Keilim, Chapter 3, Mishnah 3. This chapter has been measuring how large a hole must be before an earthenware vessel stops counting as a vessel at all. Our mishnah takes the next step: what happens when such a hole is plugged up and the vessel is put back into service? The answer turns out to depend on what was holed in the first place. There is one halachah for a whole, intact vessel that developed a hole and was repaired, and a different halachah for a broken fragment that was still usable, developed a hole, and was repaired.
A barrel that was holed, patched, and then shattered
The mishnah begins with "chavit shenikvah": an earthenware barrel that developed a hole large enough to remove it from the category of a vessel. At that moment it is tahor, incapable of contracting tumah, because halachically it is no longer a keli.
Then, "va'asa'ah bezefet": the owner sealed the hole with tar. The patch restores the barrel's ability to contain its contents, so it is a vessel once more and once again capable of receiving tumah.
Now comes the case: "venishberah", the repaired barrel broke into many pieces, and among them is the very piece that carries the tar plug. What is the status of that fragment?
The mishnah rules: "im yesh bimkom hazefet machazik revi'it, teme'ah". If the fragment bearing the tar can hold a revi'it of liquid (roughly three ounces in our terms), that fragment is fit to become tamei.
Then comes the rationale: "mipnei shelo batal shem keli me'aleha", the designation of "vessel" had never been stripped away from it. Throughout the stretch of time in which the hole had rendered it tahor, anyone who looked at it still called the thing a barrel; in the minds of those who owned and used it, it had remained the same object all along. Its release from tumah had to do with what it could no longer do, not with what it was. So the moment tar closed the gap, it went back to being a barrel in every sense.
Since the patched barrel was a full barrel again, when it later shattered its fragments are judged exactly like the fragments of any complete barrel that breaks: each shard is capable of tumah if it can still hold a revi'it. The shard that happens to contain the tar plug enjoys no lesser status than the others. If it holds a revi'it, it is susceptible to tumah.
A broken shard that was holed and patched
The second half of the mishnah moves to an object of an altogether different sort. Here we are not dealing with a barrel in one piece, but with a leftover chip of clay salvaged from a vessel that had already smashed.
"Cheres shenikav": such a leftover chip itself sustains a puncture, one through which liquid drains away, and with that puncture the chip becomes tahor. "Va'asahu bezefet", the owner spread tar over the opening to close it. But here the verdict runs the other way: "tahor". The mishnah underscores the point with "af al pi shemachazik revi'it", meaning that even where the sealed chip is able to contain a quarter-log of liquid, tumah has no hold on it.
Why? "Mipnei shebatal shem keli me'alav", the title of "vessel" had already been removed from this object at the moment the original keli shattered. From that break onward it never once earned back the name "barrel", not even during the stretch before its own puncture appeared. Granted, while it was still whole it could indeed take on tumah, since it held enough liquid to matter; yet nobody was calling it a barrel any longer. Its name now was gisterah, a serviceable chip of pottery. And once a gisterah is punctured, mending it afterward leaves it too slight to be a candidate for tumah at all.
The distinction
The mishnah is therefore drawing a line between two situations. A barrel that developed a hole and was plugged reclaims its name and standing as a barrel, so that when it later breaks, the tarred shard is treated like every other shard. A gisterah, by contrast, a fragment already parted from its parent vessel, has no name of its own to return to. Once it develops a hole, it is regarded as entirely useless, the sort of thing a person simply discards. Patching it does not help: it is batel, insignificant, and it can never recover the status of a keli or the capacity to become tamei.