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The flammable/non flammable transition is investigated both experimentally and numerically within the framework of statistical mechanics and percolation-type phase transitions. The heat flux threshold for ignition is usually estimated deterministically by fire research community. It is defined actually by ASTM standards as the average between the minimum flux allowing ignition and the maximum one which does not allow it. The probabilistic aspect of the ignition transition is demonstrated here, and a new method for the estimation of the ignition critical heat flux is proposed. Dead wheat straw and live pinus halepensis needles are investigated experimentally using a cone calorimeter and numerically using a simple physical model. The experimental data on ignition time are different from those obtained numerically. This difference is due to the fact that the numerical model assumes auto-ignition whereas experimental data are obtained by using a pilot flame. It is found that the variation of ignition time with the incident heat flux obeys a universal power-law behaviour near the heat flux threshold. The critical heat flux for ignition estimated from this law is also obtained using the ignition probability method. This critical heat flux is as low as it cannot be obtained using the ASTM standards. The power-law exponent is found compatible for both dead straw and pinus halepensis suggesting a universal phase transition. Further discussions on the comparison between flammability transition and spread/non spread transition are provided.

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