Kelim, Chapter Eight, Mishnah Three
1) Netting placed over the mouth of an oven and slightly sinking into it, and having no frame:
a) If a sheretz was in it, the oven becomes unclean;
b) If the sheretz was in the oven, the food in the netting becomes unclean, since only vessels afford protection against an impurity in an earthen vessel.
2) A jar full of pure liquids placed beneath the bottom of an oven, and a sheretz in the oven the jar and the liquids remain clean.
a) If it was inverted, with its mouth projecting into the air-space of the oven, and a sheretz was in the oven, the liquid that clings to the sides of the jar remains clean.
Section one: This netting is not considered to be a vessel because it does not have an “inside.” Therefore it does not serve as a barrier for impurity vis a vis the oven. This means that if a sheretz was in it, it defiles the oven. And if a sheretz is in the oven, then the food in the netting becomes unclean.
Section two: Since the jar is below the oven, the impurity of the oven doesn’t spread into the jar. This is true even if the jar is open and there is a hole in the bottom of the oven. As long as the jar is not in the oven’s airspace, it is not susceptible.
If they put the jar on top of the oven and its mouth is open the liquid which remains in the jar is still pure. Again, this liquid is not in the airspace of the oven, rather above it. Therefore it remains pure.