Keilim, Chapter 5, Mishnah 11. Until now the chapter has been occupied with the tannur (baking oven) and the kira (cooking stove) built of earthenware, together with all the unusual halachot that attach to them. Our mishnah turns to ovens and stoves built out of other materials entirely: stone and metal. The question it answers is whether such vessels carry the special rules of ovens, or only the ordinary rules of vessels of their own material.
The mishnah begins: "Tannur shel even veshel matechet tahor" - a tannur constructed of stone or of metal is tahor. Each case arrives at that conclusion by its own route. Stone is not among the materials that can take on tumah at all, so an oven built of stone stands permanently outside the system. Metal is another story: such an oven is not immune to tumah altogether, but the tumah of ovens has no hold on it. An earthenware oven is rendered tamei when a source of tumah touches its inner surface or passes into the space enclosed within it, while contact with its outer wall leaves it unaffected. Metal works along entirely different lines.
What pattern does it follow? "Vetamei mishum klei matachot" - it becomes tamei as a metal utensil. Metal vessels receive tumah through contact, including contact on the outside, but they are not affected by tumah passing through their airspace, which is precisely the way an earthenware oven is affected. So the two systems are almost mirror images of each other: for earthenware, the airspace matters and the outer wall does not; for metal, the outer wall matters and the airspace does not.
Several other special laws of ovens follow the same principle. There is a rule that an oven receives tumah even while it is attached to the ground, unlike ordinary vessels. That rule was said only about earthenware ovens. A stone oven is beyond tumah in any case, and a metal oven receives tumah only when it is not attached to the ground, like any other metal vessel. So too the rule that a tamei oven regains its taharah only by being broken applies to earthenware ovens alone; a metal oven is purified by being immersed in a mikveh.
The reason behind all of this is straightforward. Wherever the Torah speaks about the tumah of ovens, it is speaking about ovens of earthenware. The mishnah is therefore teaching that stone and metal ovens stand outside that entire body of law. They are governed by the general laws of vessels according to their material, and they do not inherit the exceptional stipulations that make earthenware ovens so different from everything else. A stone oven never becomes tamei; a metal oven becomes tamei exactly like any metal utensil.
When the clay takes over
Now the mishnah introduces the situation in which even a stone or metal oven is judged an earthenware one. "Nikav, nifgam, nistak" - it was pierced through, or part of its rim chipped away, or a split opened along it. "Asah lo tefelah" - its owner smeared clay over the break to close it up, "o musaf shel tit" - alternatively, he rebuilt the missing portion above with fresh clay. The ruling in such a case is "tamei": from here on the oven takes on tumah the way an oven of earthenware does.
Why? Because in its damaged condition the oven is useless, and the clay is what brings it back into service. The clay, being the component that makes the thing an oven once more, becomes the determining element, so the vessel is now identified as a clay oven. From the moment it is fired to the heat an oven requires, it counts as earthenware. Nor does this require that clay have been spread over the whole of the stone or metal body. It is enough that the clay closes the break and renders the oven fit for use again; being that basic to the vessel, it draws the entire oven after the halachot of earthenware.
How serious does the damage have to be? The mishnah asks: "Kamah yihyeh banekev?" - what size hole are we speaking about? And it answers: "Kedei sheyetzei bo ha'ur" - large enough for the fire to come out through it. The hole must be big enough that the flames shoot out and the heat escapes, so that the oven loses its heat to the point where it cannot bake at all and is no longer an oven. Only then is the clay that seals such a hole what returns the vessel to use, and only then does the oven become like an ordinary earthenware oven.
"Vechen bakira" - and the same applies to a stove. A stone or metal kira that has a hole large enough for the fire to escape through, which was then sealed with clay, is likewise treated from then on as an earthenware kira.
The kira of stone and metal
The mishnah turns to the kira itself, whose law runs parallel to that of the oven with one notable difference. "Kira shel even veshel matechet tehorah" - a stove built of stone or of metal is tahor. Stone vessels lie outside susceptibility altogether, so such a stove is never tamei. As for metal, the tumah that governs an earthenware stove has no bearing on it. "Utemei'ah mishum klei matachot" - it is susceptible, rather, as a metal utensil: by touching a source of tumah, its outer side included, and not by tumah entering the space it encloses, which is the route by which an earthenware kira would be affected.
Then comes the exception: "Nikevah, nifgemah, nistekah" - it developed a hole, or a piece broke off, or it cracked, and the damage was in its base, so that the stove can no longer stand properly on the ground. "Asah lah pitputin" - the owner fashioned small clay supports, little feet placed underneath to hold the kira up. The ruling is "temei'ah": the stove is now susceptible as an earthenware vessel. Without those supports the kira could not be used at all, so the feet are essential and fundamental, and because they are what make the stove serviceable, the entire kira takes on the halachot of earthenware.
Why does the mishnah single out the base? A kira has to sit level so that pots can be set on it and cook. Should its floor be pierced or split, the stove tips to one side and offers no stable surface for a pot. Cooking on a kira whose base is ruined is simply impossible. The little clay legs fashioned beneath it give it back its footing, and with that they give back the ability to cook on it.
This is exactly why the halachah speaks only of a break in the base. Had the puncture been in the wall of the kira, patching it with clay would not turn the stove into an earthenware vessel. A kira operates by holding the pot above while the flame burns below it; the stove itself has no need to reach great heat. Open sides and all, the pot resting on it will still warm up and cook. A hole in the side hardly interferes, and clay that stops it up achieves too little for us to assign the stove a new identity on its account.
An oven is another matter entirely. It is meant for baking bread, and the loaves are stuck to the inner surface of the oven itself. That demands intense heat, so a sizable opening genuinely ruins the vessel: the heat pours out and nothing can bake. Precisely here the clay becomes essential. Once it plugs the hole, bread can be produced again, and the clay therefore confers upon the oven the standing of a kli cheres, an earthenware oven. Patching a hole in the side of a kira brings about no comparable improvement, and so no change of status follows.
Coating an undamaged stove
The mishnah finally addresses a stone or metal kira that was never damaged at all, but was simply plastered with clay. "Merachah betit, bein mibifnim bein mibachutz, tehorah" - if he smeared it with clay, whether on the inside or on the outside, it remains tahor. It does not become susceptible as an earthenware oven would. A stone kira remains entirely tahor, and a metal one remains subject only to the laws of metal utensils.
Once again the reason lies in how a stove is used. The pot stands over the kira with the fire kindled underneath, so the heat reaches the food from the flame itself rather than from the stove's sides. A clay coating on the walls therefore has almost no effect on the cooking a kira normally performs. In the earlier case the clay did alter the vessel's standing, because those clay legs under a broken base were what returned the stove to service. Smearing clay on a sound stove achieves nothing of the kind.
Contrast this with an oven coated in clay, which we do treat as an earthenware oven, because bread is baked against the inner walls and those walls must reach a high heat. There a clay lining raises the oven's performance to an entirely new level and enables the baking. This is why the mishnah is so careful to distinguish between a tannur, a kira and ordinary vessels: because each one functions in its own way, the halachah shifts slightly for each of them.
The mishnah closes with a dissenting view. "Rabbi Yehudah omer: mibifnim temei'ah, mibachutz tehorah" - Rabbi Yehudah says that if the clay coating was applied on the inside of the stove, the kira becomes susceptible as an earthenware vessel, while if it was applied on the outside, it remains tahor from the tumah of earthenware. In Rabbi Yehudah's view the clay on the inner surface is significant and effective, so the stove becomes an earthenware kira and receives tumah accordingly. Clay on the outside, however, does not give it that identity, and the stove stays clear of the tumah of a kli cheres.