May one Kosher a meat vessel to use for dairy or vice versa?
Through Libun: It is permitted to Kosher a vessel through Libun, and interchange its use between meat and dairy. Thus, one may Kosher a meat oven to use for dairy and vice versa. [All vessels made of earthenware, or other non-Kosherable materials, are only valid if they are Koshered through Libun or a firing oven known as a kiln, even when Koshering from meat to dairy or vice versa. However, vessels made from Kosherable materials, only require Hagala when Koshering from meat to dairy or vice versa, and this applies even if they absorbed meat or dairy without liquid. Nonetheless, using the method of Hagala to switch uses remains restricted, as explained next, while doing Libun to the vessel would be allowed even initially.]
Through Hagala: Some Poskim rule that the custom of the world is to avoid Koshering an item through Hagala for the sake of switching its use from meat to milk vessels, or vice versa. Other Poskim, however, rule that it is completely permitted to Kosher a vessel from meat to dairy or vice versa. Practically, the Ashkenazi custom is to be stringent not to do so, unless one of the following exceptions apply: [The above is only initially followed. However, Bedieved, everyone agrees that if it was already Koshered and used for the opposite food, one may continue with its current use.]
The exceptions for Hagala:
- Time of need: In a time of need, one may Kosher a vessel from meat to milk, or vice versa.
- Twelve months or does not remember status: If twelve months have passed since the vessels last use, then it may be Koshered from meat to dairy, or vice versa. [Likewise, if the vessels have not been used for a very long time, as often happens with items left in storage, and one no longer remembers the status of the vessel, one may Kosher them and designate them for whatever use one desires.]
- Koshering for Pesach: When Koshering meat or dairy Chametz vessels for Pesach use, one may now choose to designate them for the opposite food, whether dairy or meat.
- Koshering from Treif: If a vessel became Treif, after Koshering it, it is permitted to designate it for whatever use one desires. Furthermore, it is even initially permitted to Treif up a vessel in order to be able to then Kosher it and switch its use from meat to milk, or vice versa. [One may prohibit the vessel through cooking some of the opposite food in it while the vessel is not Ben Yomo. However, it is Biblically forbidden to cook meat or milk in it while the pot is Ben Yomo from the opposite use, unless the meat is poultry.]
- Koshering to Pareve: It is permitted to Kosher a meat or dairy vessel to Pareve. Furthermore, one may then later choose to designate this Koshered vessel for milk/meat. Thus, if after Koshering the vessel, one first uses it for Pareve for some time, it is then permitted to designate it for meat or dairy.
- Received from another: If one received a meat or dairy vessel as a present, or purchased a meat or dairy vessel from another, it is permitted to Kosher it prior to its initial use, if one desires to use it for the opposite food. Thus, if one inherited his mothers’ pots, he can decide to Kosher the pots and then designate them for whatever use he sees fit.
 See Hakashrus 2:5; 3:5; Yabia Omer Y.D. 3:4; 4:5
 Admur 494:16; 509:11; Michaber 509; [There they rule that one may Kosher through Libun a dairy skewer in order to use it for meat]; Magen Avraham 509:11; Shach Y.D. 95:9 in name of Tur; Shaar Hamelech Yom Tov 4:8; Hakashrus 3:5
Other Opinions: Some Poskim rule that one is to be stringent to avoid Koshering vessels even through Libun for the sake of interchanging between dairy and meat uses, just as we are stringent to avoid Hagala, as will be explained next. [P”M 451 A”A 30; Shearim Hametzuyanim Behalacha 56]
 Admur 494:16
 Biblically, Keli Cheres is Kashered through Libun Chamur/Gamur, if the Libun is done to the inside. However, Rabbinically, earthenware vessels that are not filled with fire on the inside during cooking require a firing Kiln oven and Libun alone does not suffice. Accordingly, typical a earthenware pot need a firing oven, while an earthenware oven needs mere Libun. [See Admur 451:7-9]
 Admur 494:16; and 461:1; 451:7 regarding Chametz
 Admur 451:13 in gloss; 1st approach in Shach 121:7; unlike 2nd approach in Shach ibid, based on ruling of Ramaz, 96 that requires Libun Chamur even by Basar Bechalav if it absorbed meat or dairy directly without liquid
 Magen Avraham 509:11 [omitted from Admur ibid]; P”M 452 A”A 13; 509 A”A 11; Kitzur Shulchan Aruch 56:15; Chasam Sofer YD 110; Maharsham 2:241; M”B 451:19; 509:25; Shearim Hametzuyanim 46
Background: In chapter 509 the Michaber and Admur ibid rule that one may Kosher through Libun a dairy skewer in order to use it for meat. As well, the Shach in Yoreh Deah 95:9 brings the Tur who rules that it is permitted. The Magen Avraham [509:11], however, writes that this only applies to Libun, however, regarding Hagala, the world is accustomed to avoiding switching meat and milk vessels through Hagala. The Poskim ibid rule like the Magen Avraham.
The reason: If it were to be allowed to Kosher and switch vessels from meat to dairy and vice versa, doing so can lead to confusion as to the status of the vessels and one may come to use a dairy vessel for meat or vice versa. [M”A ibid in name of Hagaon Mipuzna]
 Shach Yoreh Deah 89:22 95:9; 121:16; Shut Harosh 20:25; Aruch Hashulchan 89:17; 121:11 that this is a mere stringency and we cannot make up decrees on our own; See Shearim Hametzuyanim 56
 Hakashrus ibid
The Sephardic custom: The Sephardic custom is not to be particular against Koshering and switching meat vessels for dairy use and vice versa. [Peri Chadash Y.D. 97:1; Machazik Bracha 509:2; See Yabia Omer Y.D. 3:4; 4:5; Hakashrus ibid]
 Sefer Hakashrus 3:5 [p