Discovery and Characterisation

It was this black residue that greatly intrigued Smithson Tennant shortly after his entry into partnership with William Hyde Wollaston in 1800. The object of this famous enterprise was the production and marketing of platinum in quantity and it was soon decided that while Wollaston should pursue the study of the aqua regia solution of native platinum (so discovering palladium and rhodium) Tennant should concentrate on the insoluble matter. By the summer of 1803 he had begun his investigation, fortunately mentioning to Sir Joseph Banks, President of the Royal Society, his observation that the black powder “did not, as was generally believed, consist chiefly of plumbago but contained some unknown metallic ingredients”. Fortunately, because work on the same problem was being undertaken almost simultaneously in Paris, first by the director of the École des Mines, Hippolyte Victor Collet-Descotils, and a little later by Antoine François de Fourcroy and Nicolas Louis Vauquelin at the Muséum d’Histoire Naturelle (2, 3). All three found that the black residue contained a hitherto unknown element but they neither isolated it nor gave it a name.

Smithson Tennant, who had the advantage of a much greater amount of residue from Wollaston’s work on platinum, realised that while the Frenchmen suspected the presence of one new metal in the black powder, there were in fact two, namely osmium and iridium, and on 21st June 1804 he presented his paper, a masterpiece of clarity and conciseness, to the Royal Society. For this he was awarded the Copley Medal for that year, the highest honour in their gift, while a little later the French workers fully accepted the priority of his discoveries (4).

Tennant’s comment on iridium was something of a forecast of the difficulties to come:

Fourcroy and Vauquelin remarked similarly:

Some years later in 1814 Vauquelin wrote of iridium:

First Attempts at Melting

But by this time a new and powerful method of obtaining a high temperature was becoming available. Following Volta’s news of his discovery of the electric pile in 1800, a number of British scientists turned their attention to the construction of large batteries to yield a high voltage. Foremost among them was John George Children, who began to build large batteries in his private laboratory at Ferox Hall near Tonbridge in Kent in about 1806, while as is well known Humphry Davy in October 1807 discovered potassium and sodium by means of a discharge from a battery consisting of 250 plates of zinc and of copper each 6 inches by 4 inches. Davy was friendly with Children and he was present at Tonbridge when in August 1808 a piece of platinum wire was successfully melted by the use of a voltaic battery larger and more powerful than any hitherto constructed (7). This success encouraged Davy to build a still larger battery at the Royal Institution, but Children went on to construct an even larger model in 1813.

In that year Smithson Tennant had been appointed Professor of Chemistry at Cambridge, and while giving his mind to the preparation of his lectures in June he wrote to Berzelius, whose friendship with him arose chiefly from a visit the Swedish chemist had paid to Tennant’s estate in Somerset in 1812, about the choice of a “galvanic machine”. The letter goes on:

On that historic occasion no less than thirty-eight distinguished scientists, including Davy, Wollaston, Hatchett, William Allen, W. H. Pepys, W. T. Brande, Henry Warburton and William Babington, dined together at Ferox Hall, spent the night there and in the morning witnessed the melting of a small piece of iridium. This was achieved by means of “the greatest galvanic battery that has ever been constructed”, consisting of 20 pairs of copper and zinc plates, each 6 feet long by 2 feet 8 inches, suspended from the ceiling and then lowered into an enormous tank containing 945 gallons of dilute nitric and sulphuric acids (9).

Children was not yet satisfied, however, that he had really achieved the melting of a pure specimen of iridium, and early in 1815 he again set about its fusion, holding the metal in a cavity in a piece of charcoal floating on mercury. He had now, acting on a suggestion made by Wollaston, added a second copper plate to each cell so that each zinc plate was now faced by two of copper. This materially increased the power of the battery, and he then recorded in his paper to the Royal Society, read on 15th June, 1815:

This great occasion was greatly saddened for those present, however, by the absence of Smithson Tennant who, returning from a five-month tour of France in the February, had met with a fatal accident while on a riding excursion near Boulogne.

