Its a long time ago now when I was at college & the the beer has damaged what few brain cells I had... but I think the clay pot used in a traditional tandoor retains the heat whereas the metal in your prototype conducts heat, so the initial temperature would be the same and as the primary heat source (coals)subsides the metal would lose the heat to your lagging but the pottery would maintain a steady temperature. For example you have been using a granite tile for naan trials? take it out of your oven along with a metal grill & see how long the granite maintains the heat compared to the metal grill.
I have been wrong before tho ;D
I have been wrong before tho ;D


