משנה על אהלות 12:9

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. 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. [Its susceptibility to impurity begins] as soon as its manufacture is completed. What is regarded as the completion of its manufacture? When it is heated to a degree that suffices for the baking of spongy cakes. 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.

An old oven was within a new one and netting was over the mouth of the old [new] one: If [it was placed such that if] the old one were to be removed the netting would drop, all [the contents of both ovens] are unclean; But if it would not drop, all are clean. A new oven was within an old one and netting was over the mouth of the old one if there was not a handbreadth of space between the new oven and the netting, all the contents of the new one are clean.