Mishnah for Oholot 5:9

The following vessels protect their contents when they have a tightly fitting cover: those made of cattle dung, of stone, of clay, of earthenware, of sodium carbonate, of the bones of a fish or of its skin, or of the bones of any animal of the sea or of its skin, and wooden vessels that are always clean. They protect whether the covers close their mouths or their sides, whether they stand on their bottoms or lean on their sides. If they were turned over with their mouths downwards they afford protection to all that is beneath them to the nethermost deep. Rabbi Eliezer declares this unclean. These protect everything, except that an earthen vessel protects only foods, liquids and earthen vessels.