No further attempts to melt iridium are recorded for quite a long period, until, in fact 1842, when Robert Hare, Professor of Chemistry in the University of Pennsylvania, reported his success in this direction to the American Philosophical Society. Forty years earlier Hare had devised a hydrogen-oxygen blowpipe with which he had melted platinum, for the first time in reasonable quantity, and over the years he had gradually brought about improvements in the apparatus. His paper opens:

Unlike Children’s specimen, Hare’s iridium gave specific gravity values of 21.83 and 21.78, amply confirming his claim to high purity. Of its physical characteristics he wrote:

Now Hare had obtained some of his supplies of iridium from Johnson and Cock (as Johnson Matthey was known from 1837 until 1845) and although no published paper exists to support the contention, it is virtually certain that, as will be seen a little later, as early as 1837 iridium had been melted in the Hatton Garden refinery. In any case by 1846, when Michael Faraday was studying the magnetic properties of a wide range of metals and compounds, his records contain the following entry:

But the melting of iridium in any appreciable quantity had to await the work of Henri Sainte-Claire Deville and his partner Jules Henri Debray in Paris in the late 1850s. Their design of a furnace constructed from two cylindrical, hollowed out blocks of lime and fired by coalgas and oxygen was first used to melt platinum on a large scale in 1857 (13), and the British rights in their patent were at once acquired by Johnson Matthey. A little later the Russian government, impressed with their new procedure for refining native platinum, sponsored Deville and Debray by setting up for them larger furnaces and oxygen plants. From the Russian Imperial Mint they were supplied, in February 1860, with some eight kilogrammes of crude iridium-containing material among other residues, and “after three and a half months of incessant work” they were able to return large quantities of platinum ingots and sheet, together with an ingot of iridium weighing 1.805 kilogramme (14). To achieve this fusion they had to consume, for one kilogramme of iridium, more than 300 litres of oxygen and pure hydrogen, coal gas being useless for their purpose.

Johnson Matthey similarly put the process into effect, and were able to show in the International Exhibition in London in 1862 a melted ingot of iridium among their numerous other exhibits of the platinum metals. The report of the Juries contains the following item:

In 1879 George Matthey presented a detailed account of the refining of the platinum metals in the course of a paper to the Royal Society (15) of which he was shortly afterwards elected a Fellow. His procedure for the separation, refining and then melting of iridium was “an operation of extreme delicacy” and much too lengthy for reproduction here, but some measure of his achievement may be gained from his statement that while the metal should, if perfectly pure, possess a specific gravity of 22.39, the highest value he had yet been able to attain was 22.38.

The Earliest Applications

Turning now to the earliest applications of iridium we have to retrace our steps to the 1830s and to the ingenuity of an extraordinarily versatile English engineer. This was John Isaac Hawkins, born in Taunton in Somerset in 1772 the son of a watch and clockmaker. At an early age he emigrated to America to study at Jersey College in Pennsylvania—the institution which became Princeton University in 1896—and then turned his mind to mechanical and chemical engineering and also to the improvement of musical instruments, filing a patent in 1800 for the invention of the upright piano, a development that brought him to the attention of President Jefferson. In 1803 he returned to England in order to claim a legacy and here he embarked on a successful career as a patent agent and consultant, his further inventions including a pantograph and then, together with one Sampson Mordan, the use of small particles of “diamond, ruby or other hard substances” as pen points, the nibs then being made of horn or tortoise-shell (16). The advent of the gold nib was also due to Hawkins, who spent many years seeking a suitable material to make a fine and hard point. Then in 1833 he learned that some twelve years earlier Wollaston had provided samples of rhodium and of the native alloy of iridium and osmium to Thomas Charles Robinson, the famous maker of chemical balances and drawing instruments, requesting him to prepare pens with each material. (Robinson, of Devonshire Street, off Portland Place in London, lived quite close to Wollaston and had provided him with a balance incorporating improvements proposed by the purchaser.) Hawkins’ information was that Robinson had duly made a few pens with rhodium points (actually an alloy of rhodium and tin) but had returned the iridium alloy as too hard to be wrought into pen points. This stimulated him to investigate further, and by 1834 he had succeeded in producing the first gold pen, this tipped with two specks of iridium. Wollaston had died in 1828, and Hawkins had therefore to seek other sources of iridium.

