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. If a furnace produces great quantities of soot, some of the carbon of the fuel is not being burned, and is wasted. This problem can be remedied by seeing that sufficient air is supplied to burn all the carbon in the fuel. Gases Substances that burn in air are nearly always composed of two elements, carbon and hydrogen, or their compounds. For example, coal, coke, and charcoal are mostly carbon.