, which did nothing more than free hydrogen. But he met with much better results when he started to electrolyze molten compounds, first isolating potassium from potash and sodium from table salt in 1807. The following year Davy used electrolysis to produce elemental calcium, strontium, barium, and magnesium before capping off his remarkable string of success with the identification and naming of aluminum. He did not actually isolate aluminum; rather, as Norman C. Craig, professor emeritus of chemistry at Oberlin College, explains, "Davy had learned enough about compounds of other metals to conclude from the composition of aluminum compounds that they contained a new metal, aluminum." He first called the metal alumium, although it has evolved to aluminium in most English-speaking countries, and to aluminum in the United States. One of early chemistry's true geniuses, Davy was knighted and received a baronetcy in 1812 and became president of the Royal Society in 1820. (The society has awarded an annual "Davy Medal" in his honor since 1877.) Nevertheless, his repeated attempts to isolate aluminum metal met with no success before his death in 1829.

The honor of first producing elemental aluminum went instead to Hans Christian Ørsted, a professor at the University of Copenhagen, who in 1825 produced a tiny amount of the metal by rapidly heating aluminum chloride (AlCl₃) with potassium amalgam (an alloy of potassium and mercury) and then distilling off the mercury. Unfortunately, Ørsted's process produced too little aluminum to perform even the most basic analysis, and his experiment was difficult to reproduce. The German chemist Friedrich Wöhler tried again in 1827, one year before he pioneered the field of organic chemistry by synthesizing urea. While his aluminum experiment did not produce the lump metal he desired, he did obtain an impure aluminum powder after substituting metallic potassium for Ørsted's potassium amalgam. And there the matter rested until 1845, when Wöhler produced "gray metallic powder . . . [with] small tin-white globules [of aluminum], some as large as pins' heads," by heating potassium and aluminum chloride together in a closed system, thereby excluding the moisture that had been diverting aluminum into aluminum hydroxide (Al(OH)₃).

To Electrolysis and Back Again

By the mid-1850s battery technology had improved in output and reliability to the point that the first electrolytic production of aluminum was possible. Aided by this advance, and foreshadowing Hall's and Héroult's twinned, simultaneous discovery 32 years later, the first electrolysis of aluminum was also developed independently by two parties.

The first researcher to claim to produce elemental aluminum by electrolysis was the German Robert Wilhelm von Bunsen, who by coincidence had taken Wöhler's place as a chemistry teacher at the Higher Polytechnic School at Kassel in 1836. A man of wide-ranging interests, Bunsen ultimately became famous for developing the spectroscope and for the use of iron-oxide hydrate as an antidote to arsenic poisoning. (Curiously, he did not invent the burner that carries his name; that was the work of his assistant Peter Desaga, who improved on a design by Michael Faraday.) In 1841 Bunsen improved on an 1839 battery design by William Robert Grove, who a few years later also produced the first hydrogen-oxygen fuel cell. Bunsen lowered the cost of Grove's battery by replacing the platinum cathode with a more cost-effective carbon one inside the battery itself. With these batteries he started experimenting with electrolysis, producing pure chromium, magnesium, manganese, sodium, barium, calcium, and lithium, in addition to very small amounts of what he believed to be aluminum in 1854. But he then moved on to other areas of interest, publishing his important paper on emission spectroscopy in 1860.

The second person to experimentally reduce aluminum ions to metal by electrolysis was the Antilles-born Frenchman Henri Sainte-Claire Deville, who presented his findings on electrolytic production to the French Académie des Sciences in 1854, a week after Bunsen published his results. His work attracted the attention of Napoléon III, then titled "Emperor of the French," who was interested in the metal as a source of military armor. With Napoléon III's mandate, Deville quickly realized that the cost of zinc for anodes in the Bunsen cells he used was too high to efficiently produce aluminum through electrolysis. Instead he lowered the cost by returning to chemical methods, replacing Wöhler's potassium with sodium—that is, AlCl₃ + 3Na → Al + 3NaCl. Through this process he was able to obtain enough aluminum to produce marble-sized blobs. In 1855 he displayed an ingot of comparatively pure aluminum at the World's Fair in Paris, to great popular interest. Because the Deville process was deemed "good enough," most scientists then set aside experiments on electrolytic production of aluminum.