I guess it doesn’t burn because because all the compounds that CAN be burned, are gone! Ash is what’s left over. During burning, the chemical bonds in the fuel are broken to produce carbon dioxide, water… and ash.
Ash is what cannot be burned in the fire at the time. Sometimes ash can be burned if there is enough flammable material left in it. In fires, there is a dangerous event called a flashover when the smoke layer above the fire ignites. It can be explosive and it makes the fire get really hot. More of the ash might burn once the fire is hotter, but some of the material that is leftover really will not burn and that stays behind.
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Christine commented on :
Ash is what cannot be burned in the fire at the time. Sometimes ash can be burned if there is enough flammable material left in it. In fires, there is a dangerous event called a flashover when the smoke layer above the fire ignites. It can be explosive and it makes the fire get really hot. More of the ash might burn once the fire is hotter, but some of the material that is leftover really will not burn and that stays behind.