This is going to be a difficult one as you've already built a structure, however the best advice I could give would be to buy a pizza 'stone' which is a round fired-clay disc. Light your fire and burn enough stuff to give you embers to heat the floor, sweep to one side and put in your pizza stone, let it get hot and slap on a pizza. The alternative would be a fairly extensive remodel as you've a couple of basic design flaws.
As a lot of your heat is going to go straight up and out of your vent it's really difficult to say how the sandstone (or the mortar) will react but I suspect it'll be ok for what you want. You'll need to watch for debris falling onto the plate as no one wants a gritty pizza !
Can you make a former inside the oven with a few inch gap between it and the sandstone and then fill the gap (from the top) with refractory cement which will give you some insulation and protection for the structure. In doing this you could also look at some way of baffling the "straight up" route for heat loss?
Post by downunderdave on Mar 26, 2019 19:43:20 GMT
The sandstone is one issue; the design and lack of insulation is another. An uninsulated oven wil not achieve higher temperature or retain heat for a usable cooking time. The insulation must encapsulate the whole oven,so underfloor insulation is just as important as over the top. Because although heat by convection rises, conductive heat doesn’t care about direction and will travel from the floor into the structure that surrounds it. An updraft design has the advantage of drawing very well reducing smoke issues at start up, as well as having no entry to have to work past. It’s disadvantage is that the flames will jump straight to the flue resulting in heat loss and high fuel consumption. As is I’d expect you’d be unable to even burn off the black soot from inside the oven (around 300 C) not hot enough for pizza and the temperature drop off too fast for any retained heat baking or roasting.