Keilim, Chapter 5, Mishnah 5. This chapter has been dealing with the tanur, the clay oven, and with the question of which parts attached to it count as the vessel itself and which are merely additions standing alongside it. Earlier in the chapter (Mishnah 3) we met a rim built on top of a kirah, a stove, and the ruling there was that such a rim is not reckoned as part of the kirah. Our mishnah returns to the same kind of question, but with a different sort of rim and a different set of users, and it teaches a principle that runs through the whole discussion: a projection is judged by whether it actually serves the work of the vessel.
The rim of a household oven
The oven of those days was baked clay, open at the top, and the dough was slapped onto the inner walls to bake. Around that upper opening one could build up an added ring of clay. The mishnah calls it "musaf hatanur", the addition on top of the oven, and rules: "shel ba'alei batim, tahor", when it belongs to ordinary householders it is pure, meaning it does not become tamei together with the oven, because it is not counted as part of the oven at all.
Why not? True, the ring does something: it holds in some of the heat. But insulation alone is not baking. The loaves are stuck to the oven's walls, not to this ring, and the ring plays no role in producing the bread. It is a convenience attached to the oven rather than a working part of it, and so tumah that seizes the oven does not extend to it.
When the oven belongs to a nachtom
The mishnah continues: "veshel nachtomin, tamei", an identical ring built onto the oven of a professional baker does receive tumah. The reason given is "someich alav et hashepud", that the baker leans his spit against it. Bakers did not limit themselves to bread; they roasted meat as well, and when they did, the shepud, the iron roasting spit, was laid across that raised ring, which held it in place over the fire. Now the addition is doing genuine work for the oven. It has the status of a yad, a handle, an extension through which the vessel is used, and whatever tumah takes hold of the oven takes hold of it too. In a private home nothing was roasted in the oven; bread was all it produced, so the householder's ring assists nothing and stays outside the vessel.
Rebbi Yochanan Hasandlar's reason
A second explanation is offered in the mishnah by Rebbi Yochanan Hasandlar. He agrees that a baker's added ring is susceptible to tumah, but he traces it to a different use: the baker himself bakes on that ring "keshehu nitzchak", when he is under pressure. Picture a morning when the orders are heavy and a crowd of customers all want their loaves at the same moment. The inner walls cannot hold everything at once, so the baker slaps dough onto the raised ring as well and bakes it right there. Once the ring is itself a baking surface, it is not an accessory but a working part of the oven, and it can therefore contract tumah.
The cauldron of olive boilers and the cauldron of dyers
Two kinds of cauldron: olive boilers and dyers
Until now the distinction has been between an oven used at home and one used in a bakery. The mishnah moves on to an analogous distinction, this time between two trades that each boiled their goods in a large pot. "Kayotzei vo", along the same lines, it teaches about the "musaf hayorah", the ring added to the top of a cauldron. In the case of the "sholkei zeitim", those whose trade is boiling olives, the addition is "tamei": it is reckoned with the cauldron and becomes tamei together with it.
Olives were first pressed for their oil, and afterward boiled in order to draw out whatever oil remained in them. These were copper cauldrons, and a clay rim was often added around the top so that more olives could be packed in. The pot could be filled right up, and when the water came to a boil and rose, the rim held the water and the olives inside instead of letting them spill over. Since the rim enlarges the amount that can be boiled at one time, it genuinely serves the work of the pot and is counted as part of it.
Concerning the second trade the mishnah rules "veshel tzaba'im, tahor", that the same addition on a dyer's cauldron stays pure. The reason is that it buys the dyer nothing: he cannot dye a single extra garment because of it. On the contrary, he takes care that his liquid never reaches that height, since dye that touches the earthenware ring is spoiled and comes out inferior, so he fills the pot only partway. Why build the ring at all? Purely as a safeguard: should the pot ever surge up and boil over, the ring catches the overflow so that the dye is not lost and does not damage what stands beneath the cauldron.
Here is the heart of the mishnah. A rim that helps the vessel do its job, whether by supporting the baker's spit, by serving as baking surface under pressure, or by allowing more olives to be boiled, becomes part of the vessel and shares its tumah. A rim that merely insulates, or that merely guards against an accidental spill, is not working for the vessel; it is protecting what lies around it. Such an addition remains outside the vessel, and it stays tahor.