Charcoal is wood that has been burned with a limited amount of oxygen. Because there is little oxygen in the fire, the wood hardly burns, causing it to char. Substances contained in the wood, such as methane, hydrogen and water vapor burn, so that after the charring you are left with more or less pure carbon. This process is also called pyrolysis. Charring of wood can take several days. The final fabric is much purer and also burns better than regular wood.