What happens when wood burns?

We have a fantastic Scandinavian wood burner. 
I put in several logs on a cold night and in the morning all that is left are a few grams of ashes which I sprinkle on the garden.

As I am a bit of a dumb ass I have always been aware of the massive mass loss - logs to ashes - but never thought too much about the chemistry.
Content to leave it as “it disappeared up the chimney”.

But what does happen?

When wood burns, a flame triggers combustion by providing the activation energy needed to start a chemical reaction between the fuel (wood) and oxygen in the air.

The wood is mainly made of carbon, hydrogen, and oxygen. During burning:
* Carbon combines with oxygen to form carbon dioxide gas.
* Hydrogen combines with oxygen to form water vapor.
* These gases float away into the air.
* The small amount left behind as ash is mostly minerals that could not burn.

So the ashes are only a tiny fraction of the original mass because most of the mass became invisible gases in the atmosphere.

The Law of Conservation of Mass

This follows the scientific concept called the Law of Conservation of Mass — if you could collect all the gases and ash, their total mass would equal the original wood plus the oxygen used during burning.

The exact proportions of carbon dioxide (CO₂) and water vapour (H₂O) produced when wood burns depend on the type of wood and how dry it is.

A typical estimate for 1 kg of dry wood burned completely is:
* about 1.6–1.8 kg of CO₂
* about 0.5–0.6 kg of H₂O vapour
* a few grams of ash left behind

So by mass, the combustion products are roughly:
* 70–75% carbon dioxide
* 20–25% water vapor
* 1–5% ash and other gases

The CO₂ mass is larger than the carbon originally in the wood because oxygen from the air becomes part of the CO₂ molecules.