Ohalos, Chapter 12, Mishnah 1. This mishnah continues the discussion of an ohel that rests upon a keli, and it applies that principle to a very common arrangement: a flat wooden board laid across the mouth of an oven. Everything will hinge on one question: is that oven a finished keli or not?
The mishnah's first words are "Neser shehu nosun", a board that has been set down, and set down where? "Al pi tanur chodosh": across the opening of an oven that the mishnah labels new.
What Makes an Oven "New"
The word chodosh here is not about age. An oven is manufactured from cheres, earthenware. It is first hardened in a furnace, and afterwards it is coated with a plaster-like substance, a kind of tit. That coating only sets and hardens when the oven is fired up the first time for baking. Until that firing takes place, the oven simply cannot be used.
We have already met this rule in Maseches Keilim: keilim do not become mekabel tumah until their manufacture is complete, nigmerah melachtan. An oven that came out of the furnace but was never heated to the point that one could bake in it is still unfinished, and an unfinished keli is not ro'i l'kabel tumah. That is a tanur chodosh.
Correspondingly, a tanur yashan in these mishnayos does not mean an oven that is two or three years old. It means an oven that was already used, even a single time: it was heated to baking temperature, the tit hardened, and from that moment it is a finished keli that is mekabel tumah.
The Case
So picture the setup. There is an oven that has never been fired. Over its mouth lies a flat wooden board. A board by itself is not a keli at all, so it functions as an ohel, a roof.
The Bartenura places this oven outdoors: it is standing in a garden or in a chatzer, with nothing but sky overhead, not inside a house.
The mishnah goes on: "v'oideif mikol tz'dodov b'foseach tefach". Oideif means extra: the board is wide enough that it juts out past the oven on every side by a full tefach. The result is that alongside the oven, under the overhang, there is a covered air space measuring one tefach in each direction. Crawl in there and look upward, and you find such a pocket of roofed space encircling the oven.
Our case, then, in brief: an oven that has yet to be fired, standing in the open air, with a board across it whose overhang leaves a tefach square of covered space on every side. With the picture set, we turn to the din.
The Board as a Barrier
Now place tumah, a kezayis min hameis, in that covered pocket at the side of the oven, beneath the projecting part of the board. The mishnah's ruling is keilim she'al gabav tehorim: vessels sitting above remain tahor, and this includes a keli positioned exactly over the mekom hatumah. Run a vertical line up from the kezayis and it meets the board; a keli parked on the wood at that very point stays tahor. Why? Because the board acts as a roof, and a roof puts an end to the tumah's ascent. The tumah climbs as far as the wood and travels no further, since a tanur chodosh has no status of a keli.
The same works in the opposite direction: tumah al gabav, keilim shetachtov tehorim. Place the kezayis above the wood, and whatever keilim lie below stay tahor, even one situated squarely underneath the tumah. Just as the board halts tumah on its way up, serving as a chatzitzah, it equally holds back tumah trying to travel downward.
All of this holds only because the oven is a tanur chodosh.
When the Oven Was Already Used
The mishnah continues: u'byashan, and with an old oven, meaning one that was fired even once so that the tit hardened and it became an ordinary keli, the ruling reverses: tamei. Whether the tumah lies beneath the board or upon it, the oven itself becomes tamei, the board does not spare it, and certainly the keilim above and below the board are not spared either.
What has changed? The oven is now a completed vessel. And once it is a vessel, the board lying across it is a covering whose entire support is a keli, the same situation as the basket propped on the menorah taught earlier in these mishnayos. Such a covering inherits the din of whatever holds it up, and the din of a keli is that it can form an ohel for spreading tumah while it can never serve as a chatzitzah to hold tumah back.
Practically: a kezayis min hameis lying underneath sends its tumah up to the wood, fills the whole sheltered area, renders the oven and every keli down there tamei, and then continues right through to whatever rests above. Reverse it, with the kezayis sitting above the wood, and the tumah descends through it, floods the space below, and climbs back out again. Which side the kezayis happens to be on changes nothing at all: everything becomes tamei.
The whole matter in a sentence: an oven still in its chodosh state, never once fired for baking, carries no status of a keli, so the board above it counts as a genuine ohel, one that conveys tumah and also stops it. But grant the oven the standing of a vessel, and the board is downgraded to a covering held up by a keli, able to convey tumah and no longer able to hold it back.
Reb Yochanan ben Nuri Disagrees
The mishnah now brings a dissenting voice: Reb Yochanan ben Nuri metaher. In his view, even a tanur yashan does not count as a vessel where the halachos of tumas ohel are concerned. The board lying over it keeps its standing as a proper ohel, which means it both conveys tumah and seals it off.
His disagreement is narrow. Set the same board on top of some ordinary vessel, and Reb Yochanan ben Nuri would agree that the surroundings, the vessel included, all become tamei; the principle that a covering propped up on a keli transmits tumah and does not stop it is one he accepts entirely. What he contests concerns ovens alone.
The logic behind his position: an oven is fundamentally not a vessel, since it possesses no bottom of its own. It is nothing more than walls of cheres placed upon the ground itself. If so, how does an oven ever come to be mekabel tumah, as we find throughout the mishnayos? Reb Yochanan ben Nuri responds that its tumah is a gzeiras hakosuv, a decree of the possuk making an oven receive tumah despite its not truly qualifying as a vessel. And because it never truly qualifies, the distinction between a fresh oven and a used one falls away. The board above it is comparable to the roof of a house: a complete ohel, resting on no vessel at all, and hence one that both transmits tumah and forms a chatzitzah.
A Board Bridging Two Ovens
The same machloikes is now replayed in a second configuration. Rather than a single oven with the board hanging over on every side, we have a board nosun al pi shnei tanurim, stretched from the mouth of one oven to the mouth of another, both of them already fired, so that the wood bridges the gap. The tumah, says the mishnah, is beineihem: it lies in the gap that the board spans. The board again overhangs the ovens by a tefach in each direction all around.
The Tanna Kamma's verdict is heim tamei: both ovens contract tumah, along with whatever keilim are below the board and whatever keilim sit above it. Since these ovens have already been used, they qualify as vessels, and a board whose supports are vessels forms only the kind of ohel that carries tumah along without ever arresting it.
And Reb Yochanan ben Nuri metaher in this arrangement as well, consistent with his position throughout. A fired oven is still no vessel as far as the halachos of ohel go, so wood laid upon two such ovens constitutes a full ohel, one that carries tumah and equally cuts it off. Tumah in the gap below leaves whatever is above tahor, and tumah above leaves whatever is below tahor.
Summary
Take a board spread over a tanur chodosh, an oven not yet heated for baking, with an overhang on each side of a posei'ach tefach, one tefach square: tumah below the wood leaves the keilim above tahor, even one standing right over it, and tumah above the wood leaves the keilim below tahor, even one lying right under it. That the oven is tahor needs no saying, since as an unfinished vessel it cannot take tumah in the first place. The novelty being taught is the taharah of the keilim on either side of the board.
Where the oven is a tanur yashan, already fired at least once, the ruling flips and all of it becomes tamei. Reb Yochanan ben Nuri holds tahor here too: to him a used oven has never acquired the standing of a vessel, and mimeila the wood above it stays a complete ohel, one that is able to serve as a chatzitzah.
That identical argument recurs where one board bridges two ovens that have been used, with the tumah lying in the space they straddle: the Tanna Kamma declares the ovens tamei together with every keli above and beneath, whereas for Reb Yochanan ben Nuri tumah underneath never reaches upward, and tumah on top never reaches downward.