Why was radium once used in luminous paint for watch and clock dials?

Radium was used in luminous paint for watch and clock dials because of its property of radioluminescence. When a radium salt is mixed with a phosphor, such as zinc sulfide, the alpha particles emitted by the radium bombard the phosphor, causing it to glow in the dark.12

This application was common from approximately 1913 until the 1970s. However, the practice was eventually curtailed after the severe health risks to workers became apparent. Many employees, known as the "Radium Girls," ingested significant amounts of the radioactive material while shaping their paintbrushes with their lips. Because radium acts similarly to calcium in the body, it concentrates in the bones, where its radiation can cause anemia and bone cancer. Today, radium has been replaced by safer, non-radioactive phosphorescent materials that absorb and release light.13