Pit-firing or sager firing is an ancient process introduced by Native Americans and still practiced today throughout North America. It requires a multi-step process to prepare the pot for firing.
First, green ware (bone dry but unfired pottery) is sanded thoroughly to remove any surface scratches or irregularities and to obtain a smooth surface. The pot is then treated with water or terra-sidulata slip and burnished well with a stone, plastic bag or other implement. This results in a glassy-smooth surface which will replace the gloss of a glaze.
Next the pot is bisque-fired on a relatively low temperature (about 500 degrees).
After firing, the pot is coated with ferric chloride and sprinkled with salt or other organic material such as wood or seaweed.
The pot is then covered with tinfoil, another pot, or anything that will keep the organic materials in contact with the surface of the pot during firing.
The pot is then placed in a large pit of wood, and covered with more wood. The pit is then set on fire, covered to reduce oxygenation, and left to burn for several hours.
The finished pieces are then retrieved from the pit. Most of the tinfoil, if used, has usually burned away.