Hi Nate,
My first instinct is that it is too much soda, possibly also in combination with particular clay bodies and/or flashing slips. Based on what you have mentioned about your firing it seems less likely to be related to carbon trapping or int...
Will - I've constructed a dual chambered sprung arch cross-draft kiln with the chambers in parallel to the main axis of the kiln rather than the conventional sequential (back-to-back) configuration. I've got a Bourry-style downdraft firebox. Each chamber is about 45 cubic feet in volume. I've fired the kiln using salt or soda in one of the chambers. I'm going with soda in my next firing which will be 2-4 May. Firings have ranged in duration from 36 to 46 hours, but I'm sure I can shorten it once if figure out the optimum damper and air supply settings. I'd like to get it down to 30-36 hours.
William, I love the plate in your group of photos - nice wadding marks and salt glazing! What kind of kiln was it fired in and how long are your firings? What's the salt chamber size and how much salt did you use?