Steel wool being heated with a bunsen flame in air. Steel wool is a mat of very fine, sharp edged fibres of steel. When heated they react with air to form iron oxide in an exothermic reaction. Away from the surface of the wool, where oxygen is used up, the heat melts small sections of the fibres which are thrown out and ignite when they reach free air - seen as sparks. Steel wool can be used as a tinder in emergency situations as it will burn even when wet.