Charles Martin Hall, American chemist, who discovered an inexpensive electrochemical method for the isolation of pure aluminum from its compounds.

Discovered in 1827 by Friedrich Wöhler, aluminum, though the most common metal on earth, is always found tightly locked in compounds. Efforts to use electrolysis to reduce it failed repeatedly, and for years it remained an exotic metal used in jewelry and for such special purposes as capping the Washington Monument. The race for a commercially viable route to aluminum was won in 1886 by two young men working independently—Paul Héroult (1863–1914) in France and Charles M. Hall (1863–1914) in the United States.

Charles Martin Hall was born December 6, 1863 in Thompson, Ohio. He was son of Rev. Heman Bassett Hall (1823-1910, A.B. 1847, B.D. 1850, A.M. 1866) and Sophronia H. Brooks Hall (1827-1885, Class of 1850, Lit. Course).
 

Charles M. Hall at the age of twenty-two

In 1873 the Hall family moved to Oberlin, Ohio, where Charles Martin Hall took his preparatory work in Oberlin High School. His high school education was supplemented by one year in the Oberlin Academy including lessons in the Conservatory of Music. He enrolled in Oberlin College in 1880, and graduated with a bachelor of arts degree in 1885. Oberlin College awarded him the honorary A.M. in 1893, and honorary L.L.D. in 1910. He was a member of the Oberlin College Board of Trustees from 1905 to 1914. While a student at Oberlin (Ohio) College Hall became interested in chemistry, and more specifically in finding an inexpensive method for producing aluminum. Hall was influenced by his college chemistry professor, Frank Fanning Jewett (1844-1926), who challenged and encouraged Hall in his ongoing scientific experiments. Jewett is popularly credited with turning Hall's attention to aluminum through a classroom challenge. However, this story appears to contain more myth than fact.

Hall entered Oberlin College in 1880 and met Jewett during that first fall when he went to the laboratory in Cabinet Hall to buy some glass apparatus and chemicals. During the year Jewett and Hall talked about Hall's interest in developing a new process for extracting aluminum. Hall did not take a formal course in chemistry until his junior year (1882-83), the standard time for such study in the 1880s. He was not on the rolls of the college for the 1883-84 year and may have spent much of that year working in Jewett's laboratory or his own home laboratory. In the chemistry class Hall heard Jewett's talk about aluminum and the challenge to find an economical method for preparing this element. Jewett said, "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 said to some fellow students, "I'm going for that metal." Not only did Jewett challenge Hall, but he also supplied laboratory space, materials, and up-to-date chemical knowledge. From his experience in mineralogy Jewett may have provided the crucial idea of using cryolite, as one writer has suggested.

Professor Jewett played a crucial role as Hall's mentor. For his day Jewett was exceptionally well educated in chemistry and exceptionally well travelled. He had received an undergraduate degree from Yale in 1870 and had returned to Yale for graduate work in chemistry at the Sheffield Scientific School. He then studied at Göttingen in Germany, as was essential for the best training in science at the time. There he met Friedrich Wöhler, one of the two scientists first to isolate aluminum metal in the 1820's. From Wöhler, Jewett learned directly about aluminum and was stimulated to obtain a small sample of this metal. Jewett returned to the U.S. to work for a year as a private assistant to Wolcott Gibbs at Harvard. Then, after nomination by the president of Yale, he taught chemistry for four years (1876-1880) at the Imperial University of Tokyo, Japan. He assumed the position of Professor of Chemistry and Mineralogy at Oberlin in 1880.
 

After graduation from Oberlin in June 1885 Hall continued the work begun with Jewett. Working in his Oberlin woodshed laboratory with encouragement from his older sister Julia Brainerd Hall (1859-1926), he pursued the idea that aluminum could be reduced from its ores through electrolysis. Hall was aware of Richard Grätzel's success in obtaining magnesium metal by using an electric current in a magnesium chloride melt as reported in the Scientific American in 1885.

On February 23, 1886 in the woodshed behind his family's home at 64 East College Street in Oberlin, Charles Martin Hall produced globules of aluminum metal by the electrolysis of aluminum oxide dissolved in a cryolite-aluminum fluoride mixture and repeated this esperiment the next day for his sister Julia to witness. This achievement was the culmination of several years of intensive work on this problem.

