משנה על במדבר 17:29

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. Rabbi Yohanan ben Nuri says: even those made of pieces of vessels. 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. [If they were made] from ordinary nails: Bet Shammai says: they are unclean, And Bet Hillel says that they are clean.