For　slide　gate　plate　with　a　carbon　component，a　reduction　firing　method　is　applied　to　achieve　superior characteristics．However，a　part　of　the　carbon　material　added，or　a　part　of　the　binder　used，vaporizes，which results　in　a　porous　structure．Usually，plates　are　impregnated　to　reduce　their porosity　and　to　gain　strength． Another　method　is　to　examine　and　adjust　the　raw　material　composition．However，there　are　not　many　cases reported　which　examine　the　grain　size． The　present　study　shows　that　the　material　adjustment　base（l　on　the　closed　packing　theory　results　in　a　dense plate　even　with　reduction　firing．This　is　proved　by　the　actual　performance　of　the　plate　produced　by　this　method， was　successfully　compared　with　that　of　an　impregnated　plate．

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Accurate　value　of　weight　loss　of　magnesia carbon　brick　in　air　was　obtained　continuously　with　a　thermobalance at 1273，1473 and 1673K．The　rate　of　oxidation　was　studied　kinetically　by　application　of　topochemical　reaction model　to　the　oxidation　process．It　was　concluded　that　the　oxidation　process　of　magnesia carbon　brick　in　air　was mainly　controlled　by　the　diffusion　of　O2through　the　porous　decarburized　layer． Furthermore，the　effective diffusion　coefficients　through　the　pores　in　the　decarburized　layer　were　0．25－0．56×10-4 m2・s-1 and　the　activation energy　of　effective　diffusion　coefficient　of　O2 was 36，9 kJ・mol-1．

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