Ruddy radiation

The dangers of radium lume need careful consideration and treatment

- MARK MCARTHUR-CHRISTIE

BEFORE THE EARLY years of the 20th Century, if you wanted to tell the time in the dark you had a choice: commission a montre à tact or a repeater from a posh watchmaker like that nice M Bréguet. As the name suggests, you felt the time on the dial of a montre à tact, whereas a repeater chimed the hours and minutes. Neither was cheap or easy to make. So once Marie and Pierre Curie isolated the glow-in-the-dark wonder-element radium, watch and clockmakers (car and aircraft instrument manufacturers, too) were eager to put the new substance to work. 'Does your watch tell the time in the dark?' asked US firm Ingersoll before extolling the advantages of its new radium-lumed Radiolite watches.

Radium-226 on its own isn't much use as a paint for watch dials and hands; it's simply not bright enough. But add some zinc sulphide and a solvent carrier and you've got a paintable compound that'll glow happily for perhaps 1015 years. The radiation from the radium slowly breaks down the zinc sulphide, eventually dimming the glow to nothing.