Underground Air Flow Pattern
The potter can allow air to flow under piles of coals through underground ducts controlled from outside the kiln (at positions A, B, and C in this case). Of course this enhances combustion of the coals, but as is often the case, there is more to the story: pots can be placed on top of the air ducts and held in place with lumps of fireclay. The tops of the pots get a very rough black glaze effect from the coals, and the bottom sides get various "oxidation/reduction" reactions which can make dramatic colors. These pots are highly prized among Bizen aficionados, and are called "korogashi" (lit. "tipped over") in the Bizen dialect, because they are usually placed on their side for the fire. They are also called "yo-hen" (lit. "kiln-changed"). Thus, one pot can have two quite different "moods", either of which can be displayed at appropriate times- weddings, funerals, bright and happy or dark and sad.