Koshering an Oven for Pesach

See the main page: Kashering the Kitchen for Pesach.

1. Why do we need to kosher an oven in the first place?
   1. The concern to use an oven if that there’s food that fell on the walls and floor and got absorbed directly with the heat of the fire. That would require libun.
   2. Also, there’s a concern that there was steam from the food that got absorbed into the walls.
2. How do we need to kosher to avoid those concerns?
   1. If the concern is the food that fell we need libun.
   2. If the concern is zeyia we should logically only need hagalah, but some poskim require libun for that.
   3. What type of koshering does zeyia need?
      1. Or letzion holds that libun is necessary based on Tur and S”A 451:15.
      2. Minchat Shlomo 2:51 based on Bet Meir 461:1 argues that hagalah is sufficient because only liquids have steam. What about tur and s”a 451:15? That was very close zeyia and would require libun but an oven zeyia only needs hagalah. Also libun kal is enough which is having it get to yad soledet on both sides (Rabbi Akiva Eiger 451:4, Shulchan Aruch Harav 451 unlike Pri Megadim that you need kash nisraf). Chut Shani Pesach 10:2 agrees that zeyia only needs libun kal. Pesach Khilchato 1:8 fnt. 11 quoting Rav Elyashiv says ovens only need libun kal. Binat Tzvi explains that he’s only talking about zeyia and not spills. Halachos of Pesach by Rabbi Ribiat p. 353 writes that for zeyia libun kal is sufficient.
3. Can we ignore the first reason because of rov tashmisho?
   1. Ashkenazim are strict for the miyut tashmish of the utensil initially and not after the fact (Rama 451:6). If there’s no other options we can be lenient since the ikar hadin is that we follow rov tashmisho (Binat Tzvi p. 81; similar to Gra on S”A YD 108:8 that shaat hadachak is like bedieved).
      1. We can’t rely on rov tashmisho when there’s a beliya yadua (Binat Tzvi quoting Rav Elyashiv)
         1. The Maharam Chalavah Pesachim 30b s.v. vhilchata writes that following the main use of a utensil is only effective if we’re unsure if the utensil was ever used for the other type and we’re allowed not to be concerned for an abnormal use. This is also the opinion of Tosfot Chullin 8a s.v. shlibna, Meiri Pesachim 30b s.v. hasakinim, and Raah cited by Nemukei Yosef Pesachim 30b. However, the Ran Chidushim Pesachim 30b s.v. vhilchata absolutely holds that we follow the majority of uses even though it is certain that it was used for the other type of use. Ramban Avoda Zara 76a s.v. umah sh’amru and Rashba a”z 76a s.v. vkatav agree. This is also implied by Rabbenu Dovid Pesachim 30b s.v. vhilchata. Kaf Hachaim 451:100 follows that approach and cites many who agree including: Sharei Kneset Hagedola 451:6, Pri Chadash, Olot Tamid, Eliya Rabba 451:17, Chok Yakov 451:31, Bet Dovid 212, Gan Hamelech 53, Erev Hashulchan 451:11, Chemed Moshe 451:12, and Shulchan Aruch Harav 451:31.
      2. We can’t rely on rov tashmisho to exclude spills because spills are common (Binat Tzvi)
         1. Proof is Mishna Brurah 451:34 who says that a tripod which you spill on needs libun even though rov tashmisho is to hold pots and asur even bedieved if you did hagalah!
         2. Proof is Shulchan Aruch 451:20 who says that you need hagalah for table because things spill but rov tashmisho is for cold!
         3. Chut Shani rejects that answer differently. There the table doesn’t have any shimush with food besides spills. [It seems that you can answer the same for the tripod.] Binat Tzvi argues that they used to put cold food directly on the table more than hot food spilled on the table.
         4. Or Letzion rejects both of these answers.
4. How hot does libun chamur need?
   1. Rabbi Yitzchak Abadi (Or Yitzchak 1:302) writes that libun chamur is 350 and 400 at most.
   2. Everyone else assumes it is 800/900 degrees (Binat Tzvi p. 79).
5. Kbolo kach polto
   1. R’ Soloveitchik (cited by Rabbi Jachter, https://www.koltorah.org/halachah/can-one-kasher-a-conventional-oven-by-rabbi-howard-jachter), Arugot Habosem 119. Rav Sheinberg (Binat Tzvi p. 81 quoting Tzohar 15). Binat Tzvi p. 81 brings a proof from Rabbenu Yonah quoted in Tur and Shulchan Aruch 461:1 is a proof that even for libun we use kbolo kach polto. He quotes Yad Yehuda 97:14 who argues with this proof.
   2. Igrot Moshe YD 1:60, Pri Megadim E”A 451:30, Or Letzion 3:10:2, Minchat Yitzchak 3:66 (with respect to pesach) argue that libun needs to burn beliyot all the way so you can’t apply the principle of kbolo kach polto to figure out the specific temperature for the koshering. It is only relevant to hagalah.
6. Hetera baala
   1. Many rishonim hold that chametz is only hetera baala and therefore libun kal is enough. Yachava Daat 2:63 says that if there’s no other option you can rely on those rishonim to kasher oven with libun kal.
   2. Shulchan Aruch 451 and meforshim all assume chametz is isura baala even though there’s stirah to this in S”A 452.
7. How can libun kal work if there's an cheres lining of the oven?
   1. Rav Moshe Feinstein (Mesoret Moshe v. 1 p. 230) is quoted as saying that indeed this is an issue and it isn't possible to kasher an oven with porcelain lining. Rav Willig (April 1 2020, min 31-34) explained that the thin layer of porcelain isn't a problem and libun kal, by putting it up to the highest temperature it reaches, works for an oven for pesach.