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The idea is to make a weak mortar; strong enough to hold the bricks in place while you build the dome, but weak enough to allow swelling once the floor bricks get hot and start to expand.
As the plans say:
"The underfloor is paste, the texture of a sticky mortar, comprised of 1 part fine sand and 1 part fireclay (photo 2). Spread the underfloor using a notched trowel as the ridges will make it easier for you to get your floor perfectly level (photo 3)."
I did this just yesterday, as a matter of fact, and it worked quite well. Be sure to use a 1/4" X 1/4" notched trowel and have a rubber mallet on hand to tap each brick into place.
There is nothing quite so satisfying as drinking a cold beer, while tending a hot fire, in an oven that you built yourself, and making the best pizza that your friends have ever had.
I too did this yesterday! I found that I had to make the paste much wetter than I thought to get it to go through the notched trowel. I also needed much less than I made. I had to figure out how to throw away a big sack of wet mortar. I must have 20 lbs of fireclay leftover as well.