Pyrophoric

A pyrophoric substance is a substance that ignites spontaneously, that is, its autoignition temperature is below room temperature. Examples are iron sulfide (FeS) and many reactive metals including uranium, when powdered or sliced thin. Pyrophoric materials are often water reactive as well and will ignite when they contact water or humid air. They can be handled safely in atmospheres of nitrogen or argon. Most pyrophoric fires should be extinguished with a class D fire extinguisher.

The creation of sparks from metals is based on the pyrophoricity of small metal particles. This can be useful, including: the sparking mechanisms in lighters and various toys, using ferrocerium (the "flint" in lighters); starting fires without matches, using a firesteel; the flintlock mechanism in firearms; and spark testing metals.