We have built the base and created a ring to contain the insulation layer. This layer consists of glass bottles packed with wood chips covered in a clay slip. Clay slip is clay and water mixture the consistency of runny pancake batter. You take the slip and completely cover the wood chips. This process fireproofs them, cements the bottles in place and creates air pockets to trap the heat with.
Once we got both layers done we packed the whole thing in more wood chips. In all we used about 50 lbs of clay and half a bale of wood chips.