Kelim, Chapter Seven, Mishnah Three
1) A double stove which was split into two parts along its length is clean.
a) Through its breadth is unclean.
b) A single stove which was split into two parts, by its length or by its width, it is not susceptible to impurity.
2) As to the extension around a stove, whenever it is three fingerbreadths high it contracts impurity by contact and also through its air-space, but if it is less it contracts impurity through contact and not through its air- space.
3) How is the air-space determined?
a) Rabbi Ishmael says: He puts a spit from above to below and opposite it contracts impurity through the air-space.
4) Rabbi Eliezer ben Jacob says: if the stove contracted impurity the extension is also unclean, but if the fender contracts impurity the stove does not become unclean.
Section one: If a double stove was split lengthwise it is rendered unusable since each opening is cut in half. Therefore it is pure. However, if it split by its breadth, it is pure because it is still usable. In contrast, a single stove is rendered useless if it split, no matter how it is split. Therefore, in all cases it is pure.
Section two: The “extension” is an earthenware base that surrounds the stove. Occasionally pots were placed on the extension. If it is three fingerbreadths high, the minimum height of a stove, then it is subject to impurity through contact and through airspace. This means that if a defiling agent enters the stove’s airspace or touches the inside of the stove, the extension is also impure. If it is less than three fingerbreadths high, then it is impure only if the stove is defiled by touching and not by airspace. It seems that in this case the extension is not a vessel in and of its own right, but only a “yad” an “appendage” of the stove. A “yad” cannot be defiled through airspace.
Section three: The mishnah questions how we determine the airspace of the extension. This is a problem because the extension surrounds the stove which is much higher than the extension. Rabbi Ishmael says that we take a spit and lay it diagonally from the top of the stove to the top of the extension. If a defiling agent enters its airspace below this diagonal line, it transmits its impurity.
Section four: Rabbi Eliezer ben Yaakov considers the extension to be an appendage to the stove. Hence, if the stove becomes defiled, its extension is always impure, even if the stove contracted impurity through its airspace and the extension is less than three fingerbreadths high.
However, an appendage cannot transmit impurity to the main vessel and therefore if the extension contracts impurity, the stove remains pure.