Tritium

Tritium is a radioactive form of hydrogen once used in luminous paint on watch dials and hands, before the industry moved to non-radioactive luminescent materials.

Tritium lume loses its glow over time and often ages to a cream or brown tone, which is part of the appeal of many vintage watches. Dials from this era are sometimes marked to indicate the material.

For a dealer, tritium markings and ageing help place a watch in time and judge whether its dial and hands belong together. Lume that looks too fresh on a tritium-era watch can indicate a relume or replacement.