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. 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. Radioactive paints are not commonly available today. 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. 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. If you are a vintage clock collector, this movie is a must-see. ![]() 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 have not seen the movie “Radium Girls” (presently on Netfix), you should take the time to see it. Clocks produced in the early 1900’s are particularly concerning in this regard. 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. We recently received an inquiry and decided to post the response here for general discussion. Today, most glowing watches use a radioactive isotope of hydrogen called tritium (which has a half-life of 12 years) or promethium, a man-made radioactive element with a half-life of around three years.Folks sometimes write us to ask about the risks of owning a clock with a luminous dial. In the past, the radioactive element was radium, which has a half-life of 1600 years. In these products, the phosphor is mixed with a radioactive element, and the radioactive emissions (see How Nuclear Radiation Works) energize the phosphor continuously. The most common place is on the hands of expensive watches. Occasionally you will see something glowing but it does not need charging. The phosphor is mixed into a plastic and molded to make most glow-in-the-dark stuff. It has a much longer persistence than Zinc Sulfide does. Strontium Aluminate is newer - it's what you see in the "super" glow-in-the-dark toys. Two phosphors that have these properties are Zinc Sulfide and Strontium Aluminate. To make a glow-in-the-dark toy, what you want is a phosphor that is energized by normal light and that has a very long persistence. The length of time that they glow after being energized (known as the persistence of the phosphor). ![]()
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