pizzaneer- here's some shots of my kiln. The firebox is a "Bourry box"- 40" wide wood is suspended above the coal bed- primary air is sucked through the wood charge, so it effectively burns upside down. The fire gets superheated as it approaches the coal bed. The firebox shot shows the "throat" area as well. This firebox design has a number of advantages,including the ability to put a lot of wood in, and that the wood shields you from the intense heat in the kiln (some other designs will catch you on fire from radiative heat if you aren't careful).
The pic with me loading shows the flue area- the vent to the chimney. They go all the way across to even flow, and I have "passive dampers" that I can use to direct heat left and right to even out a firing.
The fire pic is me next to the firebox during a "reduction" cycle- getting the burn to be inefficient at certain times actually helps with clay and glaze colors. With the draft slowed way down, the gases produced by the heating wood create a back pressure, and the hot gases combust when they hit air.