My FLIR camera maxes out at 700 degrees, so the starting temperature is above that. After 14 hours the interior temp was still over 400, and the next day it was still over 200. I baked eggs in it 36 hours after it was last fired.

The oven has a refractory cement interior and floor, with a layer of insulation top, bottom and sides. The whole thing is coated in stucco. The door is around 6 inches thick and also insulated. This gives it a tremendous thermal mass.

The only significant area of heat loss is around the door. It is a remarkable piece of engineering.