May one be lenient this year to Kasher his Keilim that need Haggalah through heating them up in an oven, or over a fire?
Some Poskim rule that heating the inside of a vessel to Yad Soledes through it sitting over a fire or inside of an oven is considered as if one has performed Libun Kal, or Hagalah to the item, and hence all items that require Hagalah can be Kashered through placing them over a fire, or inside a clean oven [i.e. no Chametz], until they reach the state of Yad Soledes. Other Poskim, however, rule that it is invalid to perform Libun through heating up an item from the outside, or from under, and rather the fire must be inside the actual pot, in the area that the cooking took place, and so is the implied opinion of Admur. Practically, one may not be lenient to Kasher vessels that require Kashering according to the letter of the law, through placing them over a fire or inside a hot oven, and rather one is to perform Haggalah to them. However, vessels that only require Kashering due to a stringency, such as new vessels, may be “Koshered” through placing them on a fire without any food inside until it heats up to Yad Soledes from the inside.
The steps for doing Haggalah to your vessels-Hagala checklist:
See here for the full details of this matter!
- Review Hagala laws and verify if the vessel is Kasherable through Hagala.
- Do not use the vessels with hot Chametz, or soaked Chametz, for 24 hours before Kashering.
- Clean the vessel with scrub and water. Remove rust and clean cracks, and do Libun Kal to required areas. Some say this should be done before it waits 24 hours.
- Dry the vessel well.
- Kasher the Hagala vessel [if was used for Chametz] during the year, prior to placing the Hagala water inside. Kasher it by boiling water inside of it to the very top, and then drop inside of it a red hot metal wrench, or hammer, causing the boiling water to overflow.
- Place clean water into the Hagala pot without any other substance, and bring to a boil. [Nonetheless, some are accustomed to place a spoiling agent, such as soap, into the water prior to Hagala, in order to make the water Pagum.]
- Prepare a second pot with cold water.
- Insert the vessels to be Kashered into the boiling water. Only insert while boiling [i.e. 100 degrees Celsius].
- One can use heat resistant gloves, or pliers, to enter the vessels one at a time into the water. One is to make sure to move the hands/pliers to a different area of the vessel while in the water. Alternatively, one is to use a perforated sack or basket to dip it in. This is the preferred method. One may insert many vessels at the same time. One may insert meat and dairy utensils at the same time if not Ben Yomo. One may not insert many vessels in a sack or box if they are touching each other.
- By a public Hagala, it is best to have two pots, one for meat and another for dairy vessels, or to never immerse more than one vessel at a time, or to make sure that all the vessels being immersed at one time are only meat or dairy, or are not Ben Yomo.
- The vessel is to remain a few moments in the boiling water.
- Immediately after removing the Kashered vessel from the boiling water, place it in cold water.
- Kasher the Hagala vessel if one wants to use for Pesach.
- Spill out the Hagala water to the drain.
 See Avnei Nezer Y.D. 110 regarding Libun Chamur that it helps for metal vessels even from the outside; Magen Halef 451:3 that metal is different than Keli Cheres; Maglei Eliyahu [shlezinger] 1:3; Mikraeiy Kodesh 80:7; Rabbanut Hareishit Leyisrael for year 5780 due to the Coronavirus as signed by the Chief Rabbi’s Rav Yitzchak Yosef and Rav Dovid Lau
 The reason: As Libun effectively burns the taste of the food absorbed in a metal vessel, and hence so long as the vessel reaches Yad Soledes on both sides it is valid irrelevant of where the source of the heat derived from, and only by earthenware are we stringent that the heat be from its inside being that its absorbance of taste is much stronger than in metal. Furthermore, just as the pot absorbed the taste through an external fire, so too it can expel it through an external fire. [See Poskim ibid]
 P”M 451 A”A 10; Beis Shlomo Y.D. 164; Imrei Yosher 2:144; Rav SZ”A in Minchas Shlomo 2:67 and Halichos Shlomo Nissan 3:1; Siddur Pesach Kehilchaso 5:3 footnote 10 in great length from the Poskim; Piskeiy Teshuvos 451:3 that this form of Koshering is completely worthless; Rav Yehuda Henkin in Noam 20:169
 The reason: As Kebolo Kach Polto, and hence just as the vessel absorbed the taste through having food cooked inside of it, so too it can only expel all its taste through having the fire placed inside of it. [See Admur 451:8; 11; 38];
 Implication of Admur 451:10, 16, and 44 who emphasizes the placing of the coals inside the vessels and so can also be implied from Admur 451:70 who requires the melted led to reach all areas of the pot from inside and does not suffice with the burner on the outside. So rules Admur regarding doing Libun Gamur to earthenware vessels that it must be done from the inside, with the fire inside the vessel: 451:8 and 11 and 65; So also rules Admur regarding doing Hagallah to metal vessels that it must be done from the inside, with the water inside the vessel: 451:38 and 452:2; So also rules Admur regarding earthenware that the coals must touch all the surfaces of the inside of the vessel: 461:1; 494:16 in parentheses [See regarding the rule of Cham Miktzaso Cham Kulo: Michaber and Rama 94:1 and 98:4]
Possible contradictions in Admur: See Admur 451:38 that doing Libun over a metal patch helps, however in truth one can argue that one cannot compare a patch to the actual bottom of a vessel. See also Admur 451:70 that heating up the pot from under it in order to coat its inside with led is valid as Libun Kal, although in truth there it is valid because the led inside serves as the fire material, and on the contrary, Admur explicitly requires the led to reach all areas of the pot inside. See also 509:11 where Admur allows doing Libun to a dairy pot on Yom Tov through leaving it in the oven, which implies that external heat suffices, although in truth one can argue that it refers to one who turns over the pot over the coals, and hence the coals are inside. Alternatively, we are more lenient by “Heter Bala” or by Libun Kal”
 Rav SZ”A ibid