Kelim, Chapter Eleven, Mishnah Three
1) If vessels are made from iron ore, from smelted iron, from the hoop of a wheel, from sheets, from plating, from the bases, rims or handles of vessels, from chippings or filings, they are clean.
a) Rabbi Yohanan ben Nuri says: even those made of pieces of vessels.
2) From [vessels that are made] of fragments of vessels, from pieces of old vessels, or from nails that were known to have been made from vessels, are unclean.
3) [If they were made] from ordinary nails:
a) Bet Shammai says: they are unclean,
b) And Bet Hillel says that they are clean.
Section one: Vessels that are made from any of these materials are pure (unless they contract impurity). We need not be concerned that they come from material that was at one time impure because all of the things that are listed here are not “vessels” and therefore they are not susceptible to impurity. In other words, if a vessels comes from recycled material that could never have been impure, it is itself pure.
“Pieces of vessels” are pieces that are cut off of the raw metarl material when vessels are being made. Rabbi Yohanan ben Nuri rules that these too cannot be impure, and therefore vessels made of them are pure.
Section two: In this case the older vessels may have been impure before they were broken and recycled. Therefore, the new vessels made from them must also be treated as impure, for when the old vessels were forged into new vessels, they return to their former state of impurity (see mishnah one).
Section three: In this case, we don’t know whether the nails were once parts of vessels, in which case they are susceptible to impurity, or were from nails that were never parts of vessels, in which case they are not susceptible to impurity. Bet Shammai rules strictly and renders them unclean lest they came from vessels that were once impure. Bet Hillel rules leniently, reasoning that there is a double-doubt in this case. The nails may not have ever been part of vessels, and even if they were, there is the possibility that the previous vessel was clean.