Kelim, Chapter Five, Mishnah One
The next five chapters of Mishnah Kelim will do with the purity of oven and stoves. The oven referred to in the mishnah was like an inverted pot, with a narrow top to let a little air escape and enter. It had no floor, but rather the earth served as the floor. It was used primarily for the cooking of flat bread, which would be stuck to its sides.
1) A baking oven originally must be no less than four handbreadths [high] and what is left of it four handbreadths, the words of Rabbi Meir.
a) But the sages say: this applies only to a large oven but in the case of a small one it originally can be [any height] and what is left is the greater part of it.
2) [Its susceptibility to impurity begins] as soon as its manufacture is completed.
What is regarded as the completion of its manufacture?
a) When it is heated to a degree that suffices for the baking of spongy cakes.
b) Rabbi Judah says: when a new oven has been heated to a degree that sufficed for the baking of spongy cakes in an old one.
Section one: According to Rabbi Meir for an oven to be susceptible to impurity, its original size must be at least four handbreadths. If it was originally higher than four handbreadths, and then it broke and not all of it remains, the minimum height for it to be susceptible to impurity is still four handbreadths.
The sages make a distinction between ovens that were intended to be large from the outset and ovens that were originally intended to be small. When it comes to the large oven they agree with Rabbi Meir. However, they hold that some ovens were intended to be small from the outset. Such an oven can originally be of any height for it to be susceptible to impurity and if it breaks as long as a majority of it remains it is still susceptible to impurity.
Section two: The oven is susceptible to impurity once its manufacture is complete. The rabbis debate what is considered the completion of its manufacturing. According to the first opinion, the oven is susceptible once it has been forged sufficiently for it to be used to cook spongy cakes, which can be cooked at a relatively low temperature. Rabbi Judah slightly modifies this. Sponge-cakes would require more heat in a new oven than in an old one. If the new oven had been heated up enough for an old oven to cook sponge-cakes, then it is already susceptible.