from http://www.onagocag.com/fire.html

The use of flint (SiO2) and pyrite (FeS2) as an ignition source was common. Iron in pyrite is in the +2 oxidation state, and may readily oxidize. Upon striking flint with pyrite small pyrite particles detach. The energy provided by the friction from the strike is sufficient to heat the pyrite particles so that they burn in air to form an iron oxide, possibly Fe3O4. The spark may cause ignition by one or both of two mechanisms: it either provides energy to the system directly by thermal transfer or acts as an activating centre whereby it causes the fuel to become reactive due to energy/electron transfer.

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