When a radium salt is mixed with a paste of zinc sulfide, the alpha radiation causes the zinc sulfide to glow, yielding a self-luminescent paint for watches, clocks, and instrument dials. From about 1913 up until the 1970s, several million radium dials, coated with a mixture of radium-226 and zinc sulfide, were manufactured.
Radioluminescent paint was invented in 1908 by Sabin Arnold von Sochocky and originally incorporated radium-226. The United States Radium Corporation was founded in 1914 in New York City, by Dr. von Sochocky and Dr. George S. Willis, as the Radium Luminous Material Corporation. The company’s luminescent paint, marketed as “Undark”, was a mixture of radium and zinc sulfide; the radiation caused the sulfide to fluoresce.
At the time, the dangers of radiation were not well understood. Around 1920, a similar radium dial business, known as the Radium Dial Company, a division of the Standard Chemical Company, opened in Chicago. It soon moved its dial painting operation to Ottawa, Illinois to be closer to its major customer, the Westclox Clock Company.
Several workers died, and the health risks associated with radium were allegedly known, but this company continued dial painting operations until 1940. Radium is a radiological hazard, emitting gamma rays that can penetrate a glass watch dial and into human tissue. 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 watch faces and later suffered adverse health effects from ingestion. In 1928, Dr von Sochocky himself died of aplastic anemia as a result of radiation exposure. Thousands of legacy radium dials are still owned by the public and the paint can still be dangerous if ingested in sufficient quantities, which is why it has been banned in many countries.
The Brooklyn Daily Eagle – Brooklyn, New York · Sunday, November 25, 1928
The Radium Girls were female factory workers who contracted radiation poisoning from painting watch dials with self-luminous paint. The incidents occurred at three different factories: one in Orange, New Jersey, beginning around 1917; one in Ottawa, Illinois, beginning in the early 1920s; and the third facility in Waterbury, Connecticut, also in the 1920s. After being told that the paint was harmless, the women in each facility ingested deadly amounts of radium after being instructed to “point” their brushes on their lips in order to give them a fine tip; some also painted their fingernails, face, and teeth with the glowing substance.
The women were instructed to point their brushes in this way because using rags or a water rinse caused them to use more time and material, as the paint was made from powdered radium, gum arabic, and water. Five of the women in New Jersey challenged their employer in a case over the right of individual workers who contract occupational diseases to sue their employers under New Jersey’s occupational injuries law, which at the time had a two-year statute of limitations, but settled out of court in 1928.
The Sheboygan Press – Sheboygan, Wisconsin · Monday, May 14, 1928
Five women in Illinois who were employees of the Radium Dial Company (which was unaffiliated with the United States Radium Corporation) sued their employer under Illinois law, winning damages in 1938. The phosphor degrades relatively fast and the dials lose luminosity in several years to a few decades; clocks and other devices available from antique shops and other sources, therefore, are not luminous anymore. However, due to the long 1600-year half-life of the Ra-226 isotope, they are still radioactive and can be identified with a Geiger counter.
Washington C.H. Record-Herald
Washington Court House, Ohio Friday, April 08, 1938
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.