(Comb) Oxidation of solid material without a visible gas phase flame. A stage in the ignition of a solid occurring before sufficient volatile fuel has been generated to sustain a gas-phase flame. Note: Glowing combustion appears to be a heterogeneous surface reaction related to pyrolysis. Radiative transfer is important in such systems. For example, glowing combustion can be initiated by a thermal pulse but will die out when the radiation is removed. Steady-state glowing combustion of coal or char-coal requires a coating of ash, which reduces surface radiation loss while allowing oxygen to diffuse in. A pile of such material will also burn in the absence of ash when radiation losses are compensated by radiation absorbed from other parts of the pile. Glowing combustion rarely initiates a fire, so a fire retardant in many cases favors glowing combustion rather than flaming combustion. Cf: Smolder