Uranium's Scientific History 1789 - 1939

This paper was presented by Dr. Bertrand Goldschmidt at the Fourteenth International Symposium held by the Uranium Institute in London, September 1989. This particular symposium had as its theme the bicentenary of the discovery of Uranium by the German chemist Professor Klaproth in September 1789.

It is a relief and a treat for a Frenchman to be asked in 1989 to speak about a bicentenary other than that of the French Revolution. The bicentenary of the discovery of uranium coincides with the fiftieth anniversary of the discovery of fission, an event of worldwide significance and the last episode in the uranium-radium saga, which is the main theme of this presentation.

The story starts at the beginning of the sixteenth century in the unexplored mountainous region separating Bohemia from Saxony. This area was then covered by an impenetrable virgin forest, a refuge for wolves and bears. Human settlements advanced slowly along small rivers, until the discovery of silver in one of the rivers triggered the first precious metal rush in history. This led to the foundation of the town of Sankt Joachimsthal (the valley of Saint Joachim), soon to become the largest mining centre in Europe with the relatively large population of 20000 inhabitants, at a time when Prague, the capital of Bohemia, had around 50000.

The discovery of silver led to the minting of some two million large silver coins called Joachimsthaler, later known more simply as Thaler. These became a currency accepted worldwide, and their name is, by slight distortion, the origin of the word dollar.

The prosperity of Joachimsthal was short lived, however. By the middle of the sixteenth century, depletion of the mines and a lack of pumping machines needed for deeper mining made it difficult to compete with silver from the new American colonies, which was arriving in increasing quantities on the European market. After plague and destruction from the Thirty Years War, Joachimsthal was almost a ghost town in the first half of the seventeenth century.

However, the mines, which were the property of the Hapsburg crown, never closed. The exploitation of bismuth and cobalt deposits continued, and improvements in mining technology allowed silver production to pick up again during the eighteenth century. But it was a discovery in chemistry which was to give to the town and to the mines a second wind and a new direction.

The miners had long since detected in the mines a shiny black mineral whose presence seemed incompatible with that of silver and had nicknamed it pechblende, from the German words pech, which means either pitch or bad luck, and blende, meaning mineral. The first complete analysis of pitchblende was carried out in 1789 by a talented self-educated German chemist, Martin Klaproth.

Having extracted from pitchblende what he called 'a strange kind of half metal' (he had only isolated its oxide), he resisted the temptation to give his own name to the new element, which was quite customary at the time. It is due to his modesty that we do not today have to use the rather cumbersome name Klaprothium.

He made his discovery during the summer, while the people of Paris were storming the Bastille prison, and his paper was presented to the Berlin Academy of Sciences on 24 September 1789. He proposed, until a better name was found, to call his element after the last planet to have been discovered, thus rendering homage to his compatriot William Herschel. Herschel was a musician who had emigrated to England and had become both the director of the orchestra at the celebrated spa, Bath, and a first class astronomer. His fame was crowned by the discovery of a new planet, named Uranus after Urania, the muse of astronomy and geometry. This muse was thus doubly honoured when Klaproth called the new element 'uran', which in its final form became uranium, a name which is today known worldwide while Klaproth's own fame has faded.

During the following hundred years the history of the new element was not particularly dramatic. It was found in many places in the world, but always in deposits less rich than those at Joachimsthal. Fifteen years after Klaproth's discovery uranium had been found in Cornwall in Britain, in Morvan in France, and in Austria and Romania. By the end of the nineteenth century about 1000 papers had been published on geological and mineral occurences of uranium around the globe.

The metal, as dense as gold, was first prepared, with some difficulty, in 184l by the French chemist Eugène Peligot, using a thermal reaction of tetrachloride with potassium. Later, in 1870, an important fact was established: uranium is the last and heaviest element present on earth. This was demonstrated by the Russian chemist Dimitri Mendeleev in his famous periodical classification of the elements by chemical properties and increasing atomic mass.

