Keilim, Chapter 6, Mishnah 2. This chapter deals with improvised cooking arrangements: not a finished, manufactured oven, but stones and clay set up on the spot so that a pot can be balanced over a fire. The question in each case is whether such an arrangement counts as an earthenware oven or kirah, which is susceptible to tumah, or whether it remains an ordinary stone that tumah never touches. Our mishnah continues that discussion.
A stone that shares the work with an oven
The opening case is introduced with the words "Ha'even", the stone, "shehaya shofet aleha", upon which a person would settle his pot, "ve'al hatanur", and simultaneously upon an earthenware tanur that stood next to it, the flame burning in the gap separating them. The mishnah lists the parallel situations as well: "aleha ve'al hakirah", the pot straddling this stone together with an earthenware kirah, and "aleha ve'al hakupach", the pot straddling the stone and a kupach. Each of these receives the identical verdict, "teme'ah": halachah regards the stone as a component of the cooking installation, and it is therefore capable of receiving tumah.
Why should a stone become tamei? Because this stone was fixed to the ground with a layer of clay packed around its base, and that clay coating gives it the status of earthenware. Once that is so, the whole improvised cooking setup is viewed as one earthenware kirah, and the stone is included in it.
Consider how striking this ruling is. All that turned the stone into a limb of an earthenware oven was its being cemented into the ground next to that oven or kirah. We are not told to investigate whether the tanur was originally installed there for solo use, with nobody imagining it would ever be paired off with a neighboring stone; that question is irrelevant. The determining factors are that the stone was positioned there for the purpose of cooking, and that in practice the two serve together as a single cooking place. That practical partnership folds the stone into the installation, giving the whole thing the halachic identity of an earthenware oven that can receive tumah.
When the partner is only stone
The mishnah then turns the case around: "Aleha ve'al ha'even", the clay-fixed stone is used as a cooking place by balancing the pot on it and on a second, freestanding stone beside it, one that was never fastened down with clay. Or "aleha ve'al hasela", on it and on bedrock, a rock protruding from the ground that was never detached from it. Or "aleha ve'al hakotel", on it and on a stone wall. In all three the ruling is "tehorah", pure.
The reason is that only the stone plastered to the ground with clay can carry the status of earthenware. The other partner is plain stone, nothing more. Since the cooking place as a whole is therefore not an earthenware vessel, it is not susceptible to tumah at all.
A wider rule emerges from this ruling. Tumah applies only to an oven or stove of the sort that is demolished by breaking, which leaves out installations built of stone or fashioned from metal, since nobody shatters such things. The Torah's language is the source: "tanur vekirayim", an oven and a stove, "yutatz", shall be broken apart. Stone and metal are not objects one breaks apart, so they fall outside this category entirely and stay pure.
The stove of the nezirim
The mishnah adds: "Vezo hayeta kirat hanezirim sheb'Yerushalayim shekineged hasela". This was precisely the arrangement of the nezirim's stove in Yerushalayim, which was set against the bedrock so that it could never become tamei. When a nazir completed his nezirut he brought three korbanot, one of them a shelamim, and the meat was cooked in that designated place in Har Habayit, the chamber of the nezirim.
The cooks' row of stones
The mishnah closes with "kirat hatabachim", the cooking setup used by butchers who sold meat to the public and had to prepare great quantities in one session. The case is described as one where the cook lines up "even betzad even", arranging stone next to stone in an extended row, each pot resting across two neighboring stones with a fire kindled in each gap. The halachah in such a row is "nitmet achat mehen, lo nitme'u kulan": should one of these units contract tumah, say a dead sheretz landed inside it, the remaining units along the row stay pure. Each pair of stones constitutes its own independent stove, so tumah in one pair travels no further.
So this mishnah teaches us both how a mere stone can be drawn into the identity of an earthenware stove and become susceptible to tumah, and where the line is drawn: without clay fixing it to the ground, a stone stays a stone, untouched by tumah, as the nezirim's stove in the Beit Hamikdash was deliberately built to be.