Radium dial on an alarm clock. Manufactured in the early 1900's this clock has a highly radioactive radium paint applied to the hands and numbers. The radium combined with zinc sulfides to allow the face of the clock to continuously emit light. Once the health risks of the painted dials became known, radium was discontinued from use. The radium is a mix of several different isotopes and is a strong emitter of alpha, beta, and gamma radiation.