Charcoal is usually produced by slow pyrolysis, the heating of wood in the absence of oxygen. The traditional method uses a clamp. This is essentially a pile of wooden logs leaning against a chimney (logs are placed in a circle). The chimney consists of four wooden stakes. The logs are completely covered with soil and straw allowing no air to enter. It must be lit by introducing some burning fuel into the chimney; the logs burn very slowly and transform into charcoal. If the soil covering gets torn (cracked) by the fire, additional soil is placed on the cracks. Once the burn is complete, the chimney is plugged to prevent air from entering. The true art of this production method is in managing the sufficient generation of heat (by combusting part of the wood material), and its transfer to wood parts in the process of being carbonised. A strong disadvantage of this production method is the huge amount of emissions that are harmful to human health and the environment.