Adding Hearth Insulation Layer

Above the layer of Rocks we added the bottom insulation layer which insulates the hearth (the oven floor) from the base of the oven.
We chose to make the insulation out of empty glass bottles set in a combination of grass clippings (which we collected from a large pile which remained from mowing a soccer field) with clay slip (clay mixed with water to make a clay 'soup').
The layer of insulation is 5 inches thick.
In the pictures can be seen: Grass clipping mixed with clay slip, and the finished insulation layer.

Filling Center With Rocks

We collected rocks and pieces of broken concrete from near our house and filled the middle of the concrete ring with them. We made sure the rocks weren't loose so they could support the weight of the rest of the oven which will be built upon them. These rocks also act as a capillary break, which prevent rainwater from reaching the oven from the wet ground bellow.