Kelim, Chapter Ten, Mishnah Five
1) If [the outer layer] a jar had been peeled off but its pitch [lining] remained intact,
2) And similarly if pots of fish brine were sealed up with gypsum at a level with the brim:
a) Rabbi Judah says: they do not protect.
b) But the sages say: they do protect.
Section one:The outer layer of plaster peeled off the jar, but the inner layer of pitch remains, thereby sealing the jar off at the place where the outer layer peeled off.
Section two: The type of fish pot referred to here has a lip below the rim and inside the pot. The person sealed the pot at this point, in other words within the pot.
It turns out that in both of these cases the seal is inside the pot or jar and not external to it. According to Rabbi Judah these containers do not protect their contents because the lid must be on the jar. This is a very literal reading of Numbers 19:15”a tightly fitting lid on it.”
The other sages do not read the text quite as literally, and therefore claim that these types of lids or seals do protect their contents from impurity.