Keilim, Chapter 3, Mishnah 6. Our chapter has been examining the point at which a substance smeared onto an earthenware utensil stops being a separate material and becomes part of the utensil itself, with real consequences for tumah. The rule taught just before this mishnah was that a coating joins the vessel only when the vessel depends on it structurally: if the utensil is cracked or broken to the point that it would come apart were the coating not holding it, then the coating is treated as part of the utensil. Here the chachamim add an important qualification. There are cases in which a coating is reckoned as part of the vessel even when the vessel is perfectly whole and needs no help staying together.
Grass Sealing on a Large Water Container
The mishnah opens: "Yavlit shetoflin bah et hapitasin, hanogea bah tamei." The yavlit is a species of grass that was used as a sealant, and the pitas is a large earthenware container. The grass was applied to the container not to repair a break but to keep it from seeping. A vessel of this size was used, for example, aboard a ship, where many passengers had to be supplied with water over a long stretch of time, or where wine was stored for the voyage.
Regarding such a container the mishnah rules hanogea bah tamei: if the vessel itself has become tamei, then food or drink that comes into contact with the grass coating becomes tamei as well, exactly as though it had touched the earthenware. The reason is that a container of this type requires its sealing even in perfect condition. The coating is not an emergency measure applied to a damaged utensil; it is a permanent fixture, always present, without which the vessel cannot do its job. For this reason even the chachamim, who in the previous mishnah limited the rule to a coating that holds a broken utensil together, agree here. Where the coating belongs to the vessel by its very nature, it is part of the vessel regardless, and so it contracts tumah together with it.
The Stopper of a Barrel
The mishnah now moves to a different object: "Migufat hechavit eina chibur." A plug set into the opening of a barrel to close it off does not count as attached to it, and that holds true even where clay has been packed around the plug to hold it firmly. The reasoning is simple. No one ever meant this plug to stay in place permanently. It sits there only until someone wants to get at the wine inside, and at that moment it comes out again. Since it is temporary by design, it never becomes part of the vessel at all, and tumah that strikes the barrel leaves the plug untouched.
The Plastering of an Oven
The closing clause deals with ovens, which in those days were also fashioned out of earthenware. Their exterior surface was covered over with a coat of clay that served as insulation, so that once the oven had been fired the warmth within would be retained for many hours instead of escaping quickly. The mishnah rules: "Hanogea betipulo shel tanur tamei." Should the oven be tamei, whatever food or drink makes contact with that clay covering becomes tamei as well.
The principle is the same one we met with the sealed water container. An oven needs its clay covering in order to function properly, even when nothing about the oven is damaged. Since the coating is essential to the oven's normal operation, it is regarded as part of the oven, and it therefore takes on tumah whenever the oven does, and in turn conveys that tumah to whatever food or drink comes into contact with it.
The mishnah has thus given us two ends of a single line. A stopper meant to be pulled out remains an outsider to the vessel even when clay has been packed around it, while a sealing or plastering that the vessel requires in order to serve its purpose is absorbed into the vessel completely, whole or damaged, and shares its status in tumah.