FAO - Earth Pits for Charcoal Making
Using earth as a shield against oxygen and to insulate the carbonising wood against excessive loss of heat is the oldest system of carbonization and surely goes back to the dawn of history. Even today it is perhaps used to make more charcoal than any other method. It is, therefore, worthy of careful study to find out its advantages and disadvantages. Obviously it keeps its place because of its low cost. Wherever trees grow, earth must be available and it is natural that mankind turned to this cheap available non-combustible material, as a sealing material for enclosing the carbonising wood.
There are two distinct ways to use an earth barrier in charcoal making: one is to excavate a pit, put in the charge of wood and cover the pit with excavated earth to seal up the chamber. The other is to cover a mound or pile of wood on the ground with earth (5, 12, 16, 19, 20, 21, 28). The earth forms the necessary gas-tight insulating barrier behind which carbonization can take place without leakage of air, which would allow the charcoal to burn away to ashes. Both methods, if skilfully carried out, can produce good charcoal within their technological limitations.