Thin　flacks　of　yttrium　hydroxide　coagulated　in　a　manner　similar　to　houses　of　cards　at　again　temperatures　lower　than　room　temperature．Increase　of　againg　temperature　resulted　in　not　only　increasing　the　thickness　of　the　flacks　concemed　but　also　breaking　the　coagulation．Fine　disks　of　yttrium　hydroxide　appeared　above　50℃．Sulfate　ions　enhanced　sinterability　of　yttria．The　yttria　powder　fabricated　was　mixed　with　a　commercially　available　alumina　powder　in　the　ratio　of　yttrium　aluminum　garnet，YAG．Many　inclusions　and　pores　were　noticed　in　the　powder　compact　sintered　at　1600℃．Yttrium　ions　were　rich　in　some　inclusions　and　poor　in　the　rest．　Such　inclusions　and　pores　disappeared　by　firing　at　1700℃　for　1h，and　transparency　was　achieved　in　the　case　where　the　highly sinterable　yttria　powder　was　mixed　with　the　alumina　powder　with　a　particle　size　of　0.3μm．

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