Today i finished the base for the cob oven.
The Insulation Layer
We want to retain as much heat in the thermal mass of the oven as possible. I made an insulation layer below the fire bricks that will be the bottom of the oven to keep the heat from being conducted into the oven base. The principle here is the same as with double paned windows. Air is a poor conductor of heat. First i placed empty bottles and jars on their sides. The insides of the bottles and jars are filled with air. Then I filled the spaces between them with a mixture of sand, clay and a good bit of straw with a little portland cement mixed in for strength. Straw is hollow so it is a poor conductor of heat as well.
The Final Layer That Will Support The Fire Brick.
I finished off the base with a 1 1/2" layer of structural cob which is just clay and sand in a ratio of 1 part clay to 2 parts sand. I determined the ratio by making test bricks a few days ago. I made same sized bricks using different ratios of clay to cob. 1:1, 1:2, and 1:3. I dried them in the oven using low heat. The 1:1 ratio brick shrank a lot. The 1:3 brick turned out deformed and slightly crumbly. The 1:2 ratio brick is perfect! That is the ratio we will use when we make the cob tomorrow.