(Beraisa - R. Elazar): Mei Chatas that became Tamei is Metaher. We learn from a Nidah. The water is Metaher her (from Tum'as Mes), even though she is Metamei it when it touches her.

Avodah Zarah 75b (Mishnah): If one bought a Nochri's knife, he does Shifah (rubs it on a sharpening stone), and it is Tahor.

(Beraisa): If a Yisrael buys a used Keli from a Nochri, if it was used with cold food, such as cups and flasks, he rinses and immerses it;

If it was used with hot liquids, such as pots and kettles, he does Hag'alah and immerses it;

If it is used to cook over a fire without liquid, such as spits and grills, he does Libun (makes it glowing hot) and immerses it.

(Rava): "Kol Davar Asher Yavo ba'Esh Ta'aviru ba'Esh v'Taher" obligates an additional Taharah after Kashering.

76b (Mishnah): One does Shifah to a knife, and it is Tahor.

(Rav Ukva bar Chama): Also, he inserts it 10 times in the ground.

Shevor Malka (a Nochri) inserted his knife 10 times in the ground, cut a piece of Esrog and gave it to Mar Yehudah.

Rambam (Hilchos Ma'achalos Asuros 17:5): It says "Ta'aviru ba'Esh v'Taher." Fire only purges absorptions; Chachamim said that v'Taher adds another Taharah after passing it through fire to Kasher it.

Rosh (Avodah Zarah 5:36): The Rashbam says that he must do Hag'alah or Libun first. If he immersed it and then did Hag'alah, this is like Tevilah while holding a Sheretz. This is wrong. Tevilah is not due to Tum'ah. Rather, it is a Gezeras ha'Kasuv. One may immerse it and use it for cold food, and if later he wants to use it for hot food, he will Kasher it. Hechsher works half at a time. Haza'ah on a Nidah is Metaher (her from Tum'as Mes, even though she still needs to be Metaher from Nidah). Similarly, Tevilah works before Kashering. The Gemara mentioned Kashering first because it is the primary Hechsher, to purge the (absorbed) Isur. Also, for Kelim used for cold food we must teach washing before Tevilah, due to Chatzitzah.

Rosh (Yoma 8:24): Rav Yitzchak taught that one must be Metaher himself on the Regel, i.e. from all Tum'ah, including Tum'as Mes. Nowadays the entire year a Ba'al Keri (one who had an emission) does not immerse, so there is no obligation to immerse for the Regel. The custom is to become Tahor from Keri for Tefilas Yom Kipur.

Torah Lishmah (512): Now we are Tamei Mes. How does Tevilah help for Keri? This is like Tevilah while holding a Sheretz! The Rosh does not mean that since one is Tamei Mes, Tevilah will not help for Keri. Rather, since he remains Tamei Mes, he does not fulfill R. Yitzchak's teaching. Some say that Tevilah of a Nochri's Keli before Hag'alah does not help. There is different, for it is possible to Kasher it first. All agree that Tum'as Keri takes effect on Tum'as Mes, and that Tevilah helps for it. Bechoros 27a says that one immerses to eat Terumah of Chutz la'Aretz (even though he is Tamei Mes).

Avnei Nezer (YD 112:5): The Torah explicitly says that the first time a Metzora immerses, his Tum'ah decreases, but he is still Tamei like a Sheretz for seven days until immersing a second time.

Ran (Avodah Zarah 39b DH Tanu): The Yerushalmi says that one must immerse them because they left Tum'as Nochri and entered Kedushas Yisrael. Therefore, one must Kasher it first. If he immersed it first, this is like Tevilah while holding a Sheretz. However, the Ra'avad says that this is only l'Chatchilah.

Ritva (Yevamos 47b DH Shehuyei): Tevilah of a convert does not help before he is circumcised, even b'Di'eved. It is like Tevilah while holding a Sheretz. Regarding Kelim of a Nochri, b'Di'eved Tevilah works before Kashering. Here is different, for the foreskin is real Tum'ah.

