[dmun]

I built my oven with firebricks on edge, for a thickness of 2 1/4 inches, which apart from Christo, who followed my plan, may be the thinnest oven built here. This is covered by a one inch blanket, and a fill of the enclosure with vermiculite concrete, which is as little as an inch or two at the edges and about eight inches on top. My theory was that this was more or less the thickness of the pre-cast refractory ovens, and would behave in more or less the same way. It also had more or less the same wall and floor thickness. This is clearly a low mass oven. It seems to work fine, although I'm a little nervous about driving enough heat into the firebrick layer for the hours-long retained heat turkey cooking. Other makers have gotten a low mass approach to the dome by using 1/3 bricks instead of half bricks.

Most of the attempts to increase mass in the pompeii design have been by adding more mass to the floor. Originally the insulating layer was put under the support slab, so that all four inches of concrete was absorbing heat. Experience showed that this wicked way too much heat away from the cooking floor, and now people who want more mass put a slug of concrete or another layer of firebrick below the cooking floor only, surrounded by insulation. Search "island hearth" for more on this approach. You can also increase the mass of your dome by covering it with a layer of refractory concrete like stucco, before insulating.

The true high mass oven uses a full eight inches of solid refractory material in both the dome and the hearth. This is the classic Alan Scott design, and is mostly used for bread baking, where commercial bakers want multiple bakes from a single firing Canuck Jim has an oven like this, and uses it for multiple bakes of bread, from high heat to low heat, year around.

All forms of oven should have as much insulation as you can reasonably plan for. This keeps the heat in the oven, and the cooking, instead of heating the enclosure and surroundings.

It all depends on what kind of cooking you want to do. The conventional wisdom here is that a low mass oven is for quick heat-ups for Pizza, and a single bread bake. A high mass oven is more useful in an environment where it is fired every day, and the heat retained for the day before is re-used.