Kelim, Chapter Five, Mishnah Six
1) If an oven was half filled with earth, the part from the earth downwards contracts impurity by contact only while the part from the earth upwards contracts impurity [also] from its air- space.
2) If he put the oven over the mouth of a cistern or over that of a cellar and he put a stone at its side:
a) Rabbi Judah says: if when heated below it becomes also heated above it is susceptible to impurity.
b) But the sages say: since it was heated, no matter how, it is susceptible to impurity.
Section one: If a person filled part of an oven up with earth, then it is less susceptible to impurity than the empty/usable part. The filled part is impure only if a defiling agent touched the upper part, whereas the empty part is impure even if the defilement only enters the oven’s airspace.
Section two: In this case a person put an unfinished oven above either a pit or over a cistern and he wedged a stone to keep it from falling. Rabbi Judah says that it must be possible to forge the oven from below in the pit or cistern. If the oven can only be heated up by using its own bottom part, then the oven’s manufacturing is not complete. Rabbi Judah holds that an oven must be attached to the ground when heated for the first time.
The other sages do not share this rule with Rabbi Judah. They hold that as long as the oven is heated up, it is susceptible to impurity.