Keilim, Chapter 8, Mishnah 6. This chapter has been exploring the airspace of an earthenware vessel and the ways in which one object placed inside another can shield what it holds from tumah. Our mishnah adds a striking case in which the very same sealed container is a barrier in one direction and no barrier at all in the other.
Two halachot form the background. First, when a corpse lies in a house, the tumah fills the entire space of the house, and everything inside it is subject to becoming tamei. Second, the Torah teaches that an earthenware vessel closed with a tzamid patil, a covering sealed all around, protects whatever is inside it from the tumah of a corpse in the house. An earthenware vessel cannot receive tumah through its outer walls, and so tumah that fills a house has no way to reach into a sealed earthenware container. Our mishnah now teaches that this same sealed container shields its contents even from the tumah of a tamei oven in which it is sitting.
The Case of the Sheretz
The mishnah begins: "Beit se'or mukaf tzamid patil," a sourdough container of earthenware that is closed with a covering sealed all around, "venatun letoch hatanur," and it was placed inside an oven. "Hase'or vehasheretz betocho," the container holds both sourdough and a dead sheretz, "vehakotel beintayim," and there is a partition between them. In other words, picture a single earthenware container divided into two fully separate compartments, one holding sourdough and the other holding the carcass of a sheretz, with the whole vessel sealed and set inside an oven.
The ruling: "hatanur tamei vehase'or tahor." The oven itself becomes tamei, because the sheretz is present in its airspace, and nothing holds that tumah back. The Torah taught only that a vessel sealed with a tzamid patil keeps tumah from entering; it never said that such a seal keeps tumah from leaving. So the tumah of the sheretz emerges from the sealed container and renders the oven tamei.
And yet the sourdough in the other compartment stays tahor. That sourdough sits inside an earthenware vessel closed with a tzamid patil, and such a seal blocks the tumah of the oven from reaching in. The sealed covering functions as a one way shield: tumah cannot come in, but tumah can go out.
The Case of a Kezayit of a Corpse
The mishnah proceeds: "ve'im hayah kazayit min hamet," should that compartment instead contain an olive's volume of a corpse, the ruling is "hatanur vehabayit tamei." There are two grounds here for the oven's tumah: a piece of a corpse occupies the space within it, and beyond that, the oven itself stands in a house where a corpse is found. The house too, together with whatever it contains, is tamei, since corpse tumah fills an entire dwelling.
Still, "vehase'or tahor." The sourdough remains pure, because it lies within a covered and sealed vessel, and that seal holds back both the tumah of the corpse and the tumah of the oven from entering its compartment.
When the Partition Has an Opening
The closing clause reads: "im yesh sham potei'ach tefach, hakol tamei." Should the internal wall that keeps the olive's volume of corpse apart from the sourdough contain a gap measuring a handbreadth in each direction, purity is lost across the board: the dwelling with all its contents, the oven, and this time the sourdough as well. A handbreadth square gap is exactly the route by which corpse tumah travels onward, which is how it moves from room to room beneath one roof. Once the divider is breached in this way, nothing confines the tumah of the corpse any longer, and it reaches everything present, sourdough included.
The lesson of this mishnah is the character of the tzamid patil. It is not a wall in both directions. It faithfully keeps tumah from entering what it protects, whether that tumah comes from a corpse in the house or from a tamei oven, but it does nothing to imprison the tumah of what lies inside it, and it offers no protection at all once an opening of a tefach breaches the partition within.