Kelim, Chapter Five, Mishnah Eight
1) If an oven was cut up by its width into rings that are each less than four handbreadths in height, it is clean.
2) If he subsequently plastered it over with clay, it becomes susceptible to impurity when it is heated to a degree that suffices for the baking of spongy cakes.
3) If he distanced the plastering, and sand or gravel was put between it and the oven sides of such an oven it has been said, “A menstruant as well as a clean woman may bake in it and it remains clean.”
Section one: If instead of cutting the oven into pieces from top to bottom, he cut it along its breadth into rings, each ring of the oven that is less than four handbreadths high is clean. This accords with Rabbi Meir’s opinion in 5:1, who said that if the remnants of the oven are less than four handbreadths, the oven is clean. The other sages in that mishnah would limit this to a case where the oven was larger than eight handbreadths. If it was smaller, then it remains unclean until less than a majority is left in the ring.
Section two: If after taking the oven apart he reassembles it and plasters it over with clay, the oven is not susceptible to impurity until he fires it up sufficiently so that it could be used to bake sponge cakes. This is the same rule for when a new oven is susceptible to impurity. In other words, a reconstituted oven is treated the same as a new oven.
Section three: If instead of plastering the clay directly back on the rings of the oven, he made a covering of plaster and then filled in the space with stones or gravel, the oven is not susceptible to impurity because for an oven to be susceptible the clay must be directly on the oven. In this case, a pure woman and a menstruant can bake in the oven at the same time, without fear that the menstruant will defile the oven which would subsequently defile the pure woman’s bread. It seems that this may have been a way to circumvent what was certainly a problemhow can women who are menstruationg bake their bread without the need for a separate oven?