If I was adapting that forge to burn strictly charcoal I would probably line the firpot with firebrick splits---about 1" thick and then have loose firebrick along the top edges that I could configure as needed.
You need depth to get a good reducing fire but since *all* the charcoal is burning you don't want a lot of it that's farthur away from the work piece as it's only contributing to burning your arm hairs.
So I use a firebrick corral to keep the fuel from spreding out.
Thomas
__________________
Thomas
|