Just the point of ignition of a particular object or the temperature at which it burns up or disinigrates. Just like a tree will burn at a certain flashpoint. As long as its temperature is somewhere below that flashpoint it exists as matter. When it burns up then I suggest the energy expeled reverts back to kinetic energy in the atmosphere? Not the wood itself but the energy created or dissapated by the fuel.

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The notion that man is the center of all things is as antiquated as the belief that the world was flat. - me 2009