At the time of Hall's discovery the only practical way to make aluminum metal was through the chemical reduction of anhydrous aluminum chloride by sodium metal at elevated temperatures. The reaction is

This process was a costly one owing to the difficulty of preparing water-free aluminum chloride from aluminum oxide, the natural source of aluminum, and to the necessity of making sodium metal by chemical means. (Reaction of heavy oil [~(CH₂)_n] and Na₂CO₃ at temperatures above the boiling point of sodium metal, 892 degrees Celsius.) In the early 1880's aluminum was a semiprecious metal, but Jewett had a sample of it to show his students.

Not only did Hall have to devise a method for winning aluminum metal, but he also had to fabricate most of his apparatus and prepare his chemicals. In his early experiments he tried to adapt the high temperature carbon reduction methods that were used in the metallurgy of iron and other metals of intermediate chemical activity. He also tried to reduce the aluminum in cryolite with sodium metal. In subsequent experiments Hall, working in Jewett's laboratory in Cabinet Hall, showed that the electrolysis of aluminum fluoride in water gave only aluminum hydroxide. They understood that electrolysis provided more powerful reduction conditions than did chemical methods. Today, it is well known that hydrogen in water is more easily reduced than is aluminum ion. The cathodic half reaction in the presence of aluminum ion is

The selection of aluminum fluoride for this experiment was probably a turning point in Hall's work. Using this substance was certainly not a matter of convenience because he had to prepare it from hazardous hydrogen fluoride in special lead vessels in Jewett's laboratory. Most likely Hall and Jewett chose to try the fluoride because it had not been tried before. No doubt earlier experimenters had shown that electrolysis of aqueous solutions of aluminum chloride and aluminum oxide dissolved in acid did not yield metallic aluminum.

Having shown that an aqueous system was useless for the preparation of aluminum by electrolysis, Hall turned his attention to the possibility of using water-free fused salts as solvents for aluminum oxide. (Hall knew that Graetzel had prepared magnesium metal by electrolysis of fused magnesium chloride. (Scientific American, fall 1885.)) But first, he had to build a furnace capable of producing and sustaining higher temperatures than the coal-fired, bellows-driven furnace that he had used in earlier experiments. For this purpose he adapted a second-hand, gasoline-fired stove to heat the interior of a clay-lined iron tube. (Gasoline was available in the Cleveland area because J. D. Rockefeller was in the early years of developing Standard Oil there.) Despite the high temperature in this furnace he was unable to melt the first substance he tried, the mineral fluorspar, which is calcium fluorite (m.p. 1360 degrees Celsius). He then synthesized and tried potassium fluoride (m.p. 846 degrees Celsius), sodium fluoride (m.p. 988 degrees Celsius), magnesium fluoride (m.p. 1266 degrees Celsius) and aluminum fluoride (sublimation point 1291 degrees Celsius). The potassium and sodium fluorides melted in the furnace but did not dissolve useful amounts of aluminum oxide. He was unable to fuse magnesium fluoride or aluminum fluoride.

Hall moved on to experiments with the double fluoride of sodium and aluminum, which was formulated 3NaF^.AlF₃ in his day and was known as the mineral cryolite. He knew that this material was available from natural sources. No doubt he also knew that mixtures of salts commonly had lower melting points than the higher melting of the two. Today, this substance is written with a formula of Na₃AlF₆. It is understood to be an ionic compound containing sodium ions and hexafluoroaluminate ions, AlF₆^3-. Hall synthesized his cryolite. He also had to prepare aluminum oxide. He did so from alum, KAl(SO₄)₂^.12H₂O, which was a common household substance in his day. He dissolved alum in water, precipitated aluminum hydroxide by adding washing soda (Na₂CO₃), another common household substance, filtered off the hydroxide, and dried it. Hall's older sister, Julia Hall, who had studied chemistry in college, followed the experiments closely and probably helped prepare some of the aluminum oxide. He melted cryolite (m.p. 1000 degrees Celsius) in the furnace and quickly found that it was a good solvent for aluminum oxide. He did this signal experiment on February 9, 1886 and repeated it for his sister to see when she returned the next day from a visit to Cleveland.
 

Julia Brainerd Hall, Charles's sister, who made many contributions to the discovery of the electrolytic means of producing aluminum and to the business success of the Pittsburgh Reduction Company and the Aluminum Company of America. Julia had also attended Oberlin College, taking most of the same science courses.

