Earth, which is technically referred to as fire clay in the oven building industry, is one of the materials which are used in building an oven. It is used for its outstanding heat resistance. It is used as an oven building material as well as an insulation in fire brick mad ovens. Silica sand is used to insulate the bed under the fire clay in earth oven as well as in fire brick ovens.
Even though fire clay is an excellent oven building material, it has the problem of cracking as intolerance to moisture. To avert this problem, clay is made more stable by addition of mortar.
Further information: Clay oven, Clay oven pizza, Build a wood burning clay pizza oven and Building a clay oven.