Kelim, Chapter Three, Mishnah Three
1) A jar that had a hole and was mended with pitch and then was broken again:
a) If the fragment that was mended with the pitch can hold a quarter of a log it is unclean, since the designation of a vessel has never ceased to be applied to it.
2) A potsherd that had a hole and was mended with pitch, it is clean though it can contain a quarter of a log, because the designation of a vessel has ceased to be applied to it.
Section one: A jar had a hole in it and thereby became pure, as we learned in mishnah two. When someone fixed it, it again became susceptible to impurity. Then it was broken yet again, leaving a fragment that had been fixed with the pitch. As long as this fragment can still hold a quarter of a log, it is susceptible to impurity. The reason is that although the jar broke, it is still considered a vessel, and when it was fixed it could receive impurity again. Therefore broken pieces of it that can hold a quarter of a log are susceptible to impurity.
Section two: In contrast, a potsherd that had a hole in it and was then fixed, cannot become impure because a potsherd is not a vessel, even if it can contain a quarter of a log. The rule in this matter is as follows: if a broken piece of earthenware gets a hole in it, it can never become impure again because even if it is fixed, it is not considered a vessel.