The element, named after the planet Uranus, was reported by the German chemist Martin Heinrich Klaproth to a meeting of the Royal Prussian Academy of Science in 1789. In fact Klaproth had discovered the oxide of uranium and for a long time it was regarded as a “half metal”. The element itself was not isolated until 1841 when the French Chemist, Eugene Peligoit, prepared it by reducing uranium tetrachloride with an alkali metal. It was not until 1870 that it was realized Uranium was the heaviest naturally occurring metal. (Technically this is probably not true as a natural uranium nuclear reactor is believed to have operated at Oklo in Gabon millions of years ago and if that were the case then traces of plutonium would have been formed). Back in the 1800s the fact that the metal itself had not been extracted did not deter the use of the oxide for industrial purposes, the principle of which was probably for the coloring of glass. We should appreciate that technology, as we know it today, was in its very early stages.
Klaproth had separated the oxide of uranium from pitchblende, a heavy black mineral that is essentially U3O8 but which also contained other elements such as iron, bismuth, lead and radium. At this time chemistry was just emerging from the alchemy era. For example some scientists still held to the Phlogiston Theory of combustion. It was Lavoisier, in the 1775-80 period, who showed that the process of combustion was not due to the loss of Phlogiston but rather the combination of the material with oxygen. In 1784 Cavendish’s experiments showed that the product of burning hydrogen in oxygen was water. John Dalton’s Atomic Theory, used to explain the combination of reacting substances in set proportions, was not published until early in the nineteenth century. Although Avogadro put forward his hypothesis in 1811 it was not until 1858 that Cannizzaro showed how it could be used to construct a list of atomic weights. In 1896, Becquerel discovered the natural radioactivity of uranium, by which time it had been used in glass for well over fifty years, and 1913 before Hans Geiger invented the nuclear particle counter.
Against this background we must see the chemistry of glass, certainly until the latter part of the nineteenth century, as being very hit or miss and little understood. Surely there would only be the most vague appreciation that silica was really silicon dioxide and that this was the main constituent of quartz, flint and sand. That glass was a composition of sodium, potassium, calcium and or lead silicates. That these could “dissolve” other elements such as iron, gold, copper, uranium and so on to produce colors and that the acidity of the melt or the oxidation state of the element would determine the shade of color. The glass mixes had to be found by trial and error rather than chemical theory, and once found they would be closely guarded secrets. Analysis of a rival’s product would have been exceedingly difficult and well beyond the capacity of the glasshouses of the day. In this scenario it seems likely that different manufacturers developed their products in parallel. The ordinary glass worker would not know the composition of the mixes, these would be securely held by the foreman or manager. Thus only the defection of such a key worker to a rival would lead to the compromise of formulae. It therefore seems likely that the use of uranium to color glass developed simultaneously at different places and that no one can justifiably claim credit for “inventing” the process.
When we study the old surviving recipe books we see that, unlike modern times, different and ill defined terms are used for the raw material. For example in the Whitefriars Batch Book of 1832 on the same page we see the terms “Saltpetre” and “Nitre” used in adjacent recipes when they were in fact the same chemical, i.e. potassium nitrate. Why use different names for the same item? Was it because they came from different suppliers? Yet again we see “lead” used in some recipes, “litharge”, “red lead”, and even “lead or litharge”. Did they really add lead metal, or was lead used as a generic term, did they understand the difference between red lead and litharge? Litharge (PbO) contains about 93% PB (lead) by wt. Red lead (Pb3O4), the commercial variety of which may contain up to 35% of PbO2, could have as little as 90% Pb by wt.
Ther terms “uranium” and “uranium oxide” as used in these early recipes, also have ambiguity. It was common practice to refer to other compounds by “shorthand”, thus “arsenic” almost certainly would mean arseniuos oxide and “manganese”, “manganese dioxide”. In the light of this, and the difficulty in preparing uranium metal, we assume that recipes that simply use the word uranium really refer to its oxide, but which one?
Thank you to Barrie Skelcher for letting us share your information from The Big Book of Vaseline Glass.