Kelim, Chapter Four, Mishnah Four
1) An earthenware vessel that has three rims:
a) If the innermost one projects above the others, all outside it is not susceptible to impurity.
b) If the outermost one projects above the others all within it is susceptible to impurity;
c) And if the middle one projects above the others, that which is within it is susceptible to impurity, while that which is without it is not susceptible to impurity.
2) If they were equal in height:
a) Rabbi Judah says: the middle one is deemed to be divided.
b) But the sages ruled: all are not susceptible to impurity.
3) When do earthenware vessels become susceptible to impurity? As soon as they are baked in the furnace, that being the completion of their manufacture.
Section one: The vessel referred to here has three rims, forming three circles around the vessel. Since earthenware vessels do not become impure by contact through their backs (their outside) the mishnah needs to determine which of the rims is “inside” and which is the actual rim. The basic determining factor is that the highest rim is the “real” rim. So if the innermost rim is highest, the outer two rims are the outside of the vessel and are not susceptible to impurity. And if the outermost rim is highest, then it is the “real” rim and everything inside is susceptible.
Section two: If all three rims are of the same height, then Rabbi Judah says we split them right down the middle. From the middle of the middle rim and inwards is susceptible and the outside is not susceptible. The other sages disagree and hold that anything outside of the inner rim is not susceptible to impurity.
Section three: This clause contains an essential principle in the laws of impurity. Vessels are susceptible to impurity once their manufacture is complete. At that point they become “vessels.” The manufacture of earthenware vessels is complete once they are baked in the furnace. If a defiling agent comes into contact with them before that point, they are still clean.