here's the firepot in the "model 150" base heater. notice the rust spots on it, that is where the thin red refractory paint coating had worn off over time. it begins to rust right through the firepot. I have heard from others saying theirs had holes rotted through the firepot. This is how it starts. Don't know what the original lining was made of, but it looks like a red slurry of course gritty red brick material, that could be painted on, of the pot was dipped in it.
I cleaned and relined over this surface. not too worried about it, if it cracks or burns through, I'll have it recast. if I decide to burn this stove full time to heat with, will have new grates and pot cast up using old ones as mold pattern.