Kelim, Chapter Five, Mishnah Eleven
1) An oven of stone or of metal is clean,
a) But the latter is unclean as a metal vessel.
2) If a hole was made in it, or if it was damaged or cracked, and he lined it with plaster or with a rim of clay, it is unclean.
a) What must be the size of the hole [for it to be pure]? It must be big enough for the flame to come through.
3) The same applies also to a stove.
a) A stove of stone or of metal is clean.
b) But the latter is unclean as a metal vessel.
c) If a hole was made in it or if it was damaged or cracked but was provided with props it is unclean.
4) If it was lined with clay, whether inside or outside, it remains clean.
a) Rabbi Judah says: if [the lining was] inside it is unclean but if outside it remains clean.
Section one: All stone vessels are totally unsusceptible to impurity, so a stone oven can never become impure. A metal oven can become impure but not in the same way as an earthenware oven. While an earthenware oven is susceptible to impurity through its interior space, and not from its outside, the reverse is true for a metal oven. Also, a metal oven that has been attached to the ground is not susceptible, whereas an earthenware oven is.
Section two: If the oven was in some way damaged such that a flame could go through the hole in the side, the oven is pure because it is not usable. However, it becomes susceptible again to impurity once it has been repaired.
Section three: The same general rules apply to a stove. The mishnah adds that if he makes “props” which is either a tripod for pots put on top of the stove or legs to plug the holes on the bottom, the stove is again susceptible because it is usable. As usual, once a vessel has been repaired, it is susceptible to impurity.
Section four: According to the first opinion, if he spread a clay lining over a stone or metal stove, it is still pure, meaning it is not considered an earthenware vessel. This is because the clay is not necessary and does not affect the functioning of the oven.
Rabbi Judah holds that if the clay lining is spread inside the oven it does make the stove into an “earthenware vessel” and subject to the purity rules governing such a vessel. But if he spreads it on the outside of the vessel, it is still considered to be of stone/metal and the rules governing an earthenware vessel do not apply.