Keilim, Chapter 7, Mishnah 1. To follow this mishnah we need to picture how cooking was done. A kirah, a portable stove, was frequently not set down directly on the ground. Instead it rested on a base, and the fire was kindled on the surface of that base, underneath the kirah, with the pot sitting on top of the kirah. The base and the fire and the stove and the pot were all one working arrangement. Our mishnah asks what happens when that arrangement is damaged.
"Kirot shel ba'alei batim shenifhata": the stove of ordinary householders whose base has broken open. The surface of the base, which had been flat so that the fire could burn on it directly beneath the kirah, has now caved in, so that the fire ends up sitting somewhat lower down, inside the cavity that formed.
A break of less than three tefahim
"Pahot mishelosha tefahim, teme'ah": so long as the hollow that opened up measures under three tefahim, the stove still receives tum'ah. The mishnah gives its reason: "shehu masik milematan ukedeira beshelah milemalah", the owner lights his fire down inside the depression, and the pot standing above on the stove still cooks. Since the vessel goes on doing exactly what a kirah is made to do, it retains its standing as a utensil, and a utensil that functions is susceptible to tum'ah.
A break deeper than three tefahim
"Yater mikan, tehorah": once the hollow runs deeper than that measure, the kirah is tahor and can no longer take on tum'ah. Flames burning at the bottom of so deep a pocket are simply too distant to cook whatever is standing above them on the stove. The vessel has forfeited its purpose, and something that can no longer carry out its function does not count as a utensil where tum'ah is concerned.
Filling the cavity with a stone
"Natan even o tzeror, tehorah": suppose he drops a rock or a pebble into the deep cavity to raise the level. Now the fire can be kindled on top of that stone, higher up, close enough once again to cook the pot resting above on the kirah. Nevertheless the mishnah rules that the kirah is still tahor. A loose stone that merely sits in the break does not repair the structure. The fire is supposed to rest on the base itself, and this base is still a broken base; the stove is still regarded as a broken utensil, and it stays pure.
Sealing the stone with clay
"Meracha betit, mekabelet tum'ah mikan ul'ehan": if he smeared clay over the stone and thereby bonded it to the base, the repair is now part of the utensil itself. The kirah has been made whole again, and from this point onward it is once more fit to receive tum'ah.
Rabbi Yehuda's proof
Our mishnah now ties this ruling to an argument recorded back in Perek Heh. A clay oven is not treated as a completed vessel, and does not enter the halachot of oven tum'ah, until it has undergone its first firing and reached the proper heat. There Rabbi Yehuda and the Hachamim differed over an oven positioned above a pit, with the fire burning down inside the pit. In Rabbi Yehuda's view, such an oven becomes susceptible only if flames kindled at the bottom of that pit were capable of bringing the oven to the required temperature. Structurally the oven and the pit were two separate things, yet if a fire below could genuinely heat the oven, the two are regarded as joined: once the heat accomplishes its purpose, they are viewed as a single unit.
"Zo haytah teshuvat Rabbi Yehuda", teaches our mishnah: here was the reply Rabbi Yehuda offered his colleagues, and the case under discussion was "tanur shenatano al pi habor", an oven set above the opening of a pit, or "al pi hadut", above the mouth of a cistern. His reasoning went as follows. Our mishnah has established that a stove resting upon a base is called a stove only where a fire on that base can genuinely cook the food in the pot above. Everything depends on whether the vessel is useful. Where the damage is slight enough that cooking still happens, the utensil is whole; where the flames burn too far beneath to cook anything, the stove has ceased to be a stove.
By the same reasoning, argues Rabbi Yehuda, a fire burning in a pit that cannot bake what sits in the oven above it has accomplished nothing. There is no oven, no completed work, no gemar melachah, and therefore no finished utensil that could become tamei. The kirah of our mishnah loses its capacity for tum'ah precisely because its base can no longer serve as a working platform for the cooking above, and Rabbi Yehuda carries that same logic over to the oven set above the pit.
The two cases are, in a sense, one and the same picture: a heat source set below and a vessel set above, with the whole question being whether the heat can reach. In Perek Heh the issue was reaching the required temperature to complete the oven; here the issue is whether the stove is still a usable utensil for tum'ah and taharah. Rabbi Yehuda binds them together into a single principle: if the fire can do its job, the utensil stands; if it cannot, the utensil does not.