Microstructure　of　silica　refractory　was　compared　with　that　of　magnesia　refractory　and　was　discussed．　Glass　and／or　low　melting　compounds　in　silica　refractory　are　isolated　among　tridymite　crystals　due　to　solution－precipltation　reaction　between　crlstobalite　and　tridymite．On　the　other　hand，low　melting　compounds　of　magnesia　refractory　spread　over　the　grain　boundaries．　This　difference　makes　difference　in　properties　such　as　high　temperature　strength　under　load． 　　Corrosion　mechanism　of　crown　silica　refractory　by　vapors　such　as　Na2SO4　and　that　of　checker　magnesia　refractory　by　gases　such　as　SO3　were　discussed　thermochemicaily．

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