Where does ash come from and is it a reflection of quality of combustion?

Campfire ash

People often ask me what wood ash is, and if it is somehow a reflection of the quality of combustion efficiency. It’s an interesting topic, and important for small stoves whose performance can be affected by excess ash.

As I understand it:

  • ash is the residual mineral component of wood that cannot be burned (at least not at temperatures of a wood fire) and so is left in the wake of the fire.
  • the burning temperature and quality of combustion affects how much charcoal may remain, but makes no difference to the amount of ash. (Minerals are there, and remain, however the wood is burned).
  • most woods throw similar volumes of ash, albeit according to their density (i.e. oak a bit more than pine).
  • much of the ash comes from bark around the wood, rather than the wood itself.
  • … and therefore small twigs and faggots tend to leave proportionally much more ash than larger chunks of wood, as they have a higher ratio of bark to wood.

My evidence for the last point is that when you burn small coppice sticks in a rocket stove, you see a lot of ash remaining; grates tend to clog quite rapidly. In contrast, when you burn wood you’ve split (or shop-bought kindling) that has little or no bark on it, negligible ash remains. Similarly, charcoal briquettes tend to throw more ash than lump wood, as I think they tend to be made from brashings (small twigs and agricultural waste).

Perhaps, though, there is something essentially different about the make-up of the wood in small coppice sticks, and the bark is just a distraction?

Perhaps different kinds of wood do throw different proportions of ash?

There’s not a lot of published knowledge on this matter out there, and I’d be interested in others’ knowledge, opinions and experiences…

Incidentally, have you ever noticed how much ash remains after burning paper? This is mainly the residual clay that is a constituent of paper – particularly newspaper. That’s why I don’t generally find using scrunched-up paper as very effective way of lighting fires, as they tend to leave the whole affair clogged in dense ash.

Please do comment and share your views!

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