Keilim 10:3-4

Keilim 10:3

If the lid of a jar is loose – but not so loose that it falls off – Rabbi Yehuda says that it protects its contents from impurity but the Sages say that it doesn’t. If the handhold in the lid sinks into the jar, then if a sheretz (vermin) is in the handhold, the jar is rendered unclean; if the sheretz is in the jar, any food in the handhold is rendered unclean.

Keilim 10:4

Let’s say that someone covered the mouth of a jar with balled-up or coiled reeds. If he spread clay (or one of the other approved substances) on the sides, it doesn’t protect the jar’s contents from impurity unless he also spread the clay (or whatever) on the top and the bottom. The same is true if one covered a jar’s mouth with a strip of cloth. If one covered a jar’s mouth with paper or leather and he tied it on with string, then if he spreads clay (etc.) on the sides, it protects the jar’s contents.