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Time to ignition data, usually in the presence of a pilot, for endothermically decomposing materials have been extensively measured over the last thirty years in cone calorimeter type apparatus where the material is exposed to external radiation heat fluxes in the range of 20 to 100 kW 1 m2 in a horizontal orientation. These data have been used as a) a measure of ignitibility and b) as a means to deduce material properties such as thermal inertia and critical heat flux. The approach to deduce these properties is based on conduction heat transfer without properly accounting for front or rear surface heat losses in conjunction to the thickness of the material. Thus severe confusion has been created in the fire field concerning the interpretation of ignition data. This paper delineates all the regimes for proper interpretation of ignition data so that reported inconsistencies can be explained. In the present case, the rear surface is insulated and the major contribution to heat losses is from surface radiation because the material is horizontal.