Other methods achieve similar results. Commercially available, high-strength insulative board makes a good, firm, warm “oven mattress” that conveniently solves the problem of weight bearing insulation and provides an easy way to break contact with surrounding cold. Some earthen oven builders use a lo-cost combination of empty glass bottles surrounded by an insulative mix of clay and fine organic matter (sawdust, chaff, nut shells, etc.); as the organics burn out, they leave thousands of tiny voids in the clay, making a spongey, insulative, and firm foam-like material.