Luminous Radium Paint
Some vintage watches, especially military watches, had the numerals and hands made luminous or "glow in the dark" by painting them with luminous paint. This paint was not like the luminous compounds commonly used in todays watches, which charge up in sunlight and gradually lose their glow over a few hours. The luminous paint used on WW1 era watches was made with a mixture of radioactive radium and zinc sulphide. The zinc sulphide "fluoresces" (glows brightly) when hit by radiation from the radium. This paint glowed all the time, day and night, without needing exposure to sunlight, and continued to glow for years on end, even when stored away from the light in a drawer. However, after some time, maybe twenty to fifty years, the zinc sulfide is worn out or destroyed by the radiation from the radium, so the paint no longer glows in the dark - but the radium has hardly changed and is still there, emitting radiation.
Luminous radium paint on dials was first used around 1910, before the dangers of radioactivity were fully understood. Although health problems in workers using radium paints were noticed in the late 1920s, radium paint continued to be used up until about 1950. If your watch was made before 1950, and has thick gungy paint, often off-white or yellowish, on the hands and numerals, the likelihood is that the paint was made with radium, even if it no longer glows in the dark at all.
The half-life of radium is about 1,600 years, so over the 100 or so years since a watch dial was painted with luminous paint, the radium activity will have decayed about 4%. This means that paint that was made with radium 100 years ago will still be 96% as radioactive today as the day it was made, even if it no longer glows in the dark. Radium and its fission decay products (such as radon-222 gas) have the potential to cause various health risks, and therefore watches with this paint should be handled carefully and in ways to minimize these risks. The danger is not so much from radiation received from the watch when wearing or being near to it, but more from inhalation or ingestion of paint particles or contaminated dust. If the dust is inhaled, it can sit in the lungs where the alpha radiation can cause tumours.
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There is certainly no need to panic if you have watch with radium paint. They are reasonably safe to wear on a once-in-a-while basis, but it would be sensible to not wear it all the time, and certainly don't sleep wearing it, or keep it on your bedside table. The much more significant danger occurs when opeing or working on such a watch, and one should be particularly careful not to breathe in any of the paint or its dust. The best way to make sure that there is no risk is to have the radium based paint removed and replaced by a paint that looks the same but is not radioactive. Naturally this needs to be done by someone who knows what they are doing, so don't try this at home!
Further information for persons working with watches that have this type of luminous paint is available on the HSE web site in the article NEW CONTROLS INTRODUCED ON TIMEPIECES CONTAINING RADIOACTIVE SUBSTANCES which you can get to by clicking on the link. This article states "Up until now those in the retail and antique trade have been free to dispose of damaged clocks and watches luminised with radioactive material, that are beyond repair, in the dustbin with other general refuse. Today because of a change in legislation made necessary by the Basic Safety Standards Directive (96/29/EURATOM) the situation has changed ... and you may be required, in certain circumstances, to seek Environment Agency approval under the Radioactive Substances Act 1993 (RSA 93) before disposing of such timepieces." ... "A major implication of this change in the law is that those in the retail and antique trade will now need to know what radioactive substances are on their premises and in what quantity." ... "Please note this legislation does not apply to private individuals with a related horological hobby/collection."
I bought a radiation detector so that I can test my own watches for radium based luminous paint. I am glad that I did - the first watch that I tried it on, the same black dial trench watch with plenty of luminous paint left on the dial, sent it beserk, as you can see below. I now offer a service to test your watch for radiation, including a certificate stating what I find. This would be useful for your own peace of mind, or if you are thinking of selling your watch, especially if you have had the dial repainted to remove the radium. If you are interested in this service, please drop me a line at You don't need to copy the email address, just click on it.
The first picture shows the radiation detector reading 0.14 micro sievert per hour background level, which is pretty normal, and the alarm level set by the factory at 0.3 micro sievert per hour. The second picture shows the effect of putting the watch next to the detector: the level jumps to 1.92 micro sievert per hour and the audible alarm goes beserk. In the manual accompanying the detector, it says ". . . if a dose rate of more than 1.20 micro sievert per hour is displayed, it is necessary to leave the zone urgently . . ." Although this warning refers to radiation levels which affect a whole area, rather than from a point source such as a watch where the intensity of the radiation experienced can be diminished by simply moving away from the watch, it does show that the radiation hazard from the radium is not negligble, and should not be ignored.
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