Shevet ka'Levi (4:92:3): In both cases, something blocks entering Kedushas Yisrael. Absorbed Isur is less of an impediment than foreskin, therefore, it is not Me'akev b'Di'eved.

Ran (Avodah Zarah 40a DH Lo): An absorption in a Keli that is not Ben Yomo is proper for a Ger (Toshav). If does not forbid a mixture, because it does not improve the taste. Rather, it harms the taste. Absorbed Nosar is still called Nosar, therefore, the Torah requires Bi'ur (eradication), just like we burn (intact) Nosar. For Kelim that absorbed Nosar, the Torah commanded to break a Keli Cheres or do Hag'alah for a metal Keli.

Rashi (76b DH v'No'atzah): After Shifah, it must be inserted 10 times in the ground. Some say that Shifah is rubbing it with coarse wool to remove the grease.

Rebuttal (Ramban 75b DH Masnisin): The Tana of the Mishnah should have taught everything necessary! The Tana would not omit only one matter. What else did he omit? Also, the case involving Shevor Malka is difficult (it implies that it suffices to insert it in the ground). Therefore, others say that inserting in the ground is an alternative method of Hechsher. One may do Shifah or insert it in the ground for cold usage.

Shulchan Aruch (YD 121:2): If one bought a metal Keli from a Nochri that was used with hot liquids, whether it is of metal, wood or stone, he does Hag'alah. Afterwards he immerses it, and it is permitted. If he immersed it and then did Hag'alah, it is permitted. Some say that he must immerse it again.

Beis Yosef (DH v'Chosav ha'Rashba): The Rashba says that he must do Hag'alah and afterwards immerse it. Semag says that the Rashbam says that one must do it in this order, like the Beraisa says. If he immersed it and then did Hag'alah, this is like Tevilah while holding a Sheretz. The Ri disagrees. Even new Kelim must be immersed. It is not due to Tum'ah. Rather, it is a Gezeras ha'Kasuv. The Rosh concluded like the Ri. Tosfos wrote Stam like the Ri.

Taz (1): Tevilah does not require Hag'alah. Tevilah is only to go from Tum'ah to Taharah. The latter opinion holds that if he immerses it first, it looks like Tevilah while holding a Sheretz.

Shach (4): The Levush says that if now he wants to use it only for cold food, he can immerse it and use it for cold food. If later he wants to use it for hot food, the first Tevilah will still be valid, even according to the opinion that requires a second Tevilah. Here is different, for unlikely he intended to use it only for cold, and Tevilah sufficed. Since he immersed properly, he need not immerse it again. It seems that if he immersed it with intent to eat hot food, since he must immerse it again according to the latter opinion, he may not use it even for cold without a (Kosher) Tevilah. It seems that the Rosh and Tur agree.

Pischei Teshuvah (4): Tif'eres l'Moshe disagrees. He holds that the latter opinion holds that even if he intends to use it only for cold, Tevilah does not help before Hag'alah if the Nochri used it for hot.

Shach (5): One should immerse it again without a Berachah. See Siman 120. (In every case of Safek, we immerse without a Berachah.)

Gra (5): The first opinion learns from Nidah that one may immerse it first. The end of Avodah Zarah shows that Shifah helps for a knife (for cold food), even though it still requires Hag'alah (for hot food). Toras ha'Bayis says that it is l'Chatchilah to do Hag'alah first, like the verse "Ta'aviru ba'Esh v'Taher."

Shevet ka'Levi (6:115:3): The Ro'oh says that even according to the opinion that Ta'am k'Ikar (if an Isur can be tasted in a mixture, the mixture is forbidden) is not mid'Oraisa, we cannot immerse the Keli before Kashering. It is not due to Isur. Those who reject the proof from Nidah hold that Tum'as Mes is totally different than Nidah, therefore, one is not Me'akev the other. The Rambam and Bartenura connote that sharpening a knife suffices, and one need not immerse it. However, the Tosfos Yom Tov questioned this. Why should a knife be different than all other Kelim used for food?!

Dagul me'Revavah: I say that if it is not a Ben Yomo, all permit to immerse it first. This is unlike Tevilah while holding a Sheretz. Since not a Ben Yomo, there is no Sheretz. Mid'Oraisa it is totally permitted. Even mid'Rabanan there is no Sheretz, just they decreed not to use it lest one use a Ben Yomo.

Chasam Sofer (2:110): Even though when a Ben Yomo is used for something sharp, it gives a beneficial taste, right now the absorbed taste is detrimental and it is permitted, so this is unlike Tevilah while holding a Sheretz.

Hagahos Yad Sha'ul: Not all agree to this. The Rashba holds that even a Ben Yomo is proper for a Ger (Toshav) to eat. It is a Neveilah (i.e. the Isur is intact), just it does not improve the taste of what is cooked in it. The Isur is Batel in the majority. It is still a Sheretz, therefore, he must do Hag'alah first.

Note: Kovetz Hilchos Pesach of Rav Blumenkrantz (5769 p.66) brings opinions that one should Kasher new aluminum or steel Kelim, lest it was rubbed with Asur oil (which gets absorbed) to give a shiny appearance. If a pot was used (for hot food without Kashering), one need not Kasher it (and the food is permitted). Madrich ha'Kashrus (of Badatz Edah Charedis, Pesach 5770, p.160-161) says that one should Kasher a new aluminum or steel Keli before immersing. However, on p.25-26 it says 'the custom is to be stringent to Kasher it. If one immersed it first, there are reasons to be lenient, and one should ask a Chacham.' (Most permit Tevilah before Kashering if the Keli is not Ben Yomo. Also, R. Moshe Feinstein Ztz"l said that the decree to Kasher an Eino Ben Yomo lest one use a Ben Yomo does not apply here, for one will never use a Keli within 24 hours of when the oil was absorbed. Perhaps Hagahos Yad Sha'ul would be stringent to Kasher even new Kelim before Tevilah, but this is based on the Ran, and the Halachah does not follow the Ran - PF, with thanks to R. Y. Spitz Shlita for helping me to clarify this Halachah.)

Pischei Teshuvah (5): Yeri'os ha'Ohel 51 proved that the latter opinion is primary. He holds that one immerses it again with a Berachah.

Aruch ha'Shulchan (120:17): L'Chatchilah, he must Kasher before Tevilah. If he did Tevilah first, it is valid.

Aruch ha'Shulchan (121:9): The first opinion is primary. The Tevilah is not due to Tum'ah. It is a Gezeras ha'Kasuv.

Kitzur Shulchan Aruch (37:4): If one must Kasher through Hag'alah or Libun, he must do so before Tevilah.

Chachmas Shlomo: The Pesikta, brought in Yalkut Reuveni (Matos, on the verse "va'Yomer Elazar...") supports the latter opinion. It says that the Parshah was said through Elazar because the Kelim needed Haza'ah with Mei Chatas, and R. Elazar did the Parah Adumah. This is astounding. Moshe told the people about Haza'ah, and Elazar added Hag'alas Kelim! Rather, we must say that Tevilah and Haza'ah do not help without Kashering first. Since Elazar was involved with the Haza'ah, and it is impossible without Hag'alah first, therefore (also) he was involved also with the Hag'alah.

Question: Reuven swore to his first wife that he will not marry another wife within 30 days. Can the second wife immerse within the 30 days? This is like immersing a Keli before Kashering it. According to the latter opinion in Shulchan Aruch, even b'Di'eved the Tevilah was invalid.

Answer (Rav Pe'alim 1 YD 31): These are different. The reason to immerse the Keli again is because the first time, it still had Isur absorbed, so it is like Tevilah while holding a Sheretz. Even though Isur and Tum'ah are separate matters, since there is Isur inside, the Tum'ah does not leave through Tevilah. Here, the woman is permitted to have Bi'ah at the time of Tevilah, just the oath is an external matter forbidding her to her husband.