Keilim, Chapter 5, Mishnah 10. This mishnah picks up the subject that was opened earlier in the chapter, in Mishnah 8: an earthenware oven that was sliced from side to side into separate rings and then reassembled. The rule established there was that when a person spreads a fresh coating of clay directly over the reassembled rings, the result is treated as a new, whole oven, and it is once again fit to receive tumah. If, however, a layer of sand was interposed between the oven itself and the outer clay coating, the oven stays tahor, because the coating never truly joined itself to the rings.
Our mishnah presents the reverse arrangement. Here the clay was applied directly onto the oven, with nothing separating the coating from the earthenware. The sand was placed somewhere else entirely: between the rings themselves.
Sand Between the Rings
The mishnah opens: "chatacho chulyot," he cut it into rings, "venatan chol," and he put sand, "bein chulya lechulya," between one ring and the next. Only afterward was the exterior plastered over with clay. That plaster takes hold of every section and locks them into position, so that the oven stands upright and bakes as a single object. All the same, no ring is actually touching its neighbor: a band of sand lies between each earthenware section and the one beside it.
"Rabbi Eliezer metaher, vaChachamim metamein." Rabbi Eliezer rules that such an oven remains pure, while the Chachamim rule that it is capable of becoming tamei.
Rabbi Eliezer reasons that the sand keeps the pieces apart. Since the rings never touch, and sand does not bind one ring to another, each section remains its own separate entity. The oven therefore retains the status of a broken vessel, and a broken vessel does not receive tumah.
The Chachamim look instead at the outer casing of clay. In their view, that coating genuinely grips the sections and unites them, producing one complete oven that is no longer considered broken at all. Being whole, it is fit to become tamei.
Tanuro Shel Achnai
Our mishnah records the famous machloket that the Gemara refers to as "zeh tanuro shel Achnai." An achna is a serpent that wraps itself in loops around whatever it has seized, and the title captures the character of this debate: Rabbi Eliezer and the Chachamim returned to the very same case over and over, each bringing fresh proofs, twisting the argument around itself in coil after coil.
The debate was extraordinarily fierce, and the Gemara relates it at length. In the end the halachah was fixed according to the Chachamim, even though signs came from the heavens indicating that Rabbi Eliezer was right. The response of the Chachamim was "lo bashamayim hi": the Torah is not in heaven. The authority to determine halachah was placed here on earth, in the hands of the Chachamim, and whatever they conclude becomes the truth, no matter what may have been objectively true above. Their ruling itself establishes the new reality. When Rabbi Eliezer would not accept the decision, the Chachamim placed him under excommunication.
The Arabian Cauldron
Now the mishnah shifts to an oven of an altogether different sort, one shaped out of the earth itself: "yorot ha'Arviyin," the cauldrons of the Arabians, "shehu chofer ba'aretz," which a person hollows out of the ground, "vetach batit," and lines with clay. Ordinarily a yorah means a sizeable cooking vessel whose mouth is at its upper end, and here the word is borrowed loosely, since in this instance nothing is a vessel to begin with. What happened was this: a hollow was excavated in the soil and its inner walls were smeared over with clay. A fire was lit inside, and dough was pressed against the plastered walls to bake into bread. The name comes from Arabia, where ovens of this design were in common use.
"Im yachol hatit," if the clay is capable, "la'amod bifnei atzmo," of standing on its own, then "tamei." Should the lining be substantial enough that it requires no support from the walls of the pit around it, this oven can contract tumah, since it counts as a keli. Picture lifting the clay shell out of the ground: if it would keep its form intact, we are dealing with a real oven, and tumah applies to it.
"Ve'im lav, tahor." If the coating is thin and cannot stand independently, the oven is tahor. In that case the real oven is nothing more than the hole in the earth, and ground attached to the earth cannot become tamei. Clay that cannot hold itself up is not an oven at all; what is doing the baking here is simply the ground, and the ground remains pure.
Tanuro Shel Ben Dinai
The mishnah closes: "Vezeh tanuro shel Ben Dinai." An oven of this sort carries the name Ben Dinai, since it too produced a sweeping machloket. Round after round of legal argumentation grew out of this case, and dinai means law: the sheer volume of halachic discussion that this oven generated is what attached the name to it.