Faced with several smelting failures, I decided to apply one of the sacred tenets of practical engineering taught to me by my father: "When it won't work, cheat". I swapped out the clay forge for one made of loose-stacked firebricks, solving my spalling and insulation problems at a single blow. I was confident charcoal with forced air could get me the temperature I needed if I got a working geometry for the forge. Here's what I ended up with:
[edit post to add picture here]
Additionally a more knowledgeable copper smelter from the other side of the Atlantic Ocean, gave me some good advice about how to test for what was going wrong. (Thank you Fergus!) Armed with this information, and general his encouragement, I took another shot.
On Wednesday, 16 Jun, 2010, I completed my first copper smelt. Here's a short (and very self-important) video of the results.
I plan to do another smelt tomorrow, on a slightly larger scale, so I'll take pictures and video, and post them.