Mixed with the right kind of paint, radium would luminesce after exposure to light, so that a watch face painted with the stuff could soak up energy during the day and stay visible all night long.

ROCK SPRINGS — The watch factory workers moved with precision ... Delicate hands deftly applied radium paint, brushes flying from dials to the paint to their lips. The technique both saved ...

In the 1920s when radium paint was introduced into the manufacture of clock dials in a New Jersey factory, the dangers of the newly developed material were still to be discovered.