While we're at it, dry wood doesn't burn, either. Next time you haven't got a great deal to do, watch what goes on in the woodstove carefully. The stick of wood is the storage chamber for volatile gasses. When the wood is heated well into the 450 degree range, these gasses are driven off and ignite. After all the volatiles are driven off, the wood becomes charcoal.
Charcoal is flammable, which is why it glows red in the firebox.
When all that can burn in our once handsome chunk of wood has burned, we are left with ash. Some ash is required to insulate the hot coals and preserve them for the morning reload. Too much ash chokes out the fire, and must be removed. A sure sign of too much ash is the inability to place more than two tiny pieces of wood into the firebox. Do yourself a favor and take out the ash.