Keilim, Chapter 5, Mishnah 3. This chapter deals with the tum'ah of earthenware ovens and stoves, and our mishnah turns to a natural question that arises once such vessels are in use: what about the various additions that people build onto them? A rim on top, a warming box on the side, a little compartment for oil or spices or a lamp. When the tannur or the kirah becomes tamei, do these attachments become tamei along with it, because they are counted as part of the oven or stove, or do they stand on their own as separate items that are unaffected?
The rim of the stove
The mishnah begins: "ateret kirah tehorah." An atarah is a crown, and here it refers to a rim that a person could fix onto the top of his kirah. Its purpose is practical: it holds the heat in and covers over an area that would otherwise be left open. Nevertheless the mishnah rules that this rim remains tahor. The reason is that this is not the ordinary way a kirah is built. Most people do not fit their stoves with such a rim, and therefore even when an individual did attach one to his own kirah, it is not regarded as part of the vessel, and it does not become tamei together with it.
The "courtyard" of the oven
Next comes "tirat hatannur." The term evokes a courtyard, and that is essentially what it is: a small walled area built at the side of the oven, an open box of sorts, where fresh loaves are set down after being drawn out so that they will not lose their heat too quickly. The mishnah's ruling: "bizman shehi gevohah arba'ah tefachim," when this enclosure measures four tefachim in height, it is "mitamme'ah bemaga uva'avir." Tum'ah reaches it in two ways: by touching its inner walls, and also by entering the space it encloses even with no contact at all.
What accounts for this? The enclosure is positioned at the oven's side and functions together with it, so the halachah judges it by the rules governing an oven. Of itself it is no vessel at all: it is nothing more than an open patch beside the oven which happens to be walled in. To be capable of receiving tum'ah, therefore, it must qualify as an oven, and an earthenware oven is susceptible only from a height of four tefachim and up. Once the enclosure reaches that height it becomes tamei precisely as any other earthenware oven would. Hence the mishnah's addition: "pachot mikan tehorah." Below four tefachim it stays tahor, because it does not have the dimensions that define an oven.
Until now the discussion concerned an enclosure standing separately from the oven. The mishnah turns to the case where the two have been joined: "im chubrah lo, afilu al shalosh avanim, teme'ah." Should this side box be plastered to the oven, it receives tum'ah even if its height falls short of four tefachim, and that holds true even where the box is supported on three stones instead of sitting directly on the earth, so that the union between them is partial and imperfect rather than a full joining. From the moment it is bonded to the oven it counts as a portion of the oven itself, and it no longer requires the minimum measure of an oven in its own right.
The oil, spice and lamp compartments
The mishnah moves on to receptacles affixed to the exterior of a kirah: "beit hapach uveit hatavlin uveit haner shebakirah." The first is a holder for oil, a flask placed there so that the stove's heat should keep the oil warm. The second is a niche for spices, and the third a spot for a lamp. Each of the three assists the one who is cooking: light is needed while the work is being done, and the spices go into the food for flavor.
Regarding them the mishnah rules: "mitamme'in bemaga ve'einan mitamme'in ba'avir." Tum'ah reaches them by touch, but not by way of their airspace. This plays out in either direction. Should tum'ah come into contact with the kirah's inner surface, rendering the stove tamei, these receptacles are drawn into that tum'ah with it. Should tum'ah instead have entered the kirah's airspace with no contact taking place, the stove alone becomes tamei while the receptacles stay tahor. The same works in reverse: tum'ah that touched the interior of one of the receptacles renders it tamei and the kirah is rendered tamei together with it, but tum'ah that merely came into the receptacle's airspace without touching leaves the kirah tahor and makes the receptacle alone tamei.
Why the distinction
What lies behind this ruling is that on the level of Torah law these receptacles have no share in the kirah whatsoever, and by rights they ought not to be drawn into its tum'ah in any way. What they do is simply not what a stove does: they take no part in cooking the food, serving only to hold ingredients or to supply light while the cooking proceeds. Their status as a portion of the stove is a rabbinic institution, and the Chachamim purposely restricted it to tum'ah by touch, leaving airspace out.
The reason for narrowing the decree was so that its rabbinic character would stay visible. Were the receptacles made tamei across the board, people would end up treating them as fully identical with the kirah. The Chachamim therefore gave the enactment a distinctive contour: strict where touch is involved, and not equally strict where the airspace is concerned. There is a genuine practical consequence to keeping this in mind. Terumah and kodashim are burned on account of tum'ah only where that tum'ah carries the force of Torah law. Since these receptacles have no real share in the stove and their tum'ah is not biblical, terumah and kodashim rendered tamei by way of them may not be burned, and the built-in leniency of airspace is the signal that keeps a person from erring.
The mishnah concludes that this is "divrei Rabbi Meir," the opinion of Rabbi Meir, and that "Rabbi Shimon metaher." Rabbi Shimon holds that these holders remain tahor even when the kirah became tamei through contact on its inner surface. In his view the Chachamim never issued a decree to render the holders tamei on account of the tum'ah of the kirah, not even through maga.