For about a century and a half after Klaproth's discovery, the main application for uranium derived from the vivid colours of its oxides and salts. These were used to produce yellow glass with green fluorescence, and glazes for ceramics and porcelain in orange, yellow, red, green and black. Later, uranyl nitrate was used in early photography to give a sepia tint to prints and films, and to reinforce negative plates.

The colouring processes for glass and ceramics were for a long time the jealously guarded monopoly of Bohemia. Thus secrecy appeared for the first time very early in the history of uranium, before returning in full force in the 1940s.

The commercial use of uranium for glass and porcelain colouring resulted in a renewal of activity in Joachimsthal, and soon the production of these coloured uranium compounds became more important than that of silver. In 1855 a plant for the production of large quantities of various yellow and orange compounds was built by the Austrian chemist Adolf Patera, and the export from Bohemia of these expensive materials became an important source of revenue for the local mining industry. It is difficult to estimate with precision the quantity of uranium produced for this purpose during the nineteenth century, but it is probably in the region of 300 to 400 tonnes, of which a small fraction came from Cornwall and later from Portugal and Colorado, USA.

However, by the end of the nineteenth century the Joachimsthal deposits once more ceased to be profitable. The mine shafts were becoming deeper and deeper, while the price of coloured uranium compounds fell on the world market with the production of new colouring materials. The town's mines were once again threatened with closure by their owner, the Austro-Hungarian government. They were unexpectedly saved by a remarkable series of scientific discoveries.

At dusk on the evening of 8 November 1895, Wilhelm Röntgen, professor of physics at the University of Würzburg in Germany, noticed while manipulating a cathode tube that a sheet of paper some distance away impregnated with barium platinocyanid had started to fluoresce. Furthermore, when he put his hand between the tube and the paper, he saw an image of the bones in his hand on the paper.

He was so surprised by this discovery that he decided to study thoroughly the properties of what he later called X-rays before mentioning it to anybody. For seven weeks he pursued his experiments alone, without even his wife knowing what he was doing. She found him more and more preoccupied and tense, and even became worried about his mental health.

Those were the happy days when scientific discoveries were not announced through press conferences even before they had been made. On 28 December Röntgen decided he had untangled the problem, so he prepared a detailed paper for a scientific journal and sent reprints with an accompanying X-ray photograph of his hand to the most renowned physicists in Europe. They were thus able to check for themselves the validity of the discovery which brought Röntgen the first Nobel prize in physics ever awarded, in 1901.

The French scientist Henri Poincaré, who was one of the physicists to receive the reprint, took it to the weekly meeting of the Académie des Sciences in Paris on 24 January 1896 and showed it to his colleague Henri Becquerel. Becquerel was the third generation of his family to hold the same chair of physics at the Museum of Natural History in Paris, and worked in the same laboratory on the same subjects as his father and grandfather, in particular on the behaviour and properties of phosphorescent and fluorescent substances, including uranium salts. Poincaré suggested that, since X-rays could cause fluorescence, Becquerel should investigate whether some phosphorescent substances would emit these new rays.

During the next few weeks Becquerel experimented with various substances with no success, until he tried uranyl potasssium sulphate, which becomes phosphorescent after exposure to the sun. He found that after such exposure this compound would fog a photographic plate covered in black paper. He announced this result at the Académie's meeting on 24 February, 1986.

For the following few days bad weather prevented the sun's appearance, and Becquerel left a photographic plate and the uranium compound in a drawer. When the sun reappeared he developed the plate to check that the fogging was caused by the exposure of the compound to the sun. He expected to find the plate blank, but in fact it was considerably fogged, showing that the rays emitted by the uranium compound were independent of its exposure to the sun.

Becquerel was soon able to prove that the emission of these rays was linked to the presence of uranium, and that they would ionize air in the same way as X-rays. In exactly four months two revolutionary discoveries had taken place, X-rays and the so-called Becquerel or uranic rays, the study of which would mark the start of the era of atomic science.

While X-rays created immediate worldwide interest among scientists and physicians with more than a thousand publications in 1896, the year their discovery was announced, for two years Becquerel rays remained a scientific curiosity studied calmly by their discoverer as if scientific progress in this direction was taking a breathing space before making a leap forward.