To do electrolysis in the 1880's most people had to make batteries. Hall and Jewett made cells of zinc in dilute sulfuric acid and graphite in concentrated nitric acid. (Such cells, known as Bunsen batteries or Bunsen-Grove cells, were commonly used in electrolysis experiments before the advent of motor generators.) For Hall's experiments this was a large undertaking. Due in part to inefficiencies in the process, about 5 moles of zinc metal (~300 g) would be consumed in making one mole (~30 g) of aluminum metal. Though a good source of high current, the Bunsen battery emits noxious fumes of nitrogen oxides.(Jules Verne equipped the Nautilus with Bunsen batteries in "20,000 Leagues Under the Sea." Captain Nemo and his crew must have had leather-lined lungs.)

Hall's first attempts to prepare aluminum metal by electrolysis were carried out on February 16, 1886. He used graphite-rod electrodes dipping into a solution of aluminum oxide in molten cryolite held in a clay crucible. Because he observed gas formation around the positive electrode (anode), Hall was confident that electrolysis was occurring. However, when Charles cooled the melt and broke it open in Julia's presence, they found only a grayish deposit on what had been the negative graphite electrode (cathode). He repeated this experiment several times over the next few days. Finally, he recognized that the new material, which did not have the shiny metallic properties of aluminum, was probably silicon, which is a metalloid. Suspecting that this silicon had its origin in the silicates in the clay, he decided to fabricate a graphite crucible to use as a liner for the clay crucible. This graphite crucible was only 2 inches wide and 4 inches deep. (Because Brush Electric was developing carbon arcs for street lighting in Cleveland, Hall probably obtained his large graphite rod rather easily.) The first electrolysis experiment with this system, in which he had added some aluminum fluoride to lower the melting point of cryolite, was performed on February 23, 1886. The electric current ran for several hours. When Charles cooled the melt and broke it open in Julia's presence, they found several silvery buttons of aluminum. As soon as possible, Charles took the buttons to Professor Jewett, who confirmed that they were aluminum.

If aluminum oxide breaks, at least in part, into ions in molten cryolite, then the half reactions for the electrolysis process can be written as:

At the anode, graphite is consumed, and carbon dioxide is formed. At the cathode, liquid aluminum is formed. The liquid aluminum collects in the bottom of the crucible. In the fully developed method the heat to maintain the cryolite in the molten state is generated by the electrical resistance of the electrolyte.

The cryolite solvent that Hall found for his process has some fortunate properties in addition to being a solvent for aluminum oxide. Being ionic, cryolite is a good conductor of electricity. Although the density of solid cryolite is greater than the density of solid aluminum at room temperature, the density of molten aluminum is greater than that of molten cryolite at the electrolysis temperature. As a consequence, liquid aluminum metal collects in the bottom of the electrolysis vessel where the aluminum is protected from being reoxidized by oxygen in the atmosphere.
 

On July 9, 1886, Hall filed a patent for "The Process of Reducing Aluminum by Electrolysis." In July 1888 his application was found to be in interference with the application filed April 23, 1886 by Paul L.T. Heroult (1863-1914) of France. Héroult was the same age as Hall. He had the advantage of a motor generator to support his experiments in using large electric currents to do metallurgy. Héroult did not pursue the scale-up and commercialization of the aluminum-making process in Europe until Hall had done so in the U. S. Independently the two inventors made the same discovery at virtually the same time. Under United States patent law patent rights rested on proof of the date of discovery. Through evidence and testimony Hall was able to establish priority and was awarded patent rights in The United States.

Simultaneous Discoveries

How could it be that Paul Héroult 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 Héroult, 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 Héroult, 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–Héroult process.
 

Hall was as adept in overcoming the obstacles to the commercialization of his new electrolytic process as he was in discovering it. Within three years he and his partners were making aluminum metal in quantity at the newly formed Pittsburgh Reduction Company, the predecessor of Alcoa. Hall negotiated an option with the Cowles Electric Smelting and Aluminum Company, headquartered in Cleveland, Ohio. Cowles manufactured and sold aluminum and copper alloys produced by the electric arc smelting process. The option allowed Hall to develop the commercial feasibility of his process and gave Cowles the chance to purchase his rights. Frustrated by an apparent lack of support, Hall left the Cowles Company in July 1888.

Through Romaine Cole, a sympathetic salesman at Cowles, Hall met the noted metallurgist Captain Alfred E. Hunt (1855-1899) in Pittsburgh, Pennsylvania. Hall and Cole produced a 22-page document on the advantages of the "Hall Process," which they presented to Hunt. On the basis of that document, Hunt became one of the principal investors in the Pittsburgh Reduction Company formed in 1888.

Hall continued his developmental process, now more favored by supportive backing and adequate electrical power. The pilot plant for the Pittsburgh Reduction Company opened in Pittsburgh in September, 1888. Arthur Vining Davis (1867-1962) was hired to assist Hall. By Thanksgiving Day 1888 they succeeded in producing limited amounts of pure aluminum. The pilot plant gave way to new production facilities, based now on the internal resistive heating method which capitalized on the increased availability of electricity. Plants were established in Niagara, New York and New Kensington, Pennsylvania. The Mellon Bank of Pittsburgh was a major investor in the expansion.

In 1891, the Pittsburgh Reduction Company filed suit against the Cowles Electric Smelting and Aluminum Company on the grounds of patent infringement. Judge William Howard Taft (1857-1930) ruled in favor of Hall and the originality of his invention in 1893. Cowles appealed the decision and filed a new motion on the basis of their ownership of technology provided in the patents of Charles Bradley, which appeared to anticipate the internal resistive heating method. In 1911, after lengthy proceedings Cowles was awarded damages for infringement by the Pittsburgh Reduction Company on portions of the Bradley Patents.

In 1907 the Pittsburgh Reduction Company was renamed the Aluminum Company of America, later shortened to ALCOA. In 1890 he became its vice president. By 1914 his process had brought the cost of aluminum from twelve dollars per pound down to 18 cents a pound. Hall was a generous benefactor of his college, bequeathing Oberlin more than $5,000,000.These interests were manifested in his collection of oriental rugs and porcelain, and enjoyment of music. The success of ALCOA following the Cowles settlement allowed Hall to return to his interest in music and art.Hall also remained active in research and development, a passion which for him never ended. He filed several new patents for improvements in the production of aluminum, including one registered four years after his death. In 1911 he was awarded the prestigious Perkin Medal for outstanding achievement in applied chemistry.

Drawing of the original Hall electrolytic cell set-up in the Pittsburgh Reduction Company plant, which shows the cast iron cubicles or "pots"; the carbon anodes suspended by copper rods from an overhead copper support; and, on the floor, ingot molds.
 
 

Charles Martin Hall was a generous benefactor of Oberlin College. During his lifetime he made several direct gifts to the College relating to his personal interests such as the care of the campus and grounds. From his estate the college received over $10 million dollars for the endowment of Oberlin College, and left money for the erection of an auditorium in remembrance of his mother, Sophronia Brooks Hall. Others benefited from his legacy as well, including Berea College, the American Missionary Association, and educational programs in Asia and the Balkans, including the Oberlin Shansi Memorial Association and the Harvard-Yenching Institute. Aluminum statue of Charles Martin Hall located in the Kettering Hall of Science.

A recreation of the Hall woodshed is associated with the Jewett house. This house is part of the Oberlin Historical Society site on Professor Street just south of the Conservatory. A lecture-demonstration recreating Hall's original laboratory process was held in Oberlin in 1986 on the centennial of the discovery.

Hall spent the rest of his life developing both his process and the aluminum industry. In 1911 he was awarded the Perkin Medal for his work. Hall died in Daytona Beach, Fla., on Dec. 27, 1914.

Today the electrolytic reduction of aluminium is essentially the same process as discovered by Hall and Heroult. The electrolysis takes place in a steel vessel called a cell . The cell is lined with carbon and contains a meltor electrolyte of molten cryolite maintained at a temperature of about 965 C. Carbon blocks are suspendedabove the cell and partially immersed in the electrolyte to act as anodes, while the carbon lining of the cell acts as a cathode. Alumina is fed into the electrolyte and separates into positively-charged ions of aluminium and negatively-charged ions of oxygen. The direct current applied across each cell moves the ions in opposite directions . The oxygen rises to the anode, where it burns the carbon to form carbon dioxide. The positive aluminium ions are drawn to the negative cathode, where they lose their charge to form aluminium. Due to its higher specific gravity, the molten aluminium once separated from the oxygen settles at the bottom of the cell . At regular intervals this is extracted - or tapped - using a vacuum crucible. To sustain the electrolytic process, alumina is fed into the cells continuously to maintain a sufficient amount of dissolved alumina in the electrolyte. As the carbon anode is gradually consumed during the process, it is lowered to maintain the optimum distance between the anode and cathode surfaces, until it is burnt away and replaced.

This text has been compiled from the biographies of Hall available in the Internet: (1, 2, 3, 4, 5).
See also History of aluminium available in the Internet.