Many watches, particularly sport watches, offer luminescent numerals or markers, hands and other accents. Generally this glow — not visible in the light — is designed to make nighttime or underwater reading easier.
Making a glow-in-the-dark watch sounds simple, right? It’s not. In fact, creating a luminous effect on watches has been an arduous and, at one time, deadly task. While 18th and 19th century efforts to add sheen to watches boiled down to using crushed shimmering shells and volcanic materials and painting them on to the dials, the first real break came in the early 1900s — just a few years after Marie and Pierre Curie discovered radium.
The radioactive material — radium-226 — isolated by Curie (and said to have a “fairy-like glow” in the hue of blue) became the material of choice for use on dials for a few decades. While the radium itself does not glow, it emits numerous particles that have he effect of ionizing certain materials so that they glow fluorescent or phosphorescent.
In the first decade of the 20th century, radium-based paint was developed to achieve glow in the dark effects. Depending on the formulas, different colors of lights could be achieved, but green was the most commonly used. These radium-based paints made their way into many fields, including watchmaking. Dial makers, in particular, found that painting the hands and markers with radium would render them luminescent.
In 1914, an American company called Radium Luminous Materials Corporation (later called U.S. Radium Corporation), mined radium and began producing radio-luminescent paint. U.S. Radium Corporation and similar companies had hired thousands of workers (mainly women) to paint watch dials.
Unfortunately, radium emits alpha and gamma radiation, which is deadly if ingested. While scientists and chemists were aware of the dangers, workers painted several hundred dials per day. Sometimes, the women would use the luminescent materials to paint their fingernails or run flecks through their hair for a glowing night appeal.
The real danger, though, came in the fact that the workers would lick the tips of their paintbrushes as they were painting to keep them pointed for legible painting. Many began suffering grave sickness and died. It is said that a cover-up was enacted by the companies, which issued statements that the women were dying due to sicknesses caused by x-ray machines, or from other diseases. Finally, in the late 1920s, a group of workers — who became known as the Radium Girls — retained a lawyer willing to take their case and took U.S. Radium to court.
Eventually the factory sites we shut down and new rules were enacted. Radium for watch dials was replaced with another radioactive material called tritium — deemed 400 times less harmful that radium. Since the 1960s, tritium became the material of choice — though it was regulated that it had to have a limit of no more than 25mCi. Some 20th century watches even carry a marking “T<25” or a single or double “T” on the dial.
The use of radioactive paint was subsequently outlawed, which stirred a return investigation into original photo luminescent paints (that absorbed energy from external light sources in the UV spectrum and re-emitted it over a period of time) in the effort to find a legal and safer lumen. The search to develop strong, photo luminescent materials for use on watch dials and in other fields would not come to fruition until the 1990s when a non-radioactive substance made of a variety of elements was unveiled to the world.
That substance is called Super-LumiNova. Super-LumiNova — made in a variety of colors — has a strong glow that will last for an entire night. Daytime light will recharge it, so it can begin to emit the lumen over a period of time. Today, Super-LumiNova is the material of choice on watches, but there are options available. Check back later this week, when we will take a look at current sources and applications of luminosity.
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