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Uranium Glass(Page 2)As I will describe later, there are many examples of uranium being used in both pressed and blown glass, in green, amber, yellow and other colours right up to the start of the Second World War. These were from large glasshouses such as Walsh-Walsh, Thomas Webb, and Bagley.
It seems likely that during the war there was a moratorium on the use of uranium. Anyhow, the glass producers were on war work rather than producing fancy goods. There is considerable evidence that uranium was used in the UK after the war but probably nothing like to the extent of its pre-war deployment. I have seen a number of examples of Bagley’s design registration 849118, which was not registered until 1945, and these have all contained uranium, albeit at relatively low levels. I also know that Plowden and Thompson in conjunction with Thomas Webb were using uranium to produce borosilicate tubing for French neon light tubes as late as the 1970s. Nazeing produced an ashtray in the 1950s or early 60s, which contained about 0.28% uranium by weight. Uranium was also being used abroad, and I have found lampshades made in France in the 1980s and pieces of Fenton Burmese (USA) as re-cent as 1994. One advantage of collecting uranium glass is that it is easy to detect. Without recourse to sophisticated analysis techniques, there are two ways the collector can confirm the presence of uranium, although neither is absolutely foolproof. Used together they must provide a level of certainty, which would be highly acceptable in any antique assessment. Uranium responds strongly to ultra-violet light. This is especially so for the wavelengths close to those of visible light (near region), and lamps producing UV in this range are easy and cheap to buy. It some-times goes under the name of “black light” and is not uncommonly used for stage effects. A 150-watt bulb used for this purpose will cost about £35.
It is also used for checking “invisible marking” and the small torches used for this purpose are readily avail-able for around £15 - £20. When exposed to such light the uranium glows with a very characteristic ghostly green colour, which, once seen, is easily recognised again (Plate 3). There are three problems with using UV light. The first is that it cannot be used in bright “visible” light as this swamps the fluorescence. Secondly, in some glasses, especially those with a high lead content, the fluorescence is so weak that there is an element of uncertainty. Thirdly, I have found examples of modern glass with yellow fluorescing agents, which glow much the same as uranium. The other method is by the use of a Geiger counter or other suitable radiation-detecting instrument. This again is not foolproof for there are other sources of radiation, which might confuse an instrument. However, the likelihood of this happening can be greatly reduced by careful selection of the instrument. I have found an end-window, beta-sensitive Geiger counter suitable for this work. Its sensitivity is such that when presented to a packet of sulphate of pot-ash fertiliser, it reads one count per second on a scale of one to five. A combination of both methods gives a very high degree of confidence. There are a number of methods available for estimating the uranium content of glass. Probably the most accurate is by chemistry, but this requires a small sample to be destroyed and is not available to the ordinary collector. Another is by gamma spectrometry. Although the measurement itself is simple and non-destructive, the equipment is very expensive and technically specialised. In the 1970s some work with gamma spectrometry was reported by Murray & Haggith (Journal of Glass Studies, Corning Museum of Glass, Vol. XV, 1973), but the technique is not generally available to the collector. As an alternative, I have used a beta-sensitive Geiger counter. It enables an estimate to be made of the uranium content of glass, which, although lacking the precision of the other methods, is probably within the variation of the mixes in the earlier days. It is non-destructive and can be used almost anywhere at any time. The measurement is based on the “infinite depth” method and assumes that the sample under consideration is so thick that any increase in the thickness would not increase the reading on the counter. (Beta radiation is not very penetrating and is easily absorbed by matter. Consequently if we take a material which has a beta radioactive element evenly dispersed with in it and we measure the radiation at its surface, as the thickness increases, the radiation will at first increase but then tail off to a constant level. This is because the radiation originating in that part of the material, which is furthest from the surface, will all be absorbed before it reaches the surface.) In the case of glass this is probably only a millimeter or less, a thickness which is exceeded on most glass objects. However, caution has to be observed when the uranium layer is cased and very thin, as the “infinite depth” may not have been reached and any measurement will lead to an under-estimate of the uranium concentration. The Geiger counter is calibrated against a source of known strength, which is also at infinite depth, and from there on it is a matter of simple proportion. Ideally the calibration source should resemble the nature of the test sample as closely as possible. Hence it is better to calibrate against a glass whose composition is known. These are not easy to find, although the Thomas Webb Sunshine Amber formula is published, as is the formula for their Eau de Nil and Bristol Green (see S.R. Eveson Reflections - Sixty years with the crystal glass industry, Glass technology Vol. 31, 1990). Both these glasses were made in the 1930s when chemical control was reliable and they can therefore be used for calibration. Nevertheless, it is best to take an average of several samples that are unlikely to have come from the same batch. For example, if