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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.
 |
PLATE 2:
Group of late 19th and 20th century glass, all containing
uranium
except for the sailing boat
on the right. |
|
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.
 |
PLATE 3:
The same group of glass as seen in plate 2, but shown
under UV
light. This illustration shows how different metals respond
to UV
light. The sailing boat on the right, which responds strongly,
is the
only item which contains no uranium! The dark amber wine
on
the left hand side has twice to three times the uranium
of any of
the other items, yet hardly responds at all! The small
Burmese
hand vase, which is from Fenton, responds more strongly
than the
piece of Webb’s Burmese even though it contains
only about half
the uranium. |
|
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. |
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| Text © Barrie Skelcher and The Journal of
the Glass Association 2001. |
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