I've just discovered that two vases I've made from a cone 5-6 stoneware clay are slowly leaking onto the table!   Both were glazed inside, dry-footed. Fired to cone 6. The glaze appears to be thin in some spots near the bottom inside. Is this why water is slowly leaking through?   Should I refire them with additional glaze inside? With enough glaze inside should pots be safe to use as vases?   Thanks for any help!   ginny c.