Kelim, Chapter Seven, Mishnah Four
1) If it [the extension] was detached from the stove, whenever it was three fingerbreadths high it contracts impurity by contact and through its air-space,
a) If it was lower or if it was smooth it is clean.
2) If three props on a stove were three fingerbreadths high, they contract impurity by contact and through their air-space.
a) If they were lower, all the more so they contract impurity, even where they were four in number.
Section one: If the extension that surrounds the stove was not attached to the stove, it is susceptible to impurity only if it is three fingerbreadths high. This means that if the stove contracts impurity through its airspace or by contact, the extension will also be impure.
If, however, the extension was less than three fingerbreadths high, or it had no rim (it was smooth), then it is considered separate from the stove and if the stove becomes defiled the extension remains pure.
Section two: Some commentators explain that the three props form a tri-pod for the pot to rest on the stove, while others explain them as serving as a base for the stove itself. If they are no more than three fingerbreadths above the stove, then they are considered part of the stove and the impurity of the stove contracted either by contact or through its airspace will similarly defile the props. If the props are closer to the stove, they are all the more so susceptible to impurity. This is true even if there were four of them, meaning that one was superfluous. However, if there were five or more props, they are not considered to be part of the stove and if the stove is defiled the props remain clean.