William, Fondue is rated at 1200c , it is a standard European refractory cement. Prices have gone mad recently, pre covid I was paying £20 for 20kg. now it is £45.
Yes a top lit and square staked fire, normally takes four loads to heat a cold oven but you can re
boot a warm oven the next day with just one load.
How the oven works has not really changed for two thousand years, the Romans worked out the formula for us and it has not been improved since. Materials may have advanced but there are still complete roman ovens to be found in Italy!
So basically cold air enters through a correctly sized door and travels and a low level to
feed the fire, the fire heats the top of the dome and exits from a correctly placed chimney, as long as the propositions are right, the hot air will form a circular motion taking any smoke to the top of the dome where it is burnt off before exiting the dome.
Simple and clever.