Keilim, Chapter 5, Mishnah 8. The chapter has been dealing with the tumah of a clay oven and with the ways an oven can lose its status as a utensil. The previous mishnah discussed breaking a tamei oven by cutting it vertically, from top to bottom, so that it is no longer a utensil and becomes tahor. Our mishnah takes up a different kind of cut: not down the height of the oven, but around it, slicing it horizontally into rings.
Cutting the oven into rings
"Chatacho chulyot lerochbo, tahor." If a person takes a tamei oven and cuts it horizontally across its width, he ends up with a stack of rings, each one looking something like a large donut. In this situation the oven is tahor.
The reason is straightforward. Not one of those rings, standing by itself, is tall enough or shaped enough to be called an oven. Each ring is simply a piece of a broken oven, and a broken oven is not a utensil at all. Its tumah is gone.
Reassembling the rings with clay
"Merecho betit, mekabel tumah misheyasikenu kedei le'efot bo sufganin." Suppose the owner gathers up those severed rings, sets them one atop the other in their original order, and then spreads clay along the outside surface. Susceptibility to tumah can return. The clay fastens each ring to the next, so that what stands before him is once more a complete oven, a real utensil in every sense.
Like any new oven, it does not become susceptible to tumah on the spot. It must first be fired: heated to the point that one could bake sufganin (a soft dough cake) inside it. Once it reaches that stage, this rebuilt oven is treated exactly like any other oven and can contract tumah.
Plaster placed at a distance
"Hirchik mimenu et hatefilah venatan chol o tzeror beinteyim." Now the mishnah describes a clever arrangement. Instead of coating the broken rings directly, he stacks them back into the shape of an oven and then builds his clay plaster a few inches away from them, following the outline of the oven but not touching it. That leaves a gap between the oven walls and the plaster on the outside, and he fills that gap with sand or with pebbles, packing it in as insulation.
"Bazeh amru: haniddah vehatehorah ofot bo vehu tahor." About an oven built this way the chachamim ruled that a niddah, who carries tumah, and a woman who is tehorah may both bake their bread in it together, and the oven itself stays tahor. The niddah does put her hands inside to arrange her dough, yet no tumah passes to the oven, since halachically it remains nothing more than a shattered oven. The clay never adheres to the fragments, so it fails to unite them into one utensil.
The result is striking. The oven works: one can genuinely bake in it. But usefulness alone does not make it a utensil, and since the rings were never bound together, it never became a halachic oven at all. That is why the two women can work side by side with no transfer of tumah between them. The arrangement is, in effect, a permitted way around the problem: the oven is fully usable, yet it makes no one and nothing tamei.