For a demonstration, click here . An entire piece of wood or coal will not burn, even if there is sufficient oxygen present. Most of us have taken the ashes from a charcoal grill or fireplace. The ash, generally a mixture of minerals, is present in the fuel, but will not unite with the oxygen.
Some fuels have a lower ash content than others.
This is important to remember when buying charcoal or wood because you want the fuel with the lowest ash content, provided that it is good in other respects. Often the bottom of a pan or a skillet becomes black when it is placed over a fire. This discoloration occurs because of soot. Soot is primarily unburned carbon. The skillet becomes coated because it cools the flame, preventing the temperature from getting high enough to burn the fuel completely.