Keilim, Chapter 5, Mishnah 6. Our chapter has been occupied with the earthenware oven, the tannur, its structure, when its manufacture is complete, and how it contracts tum'ah. This mishnah presents two cases. In the first, someone fills the lower half of his oven with dirt, so that only the upper half is still usable. In the second, an oven is not built onto the ground at all, but set over the mouth of a pit and propped in place with a stone.
An oven filled halfway with dirt
The first case: "tannur", an earthenware oven, into which its owner poured "afar ad chetzyo", dirt reaching halfway up its height. Earth now occupies the bottom half, leaving only the upper half as open space fit for baking. Our question is how the laws of tum'ah apply to each of the two halves.
For the lower portion the Mishnah rules: "me'afar ulematan mitameh bemaga", from the fill downward it takes on tum'ah only by contact. In other words, should tum'ah come into physical contact with the interior surface of the hollow upper half, the oven in its entirety turns tameh, the earth-packed bottom included. But if the tum'ah is merely present within the upper cavity, hovering in its avir and touching nothing, the filled lower portion remains tahor.
And for the upper portion: "me'afar ulemalan mitameh ba'avir", from the fill upward it contracts tum'ah through its airspace as well, with no touching required. So if something tameh is held aloft inside the empty upper cavity, that half turns tameh while the half below it stays tahor.
To see why, keep in mind how a vessel of earthenware ordinarily contracts tum'ah: either by tum'ah coming into contact with its inner surface, or by tum'ah being present in the space it encloses. The functioning upper half is a full-fledged oven and absorbs tum'ah in both of those ways. The lower half, by contrast, has been retired from service. Earth fills it, nothing can be baked there, and a stretch of oven in which baking is impossible has ceased to be an oven at all. Min haTorah, then, it cannot receive tum'ah whatsoever.
The tum'ah of the lower half is therefore rabbinic in origin, a tum'ah derabbanan. It was the chachamim who legislated that the earth-packed section joins the upper section in becoming tameh whenever tum'ah makes contact with the interior of the usable half. And they drew the boundary of their decree on purpose: they did not extend it to tum'ah present in the airspace.
Why draw the line in this way? Because they wanted the rabbinic character of this tum'ah to remain visible. Had they declared that the lower half is tameh through air space as well, the dirt-filled section would have behaved in every respect like the functioning upper half of the oven, and people would have come to forget that its tum'ah is only a rabbinic institution. By limiting the decree to maga, to actual contact, and withholding it in the case of avir, the chachamim left a permanent reminder that even when it is touched, the tum'ah of the dirt-filled half is nothing more than their own enactment.
An oven set over the mouth of a pit
We have encountered more than once that an oven's gmar melachah, the act that completes it, is its initial firing: the oven must be heated until baking in it is possible, and only then can it receive tum'ah. Normally that firing takes place while the oven stands fastened to the earth. The Mishnah now turns to a firing carried out while the oven stood free of the ground.
The case: a person took a new oven and set it "al pi habor", over the mouth of a pit, or "al pi hadut", over the mouth of a cistern, and then, "venatan sham even", he wedged a stone into the gap there, between the oven's rim and the rim of the pit, so that the oven would stay put.
Picture the arrangement. These ovens, as noted earlier, had the form of an inverted pot, open below with no floor of their own. In the standard setup the oven is stood directly on the earth, clay is packed around its base, and the fire is kindled on the ground inside it. Our case is different: nothing joins this oven to the earth. The pit's mouth is broader than the oven's base, so a stone is jammed into the space between the two rims, bracing the oven so that it hangs suspended over the opening. The fire, as the Mishnah goes on to indicate, burns down inside the pit, on its floor.
Here Rabbi Yehudah rules: "im hisik milematan vehu nisok milemalan, tameh", if a fire was kindled down below on the pit's floor and the rising heat brought the oven above to the temperature needed for baking, the oven is fit to receive tum'ah. Since the flame burning inside the pit is what fired it, we regard the oven as joined to the pit, and a pit is part of the ground itself. The firing thus counts as one performed while the oven was attached to the earth, and its gmar melachah has taken place.
Rabbi Yehudah's condition matters, though. If the fire at the floor of the pit cannot bring the oven up to the necessary heat, and the only way to make the oven usable is to raise the fire, then the picture changes. This happens when the pit is deep, so that the flame at its bottom is too far from the oven and the heat dissipates before it can properly warm it. To bake in such an oven, a person would have to bring a stone, set it inside the pit, and kindle the fire on that raised platform. A firing of that sort does not connect the oven to the pit or to the ground, and according to Rabbi Yehudah it does not constitute a gmar melachah. The oven is not finished and is not susceptible to tum'ah.
Rabbi Yehudah's ruling of tum'ah therefore speaks of a shallow pit whose mouth exceeds the oven's width by only a little. In that arrangement the flame below sits close under the oven, only a thin space separates the oven from the pit's walls, hardly any heat escapes, and the fire can raise the oven to a temperature at which baking is possible.
The chachamim hold otherwise: "ho'il vehusak", seeing that the oven did in fact get fired, "mikol makom", in whatever manner this came about, it is tameh. Once it has been heated to the point of usability, its manufacture has reached completion and it can receive tum'ah, even if at the time of the firing it stood detached from the earth.