In the times of the Mishnah, there were people who boiled olives professionally in order to take olives that had already been pressed and get more oil out of them. People who dyed materials used very large cauldrons or pots for their work. They were not small; they were very large. They wanted to make sure nothing spilled over when the pots boiled. Sometimes, as the contents boiled, they would rise and spill over. They wanted to make sure that did not happen, so they would add height to their pots by sticking clay around the rim. Then the pot would be taller.
So here the question is: Is that clay rim part of the pot? Therefore, if the pot becomes tamei (ritually impure), does the rim also become tamei? Or do we look at it as a separate entity, so that it is not really a utensil and cannot become tamei? The Mishnah says: He'id Menachem ben Signai al musaf hayorah shel sholkei zeitim. Menachem ben Signai gave testimony about the addition they would add to the top of the olive boilers: the clay rim attached to the top of the pot. Shehu tamei, that is capable of becoming tamei. The rim increases the size of the pot, allowing more olive pulp to be boiled at one time. The basic function—the function of the pot—is to boil olives, and this helps with more olive boiling. Therefore, it is part of the pot, and just like the pot itself, it can become tamei.
Ve-shel tzaba'im shehu tahor (ritually pure). But the rim attached to the pot used by people who dyed materials is tahor. It cannot become tamei. When it came to this type of pot, the people who dyed materials would not add the rim to increase the amount of dye they used. They would add it to prevent spilling. They did not want the dye to spill out of the pot, so they would put on this rim. But they would not do it to increase the amount of dye they put into the pot, because they were nervous that if the dye touched the clay for too long, it would harm the dye, and dye was very expensive. So the only reason the rim was there was in case the pot boiled over a little once in a while, so things would not spill, and they wanted to save that spillage. They did not want the dye to get lost. The rim would protect it; then the contents would lower, and it would not really harm the dye. But they would never fill the pot to the top in order to get more use out of it. Therefore, since the rim does not really help with the function of the pot, which is to boil the dye, it would help only to prevent spilling. So it was not looked at as an actual, necessary part of the pot, but as a separate entity, and therefore it could not become tamei.
The whole reason Menachem ben Signai taught this idea was to tell us that the Hachamim (the rabbis) would say the opposite. They would say that the clay rim on top of the olive boilers' pot cannot become tamei, while the rim of the dyer's pot can become tamei. The sevara, the rationale behind this opinion, is that the rabbis held that nobody would use the rim to add material or to add to the pot. They would all use it just to prevent the occasional spill. It was just to protect the contents. The person dyeing materials was especially worried about spilling because dye is very valuable; it is expensive. Therefore, the rim is very essential because they do not want to lose the money. The clay rim is looked at as part of the pot, and therefore it can become tamei. But when it came to the people who boiled olives, the rim was not really necessary because olive pulp is very cheap. Therefore, it is there to protect against spilling, but it is really insignificant. It is not such an important thing, and so they looked at it as something that cannot become tamei because it is an accessory. It does not really have an important role, and therefore it is not part of the pot and cannot become tamei. This is why Rabbi Menachem came to teach us that it is the opposite: the rim of the olive pulp can become tamei. It is looked at as part of the utensil, while the rim of the dyer is not looked at as part of the utensil, and therefore it cannot become tamei.