Radioactive Elements in Glass
Thorium is derived commercially from certain monazite sands (e.g., from India). Thorium is
radioactive itself, emitting alpha particles. The resulting "daughter" products of that radioactive
decay series also produce both alpha and beta particles. Related rare earth such as lanthanum are
often produced from the same sources, with monazite being up to 25% lanthanum.
This decay process means these thoriated glass lenses can gradually become more radioactive over
time, as the more highly radioactive decay products build up in the glass. This result is counterintuitive.
You would expect the radioactivity to decrease over time. But after chemically purifying
the thorium from its ore sources, the thorium is relatively free of these daughter products. Over
time, the thorium decays, and the levels of radioactive daughter by-products builds up. Eventually
a more highly radioactive equilibrium will be reached, as in the original radioactive ores. So over
the years, your "hot" lenses are likely to get more radioactive rather than less. Surprise!
Lanthanum has two isotopes in its natural form, one of which is weakly radioactive. Another
source of rare earths such as lanthanum is cerite, which contains the element cerium. Cheap
glasses may have iron salts in them, often yielding the familiar green color of some iron salt
contaminated glasses. Cerium is often added to glass to convert iron impurities into colorless
compounds, yielding clear glass suitable for optical uses. Unfortunately, the rare earths include
some chemically very similar elements as contaminants, many of which are mildly radioactive.
The chemistry of lanthanum and its associated rare earths is so closely related that it is readily
possible to have radioactive contaminants end up in the desired lanthanum salts used in making
optical glasses. The amount of such contaminants could also easily vary from batch to batch,
depending on the degree of contamination in the original monazite or other mineral sources being
I don't think that the original levels of thorium or lanthanum specified for use in these lenses [e.g.,
in patent filings] is the cause of their radioactivity. Later lenses of the same exact design and glass
formulas, but from later batches with higher serial numbers, do not exhibit any similar degree of
radioactivity. Nor do they suffer from yellow discoloration over decades of time. So it isn't the
thorium or the lanthanum that causes the problem here. The radioactivity of these early lenses is
caused by contaminants in the ingredients (e.g., thorium salts) used to make the early lenses. It is
these radioactive contaminants which cause these lenses to be more radioactive than their later
(more purified) batches of the same design.
However, don't assume that all yellow lenses are necessarily radioactive. Many lenses turn yellow
due to aging of the Canada Balsam adhesives used to glue lenses together. Other lenses may have
coloration biases which make them slightly yellowish rather than clear, or other colors depending
on the glass. But if you have a lens using early specialty glasses (wide angles, fast lenses..) from
before the 1970s, you should consider checking your lens for radioactivity. Most school physics
labs can do this, as can local Civil Defense offices and police or fire emergency response teams.
Another important point here is that these contamination levels vary from batch to batch,
depending on the contamination in the sands used to make the impure chemicals being used. So
you have to test each lens to determine if it is radioactive, and how "hot" any given lens really is.
Tests of one lens from one batch won't apply to another lens made from another batch of
chemicals with different levels of radioactive contaminants. You have to test each lens to be sure.
Alpha particles can only go through a few inches of air, and are stopped by a sheet of paper. But
they can interact strongly with the surface of the cornea of the eye held near the glass for long
periods. This event happens when an alpha particle emitter is used in the glass of a telescope or
microscope eyepiece. Such exposures can produce cancers and radiation burns after long exposure
and daily use.
Beta particles can penetrate a few feet in air, or a range of thicknesses of cloth, paper, and other
materials. The beta particle emitters are most important photographically. Beta particles can
penetrate camera shutters and film cassettes to fog film next to radioactively "hot" lenses left in
your camera bag. Gamma and xrays can do so too, but their intensity is usually much less from
the contaminants in glass than for beta particles.
Just how radioactive are typical "hot" lenses? Studies of a half dozen Leica lenses (see postings
below) came up with 1.5 milliroentgens/hr. This figure compares to 20 milliroentgens (mrem) per
day maximum permissible dosage in many western countries. But that 20 mrem is for whole body
exposure, while the lenses mostly emit shorter range beta and alpha particles. On the other hand,
it may take just a single gamma ray to turn one of your cells into a cancerous cell and cause a
tumor. These 1.5 mrem/hr levels are surprisingly high compared to typical levels for radiation