Keilim, Chapter 8, Mishnah 3. Our chapter is occupied with the airspace of an earthenware oven and with the question of when an object placed at that airspace has an interior of its own. A true keli that is set into the mouth of an oven creates its own separate space, so that whatever lies inside it is not reckoned as lying in the oven's airspace. Our mishnah presents a further case of an object that is not treated as a keli, and therefore does not generate a separate airspace at all.
The mishnah's first case opens with a "serida", a board made of earthenware, described as "shehi netunah", positioned "al pi hatanur", over the oven's opening. Of this board the mishnah adds "veshoka'at letocho", that it sags downward into the space within the oven, and "ve'ein lah gapayim", that no lip or wall runs around its perimeter.
What is this object? It is a flat perforated earthenware board that was used for kneading dough. Because it is flat, it does not have the status of a keli. It is true that the board is firm and does not tilt, and that its surface is not perfectly level but dips slightly in the center, yet that slight dip is not enough to turn it into a receptacle. In our case the center of the board sags beneath the level of the oven's mouth, while its edges rest above the oven's rim.
What does halachah say about such an arrangement? The mishnah rules: "hasheretz betochah, hatanur tamei". Should a creeping creature be lying in the sagging center of the board, and thus below the height of the oven's rim, the oven contracts tumah. The board carries no keli status, so the depression in it counts as nothing more than a part of the oven's own space. The creature is therefore located plainly within the airspace of the oven, and not, as it would be with an authentic vessel, within an inner space contained inside the oven's space. Being there, it conveys tumah to the oven.
The opposite direction is stated as well: "hasheretz batanur, ochalim shebetochah temei'in". When the creeping creature lies within the oven, so that the oven becomes tamei, food resting in the sunken part of the board picks up tumah along with it, since nothing surrounds that food that would count as a vessel of its own. A board of this sort has no keli status, and the depression in it forms no domain distinct from the inside of the oven.
At this point the mishnah gives the governing rule: "she'ein matzilin miyad", nothing provides protection from the reach of a "keli cheres", an earthenware vessel, "ela keilim", apart from genuine vessels. Only a real keli has an inside that halachah recognizes as a domain of its own, and only such an inside guards what it holds from the tumah of earthenware. An item that happens to have some depression in it but does not qualify as a keli, whether these perforated kneading boards or the vessel fragments treated earlier, owns no airspace that could be counted as detached from the airspace of the earthenware oven or vessel. That is what stands behind the rulings just given: the insides of keilim alone are independent, and they alone hold back the tumah of earthenware, while something lacking a true interior is never granted separate status.
The mishnah continues with cases in which the oven's airspace simply does not reach the object: "Chavit shehi meleiah mashkin tehorin", a barrel filled with liquids that are tahor, "untunah lematah minichushto shel tanur", and it is set below the base of the oven. Picture an oven with a small opening at its bottom resting on the ground, with a cavity dug in the ground beneath it, and an open barrel of liquid fitted into that cavity. The mouth of the full barrel reaches up to the oven's airspace but does not enter it.
Here the ruling reads "hasheretz batanur": a creeping creature inside the oven renders that oven tamei, and yet "hechavit vehamashkin tehorin", the barrel below, together with everything it holds, keeps its purity, for nothing there ever entered the oven's inner space.
Now the mishnah reverses the picture and sets the barrel above the oven, so that a portion of it occupies the airspace of the oven's upper opening: "Haytah kefuyah", the barrel was empty and stood inverted, "al piha", upon its own mouth, with that mouth jutting into "avir hatanur", the oven's airspace. The decision is "hasheretz batanur", a creeping creature within the oven, which does render the oven tamei, and nevertheless "mashkeh tofei'ach", the film of liquid "shebeshulei hechavit tahor", clinging to the barrel's base, is not defiled. Those droplets lie above the oven's airspace rather than inside it. Any lachut, any dampness, left on the underside of the barrel, which is now turned skyward and stands outside the region of air belonging to the tamei oven, therefore stays tahor.
The barrel is pure as well, notwithstanding the fact that part of it does stand inside the oven's airspace. The reason is that a vessel never acquires tumah from a defiled earthenware oven in the first place. This is precisely why our cases have consistently spoken of ochalin: it is the foodstuff lying within a vessel that a tamei oven can render impure, while the vessels themselves are unaffected by it.