I have done bronze cast for several years with home made furnaces of varying size made from fire brick and fire clay or from fire clay, silica sand and Portland cement. The ones that have lasted the longest have been the ones out of fire brick. I usually pour my bronze when the pyrometer reads 2,450f so the furnace has to be some hotter then that. The problem with clay, sand and cement furnaces seems to be cracks that developed from heating and cooling over time. These have to be continually patched. The furnaces of both types have to dried out very slowly or expansion of moisture in the refractory will cause explosions. The Portland cement helps the castable refractory flow and mix better then the straight clay in my experience. Very little is required to help mix the sand and clay together with a minimum of water. It should not be so much pourable as packable.
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