Folks sometimes write us to ask about the risks of owning a clock with a luminous dial. We recently received an inquiry and decided to post the response here for general discussion.
Many folks might not realize that early vintage clock dials were painted with radioluminescent paint, which is a mixture of fluorescent salts (these produce the glow) and radium-226, which is a dangerous and highly radioactive element. Clocks produced in the early 1900’s are particularly concerning in this regard. If you have not seen the movie “Radium Girls” (presently on Netfix), you should take the time to see it. The movie depicts very accurately the dangers of working with and ingesting radium-based paints, the suffering, pain and death that early 1900’s dial painters experienced in their young lives, and how they moved occupational safety forward. If you are a vintage clock collector, this movie is a must-see.
One extremely important note to keep in mind is that just because the “glow” has died away and your clock no longer glows in the dark (common with pre-1940’s clocks), this is no indication that the radium is no longer present! Quite the contrary, the radiation from radium-226 has a half-life of on the order of 1000 years. It will be present despite the fact that the salts that produce the glow have been bombarded by radiation to the point where they have broken down and no longer glow. But the radiation persists!
Radioactive paints are not commonly available today. Most luminescent paints purchased today basically “store” light and re-emit it at a different wavelength using an effect generally referred to as “phosphorescence.” This is in stark contrast to the “radioluminescence” of paints developed for clock dials in the early 1900’s which largely used radium-226 as the active element providing the energy to the salts in the paint (such as zinc sulfide or strontium aluminate) resulting in the “glow in the dark” effect.
It is generally and commonly felt that since radium primarily emits alpha particles, and these are stopped by even something as thin as paper (so certainly the glass is an excellent barrier), that they are safe at about 1 ft away from the clock. Does this mean that at the right distance from the clock, radiation is acceptably low? To answer this question, we must also consider that beta particles are emitted at a lower intensity by radium-226, as well as other emissions (such as gamma rays).
We don’t really know the purity of the radium used in clocks over the years. Since it is possible that other radioactive contaminants could be present as well, these need to be studied. References such as Wikipedia indicate that there could be contaminant levels on the order 3% present in the radium used. These contaminants could possibly emit gamma rays, which are harmful and cannot be stopped without thick concrete and lead.
It is also important to consider that if the luminous paint is beginning to deteriorate and become powdery, that this powder or dust could be ingested, most notably through inhalation, and that could become a primary heath risk. Most clocks produced in the early 1900’s (1900-1940’s) are found in this state, and this is a serious concern! Clocks produced after 1950 have not deteriorated to this point in our experience, but they may be well on their way to this level of deterioration. Clocks produced through 1970 could have dials painted with radioactive materials, so we must be concerned about deterioration that will ultimately occur.
We also don’t know if and what kind of radioluminescent elements were used in the manufacture of a particular clock, especially post-1970. Depending on its date of manufacture, a clock could have also been illuminated with promethium or tritium (shorter half life and generally safer) or phosphorescent materials, more commonly available today which use no radioactive elements.
Bottom line is that the only way to know for sure is with a geiger counter. The readings when the counter is placed near the clock should be compared to general background levels in our environment (typically 0.1 to 0.2 usV/hr). It is not unusual for collectors and hobbyists to own low cost geiger counters for this and similar concerns (rock collecting, vintage aircraft instrumentation, antique jewelry, glass and glazed stoneware, food studies, etc. all involve low levels of radiation). Once you know the “dose” of radiation emitted by your clock you can be much better informed on where to store it, how often to handle it, etc.
Our measurements show that most vintage clocks are quite “hot” at close range (few inches or closer). Levels can easily exceed safe levels. But move back 1-2ft and the levels drop down to background again, or rather close. The protective glass on the clock will stop most alpha particles, but will do little or nothing to stop beta and gamma particles.
Likewise, our measurements show that the dust and fragments from crumbling and failing radium paint can be quite “hot.” We have measured levels up to 50x what is considered safe (if exposures are long term). If these particles escape past the glass, or if the glass is missing, this poses a quite high exposure rate, and also risks that you could ingest this dust which could then be quite hazardous especially if breathed in.
Harmful radiation is all around us. It’s not so much a matter of eliminating it, but rather managing it so you can optimize living with it safely. For example, when handling vintage clocks when deterioration is present, we must take extreme precautions, such as wearing protective gear and dust masks to avoid inhalation. But then, disposal of this dust poses a major concern as well.
More will be added to this post in the future.