In an account of his early efforts, given in 1875 by an American pen manufacturer, John Foley, who had received it directly from the former, the following passage occurs:

Hawkins sold the secret of his technique to an American in 1836, but manufacture of the iridium-tipped pens was continued by Francis Mordan.

In the United States several manufacturers entered the gold pen business, but the one that is remarkable in our present connection was G. W. Sheppard of Detroit who moved in about 1842 to Cincinnati. Some ten years or so later he engaged a boy whom he had noticed working in a neighbouring drug store, one John Holland. Sheppard carried out many experiments with the oxy-hydrogen blowpipe and with electric batteries in attempts to melt iridium but without success. In 1858 Holland became a partner in the business and then, on Sheppard’s death in 1862, the sole owner and manager (18). He continued the experimental work with no greater degree of success until 1880 or thereabouts, when he was faced with a demand for much larger pieces of the iridium alloy for a new type of pen. Simultaneously he observed that a specimen of iron ore very high in phophorus content melted much more readily than other samples and he quickly heated a small piece of iridium to a white heat and then threw into the crucible a piece of phosphorus. When the fumes had cleared away he poured out the contents of the crucible and found “to his joy and amazement a white, compact, hard metal” (19).

For use as pen points this material was perfectly satisfactory, but if other uses were to be found for the metal it was obviously necessary to remove the phosphorus. It was at this point that Holland invoked the help and advice of William Lofland Dudley, Professor of Chemistry at the local medical college in Cincinnati. Dudley tackled this problem with great enthusiasm, quickly removing the phosphorus by repeated heatings with lime, but equally quickly developing an interest in finding new applications for iridium. This enthusiasm led to the formation of the American Iridium Company based upon Holland’s activities but with Professor Dudley as general manager.

The activities of the company received quite a considerable amount of publicity. John Holland had filed an American Patent, No. 241,216 in May 1881 for his “Process of Fusing and Molding Iridium”, while Dudley had contributed a short account of the process to the Journal of the American Chemical Society in the same year, followed by a longer paper given to the Ohio Mechanics Institute that was reproduced in The Chemical News (20). Then in 1883 there appeared a further paper by Dudley with the rather grand title “The Iridium Industry” (21). By now several other uses had been found and are described in the paper. These included draw-plates for the production of wire, tips for hypodermic needles, the knife edges of chemical balances, and the bearings of magnetic compasses. Experiments are also described with an iridium electrode for the arc light, while one of the most important applications was the electrical contact points of telegraphic apparatus, these being found to outlast platinum contacts and not to be subject to sticking. Dudley had also carried out experiments on the electroplating of iridium and claimed that despite the difficulty associated with its insolubility as an anode he had found a suitable electrolyte. In his contribution to the chapter on iridium in the 1883 edition of the United States Geological Survey’s “Mineral Resources of the U.S.” Professor F. W. Clarke of the University of Cincinnati refers to the American Iridium Company and adds that “Professor W. L. Dudley has succeeded in electroplating with iridium, and has obtained excellent results”. The process was later improved by John Holland, who succeeded Dudley in the management of the Iridium Company.

Dudley thus made for himself a considerable reputation as the leading authority of his time on iridium, but his activities in this direction appear to have diminished in 1886 when he was appointed to the chair of chemistry at the Vanderbilt University in Nashville, Tennessee.