This leap was made by the French physicists Pierre Curie and his Polish-born wife Marie Sklodowska. Pierre Curie had previously discovered, with his brother Jacques, the piezoelectric properties of quartz and had used them to measure with precision the ionization of air. He suggested to his wife, just recovering from the birth of their first child Irène, that the subject for her doctorate thesis should be the accurate measurement of the rays emitted by uranium.

Her first results confirmed Becquerel's finding that the intensity of the rays was proportional to the concentration of uranium in the compound studied. However, she then decided to investigate uranium ores, starting with a sample of the 'unlucky' ore pitchblende. She was surprised to find that the emission of rays from the ore was far greater than would be expected from its uranium content. She came to the conclusion, published in April 1898, that this activity was due to trace amounts of a new element far more active than uranium.

She continued her work with her husband, and they were able to identify non weighable amounts of a new element which they called polonium after her native country. Assisted by a chemist, Gustave Bémont, they went on to discover a second new element, which they named radium. To try to obtain larger quantities of these new elements, the Curies wrote to Eduard Suess, the president of the Academy of Sciences of Vienna and a respected geologist, asking for recent residues from the extraction of uranium at Joachimsthal.

A few weeks later, a heavily laden horse-drawn cart delivered to their laboratory - a primitive shed at the School of Physics and Chemistry in Paris - canvas bags filled with brownish residues still mixed with earth and pine needles. This was the first and most important transport of radioactive waste. Nobody could then predict that nearly a century later radioactive wastes would have strong social and political implications.

From this first tonne of residues the Curies isolated, after two years of exhausting work, one tenth of a gram of radium bromide. After a further two years, in 1904, they had separated the first gram of radium from a further eight tonnes of residues from the Joachimsthal uranium refinery.

By this time medical applications of radium had been discovered and were developing rapidly. Pierre Curie collaborated with some well known physicians in a successful study which attracted considerable publicity. 'Curie therapy' became, together with surgery, the only means of treating deep-seated cancers before chemotherapy was introduced. Fine needles containing radium or its daughter product radon were most often used for this purpose.

This therapy contributed to the fame of the Curies, which had already built up around the extraordinary career of Marie, the first woman to obtain a PhD in science in France, and the first to be awarded a Nobel prize for physics, which she shared in 1903 with Becquerel and her husband. Later, in 1911, six years after Pierre's untimely accidental death, she was awarded a second Nobel prize, this time alone and in chemistry.

For several decades after the discovery of radium it was believed that radiation in small doses was beneficial to health, having a stimulating effect. This belief, neither proven nor disproven, was not affected by the news in the 1920s of the first casualties caused by the ingestion of radium and of the deep skin burns to the hands of those who, like Marie Curie, manipulated strong radioactive sources without precaution. Such a belief was probably linked to the presence of small amounts of radium and radon in the waters of the main health spas.

It was not the Curies but a British team working in Canada which was the first to understand that the presence of polonium and radium in pitchblende was not due to simple geological and mineral reasons, but that these elements were directly linked to uranium by a process of natural radioactive transmutation. The theory of radioactive transformation of elements was brilliantly elaborated in 1901 by the New Zealand physicist Ernest Rutherford and the English chemist Frederick Soddy at McGill University in Montreal.

Thus it was found that uranium is weakly radioactive, decaying slowly but inexorably at the rate of one milligram per tonne per year. It is transformed into inactive lead through a chain of radioelements or daughters, each of which has a characteristic disintegration rate, a constant of nature that man has never been able to alter. The proportion of each radioelement in the ore is inversely proportional to its rate of disintegration. Radium is the fifth radioactive descendant in the chain from uranium to lead, its daughter is the gas radon, and polonium is the last radioelement before lead.

While the alchemists of the Middle Ages dreamt of transforming worthless lead into gold, it is the reverse which occurs with Klaproth's element, although so slowly that uranium is still present in the earth's crust despite the great age of our planet, about 4.5 billion years. For every three tonnes of uranium in the ore there is only one quarter of a milligram of polonium and one gram of radium, half of which, once separated, will disintegrate in about 1600 years.

Uranium's Scientific History - Part 2