Mishnayos Keilim Perek 11 Mishnah 1

כלי מתכות פשוטיהן טמאים (metal vessels: their flat parts are impure) – for there is no analogy to a sack (see Leviticus 11:32; “And anything on which one of them falls when dead shall be impure: be it any article of wood, or a cloth, or a skin or a sack – any such article that can be put to use shall be dipped in water and it shall remain impure until evening; then it shall be pure; also see Tractate Kelim, Chapter 2, Mishnah 1: “Vessels of wood, and vessels of leather, and vessels of bone and vessels of glass: when they are flat, they are clean, and when they form receptacles, they are [susceptible of becoming] unclean.”).

חזרו לטומאתן הישנה – The Sages made a decree regarding metal vessels that he melted them and made from them new vessels, that they return to their former defilement/uncleanness. This is a decree lest they say that the breaking of these utensils purifies them and their immersion [in a Mikveh] purifies them. Just as their breakage does not require the coming of sunset, for behold, we use them the entire day that they were defiled and he broke them and he returned and made of hem new [vessels], so [also] their being immersed does not require the coming of sunset, and it comes to make the heave offering and Holy Things in vessels that their sunset had not as yet arrived (see Talmud Shabbat 16b – this is the ordinance of Shimon ben Shetach).

לא לכל טומאה אלא לטומאת נפש – Rabban Shimon ben Gamaliel holds that the reason that they (i.e., the Rabbis) decreed regarding metal vessels that they return to the former uncleanness is not other than a decree lest the law of the waters of purification be forgotten from the vessels, for there isn’t a person that waits seven days but rather he breaks them immediately and makes from them (i.e., the vessels) anew, therefore, they didn’t decree other than regarding defilement from a corpse alone that requires sprinkling [of a mixture of ashes of the red heifer with special water] on the third and seventh days (see Numbers 19:12: “He shall purify himself with it on the third day and on the seventh day, and then be pure.”). But the Halakha is not according to Rabban Shimon ben Gamaliel.