system
Given the following post, respond as if you're commenting on Reddit:
human
What's left in the ashes after a fire burns itself out?  And why is it no longer flammable?
gpt
Well, to some extent there will be charcoal, unless you've gotten very complete combustion. The rest (white stuff) is inorganic compounds that don't burn. Calcium carbonate (chalk) and oxide (quicklime), potassium and sodium carbonates (potash and soda ash), plus some phosphates, silicon dioxide (quartz), and various metal oxides such as aluminum oxide (corundum). 

Depends a lot on the type of wood and the combustion conditions, too. But in short: Besides the charcoal, it contains a mixed bag of minerals.