Keilim, Chapter 8, Mishnah 9. A tanur (oven) and a kirah (stove) hold a unique position within the laws of tumah, as this masechet has already established. Measured against the ordinary requirements for earthenware utensils, neither should be susceptible in the slightest: they possess no floor, their whole function depends upon the earth beneath them, and very often they are fixed into it. Pure reasoning would leave no grounds whatsoever for declaring them tamei. The Torah nonetheless decrees otherwise, and it is a gezerat hakatuv that an oven and a stove do contract tumah.
Our mishnah takes that special ruling and tests its borders. It presents installations that resemble an oven or a stove but differ in one detail or another, and asks whether they are drawn into the category of a vessel that can become tamei, or whether they remain outside it.
A clay-lined pit
The first case in the mishnah is "Bor sheyeish bo beit shefita": a pit whose walls have been coated with clay and which contains a surface able to hold a cooking pot. Such a pit is tamei. Even though it is sunk into the earth, people cook with it precisely the way one cooks with a stove, setting the pot above the heat, and so halachah classifies it as a kirah that is susceptible to tumah.
The pit of the glass preparers
The mishnah continues with "Veshel osei zechuchit": this same clay-lined pit when it belongs to glass manufacturers, who fire their raw materials inside it. Here too, so long as the pit has a beit shefita, a spot where a pot could rest, it ranks as a kirah and is susceptible. One might have argued that its main function has nothing to do with preparing meals, since it exists for the sake of producing glass. Even so, it remains fully capable of cooking, and it does serve that purpose now and then, which is enough to place it under the definition of a stove and within the laws of tumah.
The furnaces that stay tahor
The mishnah then lists installations that remain tahor: "Kivshan shel sayadin", the kiln of the lime burners, along with the kilns of the glass blowers (where the glass itself is shaped) and of the potters. Not one of them qualifies as an oven or a stove meant for cooking. The temperatures they reach are so extreme that no person would ever place food within them or above them, so they never take on the identity of a tanur or a kirah at all. Lacking that identity, such a kiln is not susceptible to tumah.
The purna
The mishnah now turns to another kind of installation, the purna. A purna is a large earthenware oven with an opening at one side, in which food is cooked on the floor rather than being baked against the wall.
To see why that detail is decisive, consider how the Torah's oven and stove function. In an oven, dough is pressed against the inner wall, a fire is lit, and the loaf stays there until the baking is finished. With a stove, the food sits in a pot placed above it while the stove supplies the heat. In each case the cooking takes place upon the utensil itself.
With the purna the arrangement is different. When it lacks a ledge, whatever is being cooked lies directly upon the earth, and that earth belongs to no utensil; it is simply ground. Since nothing is cooked upon the vessel, the purna matches neither the oven nor the stove of Torah law, and it stays tehorah. Where a ledge does exist, the food rests upon it and cooks there, which means it cooks upon the purna, and the parallel to the Torah's stove now holds. In that situation the purna is a keli and it is temeiah.
Hence the ruling of the mishnah, "im yeish lah lizbez": when a lizbez, a rim along the base of the inner surface, is present, the purna is susceptible to tumah, and where no such rim exists, it is not.
Two more names for the same ledge
Two other Tannaim arrive at the identical halachah, each using his own word for that rim. Rebbi Yehuda's formulation is "im yeish lah istegiyot": susceptibility depends on istegiyot, a border encircling the base of the purna's interior. Rabban Gamliel's term for it is "sefayot", lips that run along the interior at its base.
These are simply the various words the chachamim employed for one and the same feature, and all of them convey the identical halachah: the presence of a ledge, upon which the food actually cooks, is what turns the purna into a vessel that can receive tumah.