Kelim, Chapter Eight, Mishnah Seven
1) A sheretz which was found in the eye-hole of an oven or of a double stove or of a single stove: If it was outside the inner edge, it is clean.
2) If it [the oven] was in the open air, even if it was an olive’s bulk of corpse it is clean.
3) If there was [in the eye-hole] an opening of one handbreadth, it is all unclean.
Section one: The eye-hole of an oven is a hole made in the bottom of the oven or stove to let in air and let out smoke. It also could have been used to remove the ashes.
If a sheretz was found outside of the inner edge of this hole, the oven remains pure. This is because the hole is not considered to be inside the oven or stove.
Section two: If the oven was outside in the open air, meaning it was not inside a house or other building, and a source of impurity is found in its eye-hole, the oven is pure because the impurity cannot enter the oven. And since it is outside, there is no “tent” through which an olive’s bulk of corpse could convey its impurity to the oven (we will learn a lot more about this as we proceed). Thus the oven remains clean.
Section three: However, if there was an opening the size of a handbreadth in the eye-hole (besides its main opening) then the impurity can enter through it into the oven, and the oven is impure.