Kelim, Chapter Three, Mishnah Eight
1) A jar which which was pierced and the hole stopped up with more pitch than was necessary:
a) That which touches the needed portion is unclean,
b) But that which touches the unneeded portion is clean.
2) Pitch which dripped upon a jar, that which touches it is clean.
3) A wooden or earthen funnel which was stopped up with pitch:
a) Rabbi Elazar ben Azariah says that it is unclean.
b) Rabbi Akiva says that it is unclean when it is of wood and clean when it is of earthenware.
c) Rabbi Yose says that both are clean.
Section one: If one used more pitch than necessary to stop up an earthenware jar, only the part of the pitch that is necessary to stop up the hole is considered to be part of the jar. The unnecessary part is not part of the jar and therefore even if the jar is impure, this part remains pure.
Section two: Pitch that simply dripped onto a jar has no function and is not considered to be part of the jar and therefore it remains pure even if the jar is impure.
Section three: By plugging the funnel with pitch he has now created a receptacle. The rabbis debate whether this makes the funnel (not just the pitch) susceptible to impurity.
Rabbi Elazar ben Azaryah says that this does indeed turn it into a receptacle, thereby making it susceptible to impurity (a susceptible receptaclethis should remind you of a Supertramp song).
Rabbi Akiva notes that since pitch is made of wood extract, it can turn a wooden funnel into a receptacle. However, it does not turn an earthenware funnel into a receptacle because it is of a different kind, so therefore the funnel remains pure.
Rabbi Yose holds that since the plug is not permanent, in neither case is the funnel susceptible to impurity.