Kelim, Chapter Five, Mishnah Four
1) An oven that was heated from its outside, or one that was heated without the owner’s knowledge, or one that was heated while still in the craftsman’s house is susceptible to impurity.
2) It once happened that a fire broke out among the ovens of Kefar Signah, and when the case was brought up at Yavneh Rabban Gamaliel ruled that they were unclean.
Section one: Usually an oven is heated up from the inside, with the owner’s knowledge, after the craftsmen has put on the finishing touches. However, as long as the oven is heated up enough to cook sponge-cakes, the oven is susceptible to impurity.
Section two: Even though these ovens were not heated up intentionally, and they were heated up probably in the craftsmen’s house, they are still susceptible to impurity.