Ignition is associated with the thermal energy input into the system occurring at a rate faster than thermal energy is being lost to the surroundings. The solution of an idealized model for ignition at a plane metal surface characterizes the material in terms of the property thermal inertia, kpc. The thermal inertia for six metals Al, Cu, Fe, Mg, Ni and Ti, at their ignition temperatures is compared to the relative burn resistance of the metals in terms of BR_mp, BR_bp threshold pressure, resistance to ignition by particle impact and frictional heating ignition, pv. Thermal inertia is in agreement and appears to be suitable property for characterizing metals ignition. In addition, considerations for relaxing the assumptions associated with the idealized model are described and thermal inertia is still a valid characterization for the material.

The five measures associated with burning do not describe the dynamic system at ignition, nor is the build-up of a condensed product layer addressed by either the ideal model or relaxed model. The effect of this product layer is qualitatively discussed in terms of a Van Heerden diagram for the system. The mechanisms for providing sufficient circulation within the product layer and their representation in terms of dimensionless groups is described. The need for property data and mixing rules (for alloy properties) is pointed out.