Keilim, Chapter 6, Mishnah 1. This chapter turns to the different kinds of cooking arrangements a person may put together, and asks when such an improvised setup counts as a real stove that can receive tumah. A proper kira made of earthenware is a vessel in every sense. But what about a person who simply props something up and cooks over a fire beneath it? The Mishnah works through three cases: supports of clay, supports of metal, and supports of stone.
Three clay supports joined with clay
The Mishnah's first scenario: "Ha'oseh shelosha pitputin ba'aretz", a person forms three small clay knobs and plants them in the ground so that they stand in a triangular formation, "vechiberan betit", fastening them to one another above with clay, until the three of them together function as a three-legged base. His intention, "lihiyot shofet aleihen", is that he will be able to rest "et hakedera", the pot, upon them, with the fire burning below, so that the food inside will cook.
The verdict: "temei'ah." Halachically this contraption is a stove, and it is susceptible to tumah no differently than a kira of earthenware. A manufactured kira does the same job with greater convenience, but since these three little supports achieve precisely the same result, that suffices to give them the standing of a vessel. The practical consequence: should an av hatumah reach it, say a sheretz that enters the air space bounded by the three supports, the stove contracts tumah exactly as an earthenware kira does. And here too a minimum measure applies, just as it does for a kira: the supports have to rise a height of three etzba'ot, three fingerbreadths, before the structure qualifies as a stove.
Three nails driven into the ground
The Mishnah's second scenario: "Kava shelosha masmerin ba'aretz", he hammers three nails into the ground, "lihiyot shofet aleihen hakedera", intending that a pot should stand upon them. And the ruling here is different. Even, "af al pi", where "she'asa beroshan makom", he prepared upon their heads a resting spot, a flat bit of clay "shetehei hakedera yoshevet", upon which the pot will sit firmly, the Mishnah declares the whole apparatus "tehora", pure.
In terms of function, these three nails certainly serve as a serviceable stove. The difficulty lies in their material: what he has produced is a stove of metal, not of earthenware. The preceding mishnah taught that metal implements fastened to the ground are not susceptible to tumah, stoves and ovens included. In our case the clay at the top does bind the nails together, but the nails are simultaneously sunk into the earth, which grips them and secures their position. In other words, the very thing that unifies them into one stove also ties them to the ground. Since metal that is attached to the ground never contracts tumah, this stove is pure.
From here we understand why the first case specifically required a clay bonding. Those clay knobs were not sunk into the earth; they were only standing on it. The ground, then, was not doing the work of holding them together, so the sole way to turn them into one stove was by fastening them to one another with clay.
Stones made into a stove
The third scenario: "Ha'oseh shetei avanim kira", a person takes a pair of stones and makes of them a stove, "vechiberan betit", bonding them together with clay, and the ruling is "temei'ah." Its status is that of a stove of earthenware, and it is therefore susceptible to tumah. Stone on its own has no susceptibility to tumah whatsoever, but the clay that binds the two stones lends the entire structure the character of an earthenware stove, and with that it enters the category of vessels that can contract tumah.
"Rabbi Yehuda metaher." Rabbi Yehuda dissents and holds this stove pure, "ad sheya'aseh shelishit", unless he adds a stone as a third to the pair, "o ad sheyismoch lakotel", or else he places the two stones against a wall, letting the wall itself serve as the pot's remaining point of support. Rabbi Yehuda's reasoning: a stove built from clay knobs needs three of them before it is a functioning stove, and by the same logic a stove built out of stones is not a stove in halachah until it has three points bearing the pot.
"Achat betit ve'achat shelo betit": he did build with three stones, but while one stone was bonded with clay to another, the third was not bonded with clay to either of them. The ruling is "tehora," it is pure. According to Rabbi Yehuda it is not enough that a third component be present so that the stove can function; the parts must actually be joined to one another so that the stove is a single entity. Here they are not, and therefore this stove remains permanently tahor.