Keilim, Chapter 5, Mishnah 1. From here onward the masechet occupies itself for several chapters with baking ovens and cooking stoves and the ways they contract tumah. Our mishnah introduces the topic with two elementary inquiries: what are the dimensions that qualify a structure as an oven in the first place, and at which stage of its making does it begin to be subject to tumah?
What an oven looked like
To follow the mishnah we need the picture in front of us. The oven of the mishnaic period was made of earthenware. Imagine a hollow vessel shaped something like a large gardening pot, turned upside down, wider at the bottom and narrowing toward the top, with no floor of its own. It was set directly on the ground, and clay was packed around its outside to insulate it and hold in the heat. At the bottom there was a small opening through which wood or charcoal was inserted and the fire kindled. At the top there was a second opening, and it was through this upper opening that the dough was put in.
The bread was not laid on top of the oven. It was baked inside, on the inner walls, and generally only after the oven's heat had already been used for something else. A thin piece of dough would be pressed onto the hot inner wall, and a special utensil was used to remove the finished loaf. The clay coating on the outside kept the whole structure searingly hot.
Two exceptions the Torah makes for ovens
An oven of this kind presents a difficulty. The general rule for earthenware is that a vessel becomes susceptible to tumah only if it has a beit kibul, a receptacle that holds. An oven has no floor at all, so strictly speaking it holds nothing. Nevertheless the Torah teaches explicitly that ovens can become tamei. The oven is therefore an exception to the standard rule of earthenware.
There is a second exception here as well. The oven stands on the ground and is sealed to it with clay all along the bottom, so it is mechubar lakarka, attached to the ground. Ordinarily something joined to the ground does not receive tumah at all. Here too the Torah decrees that the oven becomes tamei despite its attachment.
The measurement of an oven
The mishnah begins: "Tannur, techilato arba'ah usheyarav arba'ah", an oven, its beginning is four and its remnant is four. Two stages are being measured, and both require four tefachim.
The first is the moment of manufacture. For a newly made oven to be considered an oven at all, so that the laws of tumah apply to it, it must stand at least four tefachim high. The second is the moment of breakage. If a tall oven cracks and part of it falls away, what is left must still measure four tefachim for it to remain an oven and remain capable of becoming tamei. Anything shorter than that cannot hold its heat well enough to bake bread properly, and so it is no longer an oven in any meaningful sense. This is "divrei Rabbi Meir", the view of Rabbi Meir.
The chachamim: a large oven and a small one
The chachamim disagree: "Bameh devarim amurim? Begadol", in what case was this said? Regarding a large oven. Their point is that the four tefachim rule was stated only about an ordinary full sized oven, the kind used for baking bread. About such an oven it is true that less than four tefachim is not an oven.
The mishnah continues: "Aval bekatan", but with regard to a small oven, "techilato kolshehu", whatever height it was fashioned at suffices, "ushyarav rubo", and if it breaks, what must survive is most of it. The oven in question here is a miniature one, the kind given to a little girl taking her first steps in the kitchen: a play oven of sorts, the ancestor of the children's toy ovens of our own day. One drops a small bit of dough inside, and nothing more is expected of it. Since that is its entire purpose, no four tefachim threshold applies to it whatsoever; at whatever height its maker built it, it counts as a keli and is able to become tamei.
The commentaries add a qualification: even here it must be at least one tefach. Below a tefach nothing at all can be accomplished with it, but from a tefach and up it can genuinely serve a child's small baking.
As for such an oven once it has broken, the chachamim do not measure in tefachim at all but in fractions: so long as more than half of what was originally built is still standing, the object retains its status as an oven and remains capable of contracting tumah, even when its surviving height falls short of four tefachim.
When does the oven become susceptible?
Now the mishnah turns to timing. Like every keli, an oven does not receive tumah until it is finished: "mishetigamer melachto", once its manufacture is complete. So the mishnah asks, "Eizehu gmar melachto?", what constitutes the completion of its manufacture?
The answer follows: "misheyasikenu kedei le'efot bo sufganin", from the point at which it has been fired sufficiently to bake sufganin inside it. Sufganin are an airy sort of baked item; the flour is worked together with water or with oil into a thin, pourable batter rather than a firm dough, and the result resembles a sponge cake. Precisely because the mixture is so loose, it sets at a gentler heat than bread requires, and the oven need never reach the fierce temperature that bread baking calls for. As soon as that initial firing has brought the oven to the level at which sufganin would bake, its production counts as finished and tumah can take hold of it.
A dissenting measure follows. "Rabbi Yehuda omer", Rabbi Yehuda holds that the moment arrives "misheyasik et hechadash", when the new oven has been fired, "kedei le'efot bayashan sufganin", to the degree at which an old oven would manage to bake sufganin. Ovens that have seen years of use bake at a milder heat than freshly built ones. Rabbi Yehuda accordingly lowers the bar further: the question is not whether this new oven has climbed to the temperature typical of a new oven, but merely whether it has reached the modest warmth that suffices for sufganin in a veteran oven. Reaching even that much heat, in his opinion, already establishes the oven as serviceable and its making as concluded.
The mishnah has thus given us both halves of the picture: the oven must be of a size that lets it function as an oven, and it must have been fired to the point where it actually can bake. Only then does this floorless vessel, sealed to the very ground beneath it, enter the world of tumah and taharah by the Torah's own decree.