[Dutchoven]

I wholeheartedly concur with dmun...I created the oven hearth slab in the manner of Alan Scott and at the time he suggested bonding the insulating slab underneath the regular concrete slab to support the oven...it works but not nearly as well as the inverse would have...insulate with the vermicrete and if you can add a layer of FB board insulation over that and then lay the floor bricks...that is what I am doing on my next build...as for the mixture it is by volume...you could be really specific and build yourself a box with the inside dimensions of 12" by 12" by 12" and measure your parts that way(would be exactly one cubic foot)...portland and vermiculite is good for the hearth insulation but I would recommend mixing vermiculite with "Type N" masonry mortar for the outside of the dome...type N is a bit softer and sticks better to vertical surfaces...
Dutch

Originally Posted by dmun

Insulate! Insulate! Insulate!

If you put your bricks directly on the support slab, it will wick all the heat out of your floor, and you will never get your oven to pizza temperatures.

You can't skip this step! If you do, you will have an oven that will not cook properly.

Be glad you asked this question first. It's so sad to tell people who have already built their domes, that there's nothing much they can do except start over.