Clay convertible by thermal treatment into crystalline mulite and silica is formed into green coherent bodies of desired size and shape such as for example, microspheres, pellets or honeycombs. The bodies are then calcined to convert the clay into a mixture of crystalline mullite and silica, thereby also hardening the bodies. Without destroying the form of the bodies, e.g., without grinding and without causing the bodies to decrepitate, pores in the size range of about 150 to 350A are formed by selectively leaching silica from the bodies. The porous mullite bodies retain porosity at elevated temperatures and are useful, for example, as catalyst supports and contact materials. Pore structure in the meso- (100-600A diameter) and macro- (600-20,000A diameter) size ranges may be controlled.
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