Keilim, Chapter 3, Mishnah 5. The mishnah just before this one dealt with a barrel that had cracked and was then coated with a substance that reinforced it and held it together. Our mishnah turns its attention to the coating itself: does the coating count as part of the utensil for the laws of tum'ah, so that it becomes tamei along with it and passes tum'ah on to whatever touches it? The mishnah begins with the case of a coating that was spread over a utensil that was perfectly sound to begin with.
Coating a Sound Earthenware Utensil
The mishnah opens: "Hatofel keli cheres habari" - one who coats an earthenware utensil that was bari, healthy and whole, with no crack and no weakness. This utensil then contracts tum'ah in the way earthenware contracts it, through its airspace: a source of tum'ah entered the hollow of the vessel without even touching its walls.
What is the halachah? Rebbi Meir, together with Rebbi Shimon, declares the layer tamei along with the vessel it covers: "metame'im." It is true that this keli would do its job perfectly well with nothing spread over it, and yet the layer is still performing a service for it, strengthening the vessel and giving it a longer life. In their view that service alone is enough to make the covering a real component of the keli. And the halachic consequence follows immediately: once the vessel has been rendered tamei through its airspace, the tum'ah spreads into the covering too, so that any food or drink coming up against that outer layer is rendered tamei precisely as if it had come up against the side of the vessel itself.
The Position of the Chachamim
"Vachachamim omerim" - the Chachamim reply that the answer is not uniform, but depends entirely on the condition of the vessel that received the coating. Where it was whole, "hatofel et habari" is "tahor"; where it was damaged, "hara'ua", the coating is "tamei".
Take first the vessel that was intact and undamaged when the layer was applied, and which afterwards became tamei. The coating remains tahor. Why? Because this keli derives no benefit from the layer that it could not manage without; the covering is in no way an essential component, and therefore the halachah does not reckon it together with the vessel. It cannot even be classified as a yad, a handle, for it plays no role in how the vessel is used. In practice, then, food or liquid touching such a layer is not rendered tamei.
Now take the second case: the keli had cracked, and the layer was applied for the express purpose of keeping it from coming apart. When that vessel becomes tamei, its coating becomes tamei together with it. In this situation the vessel is truly reliant on the covering in order to serve as a vessel at all, and whatever the keli cannot manage without is treated as an indispensable component of it, taking on the tum'ah of the vessel just as the rest of the keli does.
The Reinforcing Ring on a Gourd Shell
In the times of Chazal, the hollowed shell of a gourd or a pumpkin was commonly used as a bucket for drawing water out of a well. Since such a shell would crack when it knocked against the sides of the well or rubbed against something hard, people would fit a ring of wood or of metal around it to strengthen it. The mishnah teaches the halachah of such a ring: "Vechen bechiduk hakilu'ach" - the identical dispute between Rebbi Meir and Rebbi Shimon on one side and the Chachamim on the other applies to a reinforcing ring fastened around the shell of a gourd or pumpkin.
The first opinion, that of Rebbi Meir and of Rebbi Shimon, holds that since the band shields the shell from splitting, that protective function makes it a component of the shell. Consequently, the moment the shell contracts tum'ah, the band fitted around it contracts tum'ah as well.
The Chachamim see it otherwise. The shell on its own is already sturdy enough to serve as a vessel, and the band contributes nothing to its actual usability. On that basis the band is not reckoned as belonging to the shell, notwithstanding the protection it provides against breakage. Not being a component of the vessel, it remains tahor from beginning to end, even at a moment when the gourd itself is tamei.
This, then, is the machloket of our mishnah: Rebbi Meir and Rebbi Shimon measure the coating or the ring by whether it serves the utensil at all, while the Chachamim measure it by whether the utensil actually needs it in order to function.