Gas mantle

An incandescent gas mantle, gas mantle or Welsbach mantle is a device for generating bright white light when heated by a flame. The name refers to its original heat source in gas lights which filled the streets of Europe and North America in the late nineteenth century, mantle referring to the way it is hung above the flame. Today it is still used in portable camping lanterns, pressure lanterns and some oil lamps.

Gas mantles are usually sold as fabric items which, because of impregnation with metal nitrates, form a rigid but fragile mesh of metal oxides when heated during initial use; these metal oxides produce light from the heat of the flame whenever used. Thorium dioxide was commonly a major component; being radioactive it has led to concerns about the safety of those involved in manufacturing mantles. Normal use, however, poses no health risk.

Mechanism

The mantle is a roughly pear-shaped fabric bag, made from silk, ramie-based artificial silk, or rayon. The fibers are impregnated with rare-earth metallic salts; when the mantle is heated in a flame, the fibers burn away, and the metallic salts convert to solid oxides, forming a brittle ceramic shell in the shape of the original fabric. A mantle will glow brightly in the visible spectrum while emitting little infrared radiation. The rare earth oxides (cerium) and actinide (thorium) in the mantle have a low emissivity in the infrared (in comparison with an ideal black body) but have high emissivity in the visible spectrum. There is also some evidence that the emission is enhanced by candoluminescence, the emission of light from the combustion products before they reach thermal equilibrium. The combination of these properties yields a mantle that, when heated by a kerosene or liquified petroleum gas flame, emits intense radiation that is mostly visible light, with relatively little energy in the unwanted infrared, increasing the luminous efficiency.