The part that would not burn
What Is Left When Something Burns?
Ash is the minerals a plant pulled out of the soil, left behind because they cannot burn at all.
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Step 1 of 5
Most of a log leaves as invisible gas.
Burning turns the wood's carbon and hydrogen into carbon dioxide and water vapour, and both float away. That is why a big log leaves a small pile.
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Step 2 of 5
Ash is the mineral part.
A tree pulls calcium, potassium and magnesium out of the ground and builds them into itself. These are already fully reacted, so fire has nothing left to take.
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Step 3 of 5
It cannot burn because it has already finished.
Burning means joining with oxygen. These minerals joined with oxygen long ago, so there is no reaction left for the fire to run.
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Step 4 of 5
Black ash means the fire did not finish.
Properly burnt ash is pale grey or white. Black flecks are unburnt carbon — the same stuff as smoke, left behind rather than carried off.
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Step 5 of 5
It goes back to the soil.
Those minerals came from the ground and wood ash returns them, which is why a little of it round a plant works as a fertiliser.
Wood ash is alkaline enough that soaking it in water and boiling the liquid down was, for centuries, how people made the lye for soap.
The short version
Ash is the mineral part of the fuel — already joined with oxygen, so fire cannot touch it — left behind when everything else leaves as gas.
Try it yourself
Weigh a handful of dry twigs, burn them safely in a metal tray, and weigh what is left. Almost all the weight went into the air.
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