Luminous Radium Paint
Some vintage watches, especially military watches, had the numerals and hands on the dial made luminous or "glow in the dark" by painting them with luminous paint. This paint was not like the luminous compounds used in todays watches; it was made with a mixture of radium, which emits radioactivity, and zinc sulphide, which "fluoresces" (glows brightly) when hit by the 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. 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 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 paint, often off-white or yellowish, on the hands or dial 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-226 is about 1,620 years, so over the 100 or so years since a watch dial was painted with luminous paint, the radium will have decayed about 4%. 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. A handy calculator for the exponential decay of radioactive materials can be found at HALF-LIFE CALCULATOR. 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.
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There is no need to panic if you have such a watch. 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 don't sleep in it, or keep it on your bedside table. The more significant danger occurs when 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!
The picture on the left shows one of my watches, a black dial trench watch which has radium paint on the hands and numerals. As you can see, the thick yellowish paint is quite distinctive against the fine white details of the enamelled dial. This is typical of radium luminised dials, and should enable you to easily recognise one. I guess that they made the paint by mixing the radium and zinc sulphide into a varnish or laquer, which was applied quite thickly to get a good level of luminosity, making it difficult to produce fine detail. You can see some of the paint has flaked away from the enamel dial, particularly on the 9, just like a thick varnish would.
Further information for persons working with watches that have this type of luminous paint on the hands and dials can be found in a UK Health and Safety Executive paper HAZARDS FROM LUMINISED TIMEPIECES IN WATCH/CLOCK REPAIR by clicking on the link.
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 am planning to get the dial refinished to remove the luminous paint before I wear it. It seems a shame to remove the historic paint that has been there for so long, but having heard the frantic reaction of the radiation detector once it gets anywhere near the watch, I don't feel that I have any choice on health grounds. 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 . . ." I certainly won't be wearing this watch until the luminous paint has been removed, and I will be treating it pretty cautiously in the meantime!
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