the average of a number of readings from pieces of Sunshine Amber were “20” on the Geiger counter, then a reading of “1” on the Geiger would indicate a uranium concentration of 1.1% divided by 20, i.e. 0.055% “U” by wt. An alternative method of calibration is to use naturally occurring potassium, which is readily available in the form of potassium chloride or potassium sulphate. The specific radioactivity of these is 14.4 Bq/g and 12.4 Bq/g respectively, but this would then measure the uranium content in terms of its radioactivity rather than its weight. The percentage weight could then be obtained from the specific radioactivity of natural uranium. A problem with using potassium is that the energy of its beta ray is significantly different to the average from uranium and such a calibration could have a built in error. For this reason I have relied on calibration by known glass concentrations but used potassium as a standard against which to check the consistency of the instrument. In my use of the Geiger counter I consider the uranium estimates are within the range of +/- 15%. I am often asked “is uranium glass safe?” The short answer is “probably yes” but it needs qualification. First of all nothing is absolutely safe in this life; there is always an element of risk in whatever we do. So long as we are alive we are vulnerable; it is a fact of nature. Only if by the term safe we mean as safe as all the other risks we willingly accept in every day life, such as driving a car, flying in an aeroplane, travelling on a train, eating an orange etc., is the answer “yes”. In terms of absolute safety there may be some very small risk. It is not possible to be sure because scientists are not unanimous about the effects of radiation at very low levels. Some, and it is the official view, say that with all radiation there is a risk of biological damage, which could lead to a cancer. A minority take a different view and point to a substantial amount of evidence, which suggests that a very low dose of radiation may have net beneficial health effects. The only thing we can be sure about is that, if there is a risk, it is a very small one. At the levels of uranium that I have found, with possibly one exception, the risk is probably so small as to be undetectable. The exception is with items where the uranium con-tent is several % by weight and the item, perhaps a piece of jewellery, is likely to be in contact with the skin for (say) 20 hours per week, throughout the year. In this case the radiation dose to the skin could exceed the current control levels, but not by a lot! Why was uranium used to colour glass? If it had not been discovered until 1998 the probability is that it would not have been used at all. With possibly one exception, all the uranium colours that I have come across I have also seen in non-uranium glass. The chemistry of uranium is complex. It is has several valency states and can be either basic or acidic when forming salts. It is these properties, which enable it to give different colours according to the chemistry of its host glass. Green may be due to the four-valency state and yellow to the six-valent complex uranyl ion. (It is reported that trivalent uranium in aqueous solution gives a claret colour but I have not discovered this in glass). Literature tells of red and black glass produced with uranium but I have not yet found any examples. Back in the early 1800s uranium provided the glass-maker with new possibilities. The golden transparent yellows with their slightly oily look were then new and exciting. The greens of uranium often had that extra bit of life and sparkle, more so than the greens produced by iron. These were the new Annagelb and Annagrun of Bohemia and the Topaz of England. No doubt having discovered a new colouring agent, glassmakers started experimenting with other possibilities leading to the ivories, ambers, turquoise and Burmese. But why do we find uranium in the very pale, almost white, opaque glasses? Why do we find it in some of the lifeless greens of the depression years that are indeed difficult to tell apart from their non-radioactive alternatives? The answer was suggested by the late Dr Sheilagh Murray. It lies with the response of uranium glass to ultraviolet light. Before the days of cheap and readily available electricity for the modern lighting of today, folk would sit in their rooms with curtains open extracting the last from the twilight. Under such conditions the ultra violet part of the spectrum increases with regard to the visible light component. The result is that uranium glass gains a ghostly glow of its own. This is easy to ob-serve in an unlit modern living room, but perhaps more dramatic is the effect as darkness starts to fall over the traders’ tables at Newark and other antique fairs. In the last few minutes before the plastic sheets cover the outside displays, stop and survey the scene. Each item of uranium glass will stand out significantly from its non-uranium containing neighbours. But we also find uranium in colours where there appears to be no rational explanation. For example, it has been used in the reproduction dark green “Georgian” glass, made in the 1920s and 30s. Why was uranium used by Webb, Walsh, Stevens & Williams and others as the inner casing of items where its attraction, if any, cannot be seen? Uranium was an expensive component, so why use it where it appears to add nothing to the product? The relative cost of uranium can be judged from a recipe book from the Coalbournhill Glassworks, Stourbridge, dating between about 1860 and 1877. It indicates that in a formula for opaque yellow the uranium would have been nearly 60% of the total material cost! I have no answer but can only guess that perhaps, over the years, it had gained a personality of its own and that glass-makers, in their conservatism, were reluctant to relinquish its use.
Text © Barrie Skelcher and The Journal of the Glass Association 2001. |
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