When the cylinder is filled with charred wood, douse coals with sufficient water to quench the flames (for a 4’ X 4’ cylinder, 5 to 10 gallons should be adequate.) Be careful not to get burned by the steam.
Lid Method: Slide a fitted lid into the inside of the cylinder, so that the lid sits on top of the char, and seal the gap between the lid and the edge of the cylinder with soil. Steam leaks will indicate the need for a better seal.
Once the cylinder has cooled off entirely (this takes 2 days), tip it over to retrieve the biochar. Be very careful with fresh char, as it can hold heat for a surprisingly long time. If a still live coal is exposed to air, it will burn. Make sure there is no heat in the char before transporting.
Water Method: If there is plenty of water available close to the fire source, another way to put out the fire is possible, but it requires a significant amount of water (80-100 gallons and diligence). First, quench the flames with water and cool the cylinder by spraying the exterior with water. The kiln must then be tipped over, either by hand if it is a small kiln or by mechanical means (tractor or come-a-long) if larger. Spray the char with water while raking it around. Continue stirring and wetting the char while looking for hot spots. Any steam issuing from the pile indicates that the fire is not out. After an hour or two, return to the pile and spray and rake it again. It is highly recommended that the char be inspected at least once more before assuming that the fire is out and the char is safe to handle and transport.