The pit we use is an above ground pit. Soil in Door County is really rocks with some dirt sprinkled in, so we built our 30"x 60"x 30" pit with old bricks. Pits are usually much bigger than this; we decided to stay small so we could fire weekly.
We first fill the pit with 4-6" of sawdust and then line it with firewood. Salts (table, sea, water softener, de-icer), oxides such as copper (reds and greens), cobalt (blues) and iron (browns, blacks) and dry dog food are sprinkled on the sawdust before we tumble stack the pottery. The stacking is very important because shifting pottery can cause breakage. We sprinkle more sawdust, salts, oxides and dry dog food, banana peels, pine cones, crumpled newspaper, scrap wood, excelsior, leaves, cherry pits and walnut shells. In short: combustibles. All is covered with a layer of wood. Twisted newspaper is tucked between the cracks. Waste oil, either auto or aviation is dribbled onto the wood and the newspaper is lit.
After about 30 minutes, the potters don gloves and place an angle iron lengthwise over the pit. Using that iron as a divider, they cover the pit with metal drum lids and broken kiln shelves. The reduction in oxygen that occurs when they do this has a dramatic effect and smoldering occurs. This causes the gases and flames to force color into the pottery. Several hours later the pit is cool enough to open. Since we fire at 5 p.m., we wait until 10 a.m. the next day so that our friends can see the results. This pottery is fragile and needs special care. Wipe with a damp cloth. Do not put water into the vases. Use dried flowers instead. The hairline cracks that sometimes occur adds to the beauty and interest.