Keilim, Chapter 5, Mishnah 9. This chapter deals with the laws of the earthenware oven: when it counts as a finished vessel that can receive tumah, and when a break in it renders it tahor. The previous mishnah dealt with an oven that had been cut apart and then reassembled. Our mishnah takes up a different case: an oven that never arrived as one piece at all, but was manufactured in separate sections, designed from the outset to be assembled by the buyer.
The case is introduced with the words "Tanur sheba machutach mibeit hauman". The oven left the potter's shop already divided up: not damaged, not shattered, but deliberately produced in separate panels. That is simply how this product was made, and the purchaser is the one who joins the pieces into the circular wall of an oven.
The Hoops
"Ve'asa bo limudin": along with the panels, the maker supplied hoops. A hoop is a band that wraps around the outside of the assembled pieces and squeezes them tight, so that what were separate panels become one continuous oven wall. Picture a wine barrel: it is put together from wooden staves, and the metal rings running around its circumference are what clamp those staves into a single sealed vessel. The hoops here do exactly that work, except that they are made of earthenware, encircling the panels and locking them in position.
"Venotnan alav", the owner placed the hoops over the sections and thereby formed them into an oven. "Vehu tahor": until that moment the assembly was tahor and simply incapable of contracting tumah, because loose panels are not an oven. "Nitma": once the hoops are in position, the oven is regarded as a completed vessel and can now become tamei. The sections are gripped tightly by the hoops, they are considered joined to one another, and together they constitute a genuine oven.
Removing the Hoops
"Silek et limudav, tahor". Should the hoops be lifted away, the oven returns to a state of taharah. Once nothing is clamping them, the panels are no longer joined to one another, and what remains carries the halachic status of a shattered oven. Consequently, any tumah the oven had absorbed while it stood assembled leaves it the instant the hoops come off, precisely as tumah would leave an oven that had been broken. Halachah views it as broken, and it is entirely pure.
Putting Them Back
"Hechziran lo, tahor". And here is the chiddush of the mishnah: even if he returns the hoops to their place, the oven remains tahor and cannot receive tumah from here onward. The reason is that this oven had already been in use and had already been fired. An oven that was baked in while bound by its hoops becomes warped; the heat sets the panels into a particular shape. Once the hoops come off, the sections can never again be made to sit flush against one another. Gaps remain between them through which the heat escapes, and such an oven is therefore regarded as permanently broken, which is why the return of the hoops accomplishes nothing and it cannot become tamei.
A Coating of Clay
"Marach batit", if, after replacing the hoops, he smeared clay over them. We just established that restoring the hoops by itself no longer turns this into an oven; but now he adds something further, a layer of clay over the joins. In that case, "mekabel tumah", it is once again capable of receiving tumah, because the clay closes up the gaps between the sections and restores the oven to being whole. And he need not plaster the entire oven; it is enough that he fill in the spaces between the panels.
"Ve'eino tzarich lehasikenu, shekvar husak": no fresh firing is demanded of him before this oven can take on tumah, unlike a newly made oven, since this one has a firing to its credit already. Back when the panels were first joined, he brought the oven up to the required heat, for had he not done so it could never have been susceptible to tumah at all. That earlier heating still stands to its credit even after the hoops were taken off and replaced, and so it counts as an oven without being heated afresh.
Why This Differs From the Previous Case
One might object: we learned regarding an oven that was cut apart and then reassembled and coated with clay that it does not receive tumah until it is fired to the proper degree. Why does our mishnah dispense with the firing?
Everything depends on what is actually binding the oven together. In the earlier case, the smeared clay was the sole connector between the fragments, and firing was therefore necessary to set that clay and guarantee that the bond it created would truly hold. Here the hoops are doing the binding. The clay added afterward is not functioning as an adhesive between the panels; its entire role is to block the gaps. Firing exists to create a bond, and since no bond is being created here, firing serves no purpose. The attachment is already supplied by the hoops; they were only lacking somewhat, and the clay makes up that lack. The oven stands connected as it is, requiring nothing more than a small strengthening, and a fresh heating is therefore beside the point.