There is, however, one interesting sidelight to all this activity in America. In 1885 The Chemical News published a long article in three parts by one Nelson W. Perry dealing in some detail with the work of Holland and Dudley and including a bibliography on iridium (22). This brought a curt letter from Johnson Matthey which included these paragraphs:

In a footnote to this letter the editor of The Chemical News, Sir William Crookes, wrote:

One other reference to the early applications of iridium concerns the work of Thomas Edison in developing the incandescent lamp. Before settling for carbon filaments he had experimented with platinum and iridium-platinum, and in January 1879 he wrote a letter to Johnson Matthey, still preserved in their archives:

Further Applications

Two or three of the modern uses of iridium were first indicated many years ago, but as might be expected these were slow in development. One reasonably obvious application lay in the determination of high temperature by means of thermocouples. The principal couple, still in use today, is of course that devised by Le Chatelier in 1886, platinum versus rhodium-platinum, but clearly this could not be employed to measure temperatures higher than those approaching the melting point of platinum. One of the great pioneers in the measurement of such temperatures was Sir William Chandler Roberts-Austen, Chemist to the Royal Mint and Professor of Metallurgy at the Royal School of Mines, and in the course of one of his numerous lectures, given to the Royal Institution in 1892, he referred to other metals with higher melting points being available:

This problem of measuring very high temperatures was taken up a few years later by Wilhelm Carl Heraeus, the founder of the well-known firm in Hanau. In the closing years of his life he devised an electrical resistance furnace in which the main element was a tube of pure iridium, this furnace being used among others by Nernst to determine high temperatures in 1903. It was also supplied to the Physikalisch-Technische Reichsanstalt for the same purpose, together with thermocouples consisting of pure iridium against 10 per cent ruthenium-iridium (25). This combination was found, over the ensuing years, to require frequent calibration because of the oxidation of ruthenium, and numerous couples were tried based upon combinations of tungsten, molybdenum and tantalum, all of which are of course subject to severe oxidation. Then, in 1933 Otto Feussner, also of the Heraeus company, carried out a thorough investigation of the iridium-rhodium system and proposed a thermocouple of pure iridium against 40 per cent rhodium-iridium, thus enabling temperatures up to 2000°C to be measured in air (26). This combination is still in use at the present time.

Until recently the largest area of growth in the usage of iridium has been for crucibles for growing oxide single crystals for use in computer memory devices and in solid state lasers. The crystals, such as gadolinium gallium garnet and yttrium gallium garnet, are grown by melting pre-sintered charges of mixed oxides under oxidising conditions at temperatures up to 2100°C. However, long before such devices could be imagined or foreseen, one distinguished chemist had discovered the valuable properties of iridium crucibles. This was Sir William Crookes, the founder of The Chemical News and President of the Royal Society from 1913 to 1916, who contributed a paper to that society in 1908, “On the Use of Iridium Crucibles in Chemical Operations” (27). The paper opened:

Crookes, well in advance of his time, went on to report that his iridium crucibles, one of which still survives in the company’s possession, were resistant to the fusion of many fluxes, including caustic soda, and were un-attacked by molten lead, zinc, nickel, iron and gold.

These crucibles were lovingly prepared by Henry Andrew Kent, a man of near genius in things mechanical, who had joined the company in 1875 and who had worked closely with John Sellon in developing the fabrication side of the business. Nowadays the technique of argon arc melting followed by hot rolling makes it possible to produce ductile and malleable iridium in sheet form with a very minimum of impurities. Such sheet can then readily be formed into crucibles of capacities up to 4 or 5 litres.

But the outstanding application of recent years, and one that could never have been envisaged in the wildest dreams of those earlier workers, had to await the development of both nuclear physics and space travel. To provide an independent but reliable source of electric power for space missions radioisotope thermoelectric generators have been used since the Voyager probes were launched in 1977, the fuel consisting of plutonium-238 dioxide connected to thermocouples. The fuel is in the form of spheres and these are encapsulated in iridium to provide a secure shield of exceptionally high melting point and great strength (28). The possibility of this spectacular technique finding other applications is a matter for the future.