Thanks for the additional info and photos, Mel!
The positives for clay include, as you say, stability. The negative is drainage.
I think you still want some kind of air-break between your slab and the ground. Otherwise the concrete is going to absorb moisture which will have an effect on heating up the oven.
In my opinion, the simplest way to go would be to look at the instructions for preparing the site under the base, and also at this post about building down to the frostline. You’re not actually going to do that, and you’re not actually worried about frost, I’m assuming. But what you want is the second diagram that shows four footings sunk into the ground.
So, consider this, understanding that some folks here will undoubtedly improve upon it:
- Dig out about 6 inches of the clay, and allow about 2 inches all around the planned dimensions of your hearth.
- Sink four Sonotubes (or similar) down to your ground level (the level where you’d stand in front of the oven), and about 4-6 inches above the top of the raised area. I’d use a 6-inch tube, and make sure that the two outer curves of each Sonotube is just inside the footprint of your planned hearth.
- Use base rock as called for in the plans for the base to do these two things:
- Fill each of the Sonotubes with about 6 inches of rock.
- Fill the rest of the dug-out area to level with the top.
- Add two lengths of rebar as shown in the instructions for the frostproof footings. They should be centered, so they are about 3 inches inside the footprint of your planned hearth.
- Pour concrete into each of the Sonotubes and make sure the concrete is leveled at the top. The rebar should be sticking out of the concrete, so use safety caps!
- When dry, construct your monolith form and drill holes just to the diameter and location of the 8 rebars. Follow the instructions to bend each of the rebars 90 degrees inside your form (as though it were a slab). There is enough allowance at the corners so that this tie-in step will not interfere with the insulation void of your hearth.
In my opinion, this will give you a solid, stable base that allows air circulation and drying underneath, and you’ll be assured of a level hearth slab without a lot of fuss.
Hope this helps, Mel, and that sounds like the best Christmas present of all!