What are camping lantern mantles?

A gas lantern mantle is a device made of a mesh of fabric (traditionally cotton, now sometimes fiberglass) impregnated with chemicals, particularly oxides of thorium and cerium, which incandesce when heated by the flame of the lantern. When exposed to heat, the mantle glows brightly, thus providing light. Mantles are shaped somewhat like a truncated cone or an inverted sock.

Campers used to have to carry mantles as they were very fragile. Although newer mantles are quite durable, it's still a good idea to carry spare ones in case of accidental damage.