Ohalot, Chapter Twelve, Mishnah Two
1) Netting placed over the mouth of an oven, [so that it is] closed with a sealed lid:
a) If there is uncleanness below [the netting] or above it, everything becomes unclean;
b) But what is directly [above] the air-space of the oven remains clean.
2) If there is uncleanness directly [above] the air-space of the oven, everything directly above it even to the sky becomes unclean.
Section one: The oven referred to here is assumedly a used oven, one which is susceptible to impurity. The netting placed over it is also attached to the oven such that it forms a tightly sealed lid. This means that the inside of the oven cannot become impure.
The netting goes beyond the sides of the oven by a handbreadth, as was the case with the board in yesterday’s mishnah. As was the case there, the netting is considered to form an ohel over anything below it. If there is a source of impurity below or above the netting, everything found below or above the netting is impure because the netting is an ohel. However, the inside of the sealed oven is not defiled and therefore, anything found above the air-space of the oven is pure. This is the main difference between this and yesterday’s case. In yesterday’s mishnah even the oven, and anything in it, was impure.
Section two: If the uncleanness is above the oven, directly above its air-space, it travels only upwards. Anything underneath the netting or underneath the oven will remain pure.