This paper deals with mechanical and thermal properties in steatite, forsterite, high and low alumina and mullite together with their microstructures of fired surfaces when changing firing temperatures. The constituents and minerals produced are identified by X-ray analysis, chemical analysis and polarization microscopy, while the macro and micro-mechanical properties are estimated by bending strength and micro-Vickers hardness. The thermal shock resistance is also shown. The features of fired surfaces are discussed from the viewpoint of surface roughness and electron photomicrograph. The principal results obtained are summarized as follows:
(1) Bending strength increases as the firing temperature drops in steatite, which means the effect of pores.
(2) The separation of phases progresses as the temperature drops in forsterite.
(3) The grain size of crystal becomes smaller as the temperature drops, but bending strength is low at the lowest temperature in high alumina.
(4) The proper temperature range is shallow for low alumina because of its double con stituent.
(5) The separation of matrix and the existence of pores cause the bending strength of mullite to be low.
