Keilim, Chapter 11, Mishnah 1. Until now the masechet has been occupied largely with klei cheres, earthenware vessels. With this chapter we leave earthenware behind and turn to klei matachot, vessels made of metal. The Mishnah opens by teaching two foundational rules that belong to metal alone and set it apart from every other material.
Flat vessels are also susceptible
"Klei matachot, peshuteihen umkabeleihen teme'in": with metal vessels, both the flat ones and the ones that hold something inside are susceptible to tum'ah. This is a genuine novelty. For most materials, being a receptacle is the very condition of susceptibility. An earthenware utensil, a glass utensil, a wooden utensil and the like do not become tamei at all unless they form a container, something with an inside that holds. A flat board of wood, a flat piece of pottery, has no tum'ah. Metal is different: a metal implement does not have to hold anything in order to receive tum'ah. Flat metal objects are just as susceptible as metal containers.
Breaking purifies, but the standard is stricter
"Nishberu, taharu": if metal vessels became tamei and then broke, they become tahor. In this respect metal follows the general principle that governs all kelim, that a broken vessel is no longer a vessel and its tum'ah departs.
Here too, though, metal has its own measure. With other materials, the question is whether the broken piece still serves some useful function; if it does, it is still considered a keli, its "keli-ness" has not left it, and it therefore keeps its tum'ah. Earthenware is the classic example: as long as a shard is still fit for some use, it remains tamei. Metal is judged by a different standard. A metal utensil loses its tum'ah once it can no longer perform the task for which it was originally made, even if the broken piece can still be put to some other use. The loss of its original purpose is enough to end its status as that keli, and with it, its tum'ah.
When the pieces are made into a vessel again
The Mishnah continues: "Chazar ve'asa meihen keli." Picture someone collecting those fragments and working them back into a usable utensil. The ruling is "chazru letum'atan hayeshanah", the impurity of before settles on them once again. Whatever tum'ah had clung to that metal prior to the break now rests upon the utensil just produced from it.
This is a ruling of the Chachamim on a deRabbanan level: a vessel that was broken and then reassembled must undergo the very same process of purification that its earlier tum'ah would have required before it can become tahor.
The view of Rabban Shimon ben Gamliel
Rabban Shimon ben Gamliel narrows the scope of this last ruling. His words are "lo lechol tum'ah", not with respect to every category of impurity, "ela letum'at hanefesh", but only where the impurity originated in a human corpse. In his view, only a metal keli that had contracted corpse tum'ah, then shattered, then was refashioned, resumes the impurity it once bore. Where the original impurity came from any other source, the utensil produced from those fragments is, in his opinion, pure.
So this mishnah gives us the two signatures of metal: its susceptibility does not depend on being a receptacle, and its tum'ah is bound up with the vessel's own original purpose, so tightly that breaking it purifies it and remaking it can bring the old tum'ah back.