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Radium

A 1950s radium clock, exposed to ultraviolet light to increase luminescence
Main article: Radium dials
Radioluminescent paint was invented in 1908 and originally incorporated radium-226. The toxicity of radium was not initially understood, and radium-based paint saw widespread use in, for example, watches and aircraft instruments. During the 1920s and 1930s, the harmful effects of this paint became increasingly clear. A notorious case involved the "Radium Girls", a group of women who painted watchfaces and later suffered adverse health effects from ingestion. It is now recognised that radium paint requires great care in application, maintenance and disposal to avoid creation of a hazardous condition.
Radium paint used silver-doped zinc sulfide phosphor, usually doped with copper (for green light), silver (blue-green), and more rarely copper-magnesium (for yellow-orange light). The phosphor degrades relatively fast and the dials lose luminosity in several years to few decades, despite the long half-life of the Ra-226 isotope (1600 years); clocks and other devices available from antique shops and other sources therefore are not luminous anymore, though they are still radioactive and can be identified with a Geiger counter. The dials can be renovated by application of a very thin layer of fresh phosphor, without the radium content (with the original material still acting as the energy source); the phosphor layer has to be thin due to the light self-absorption in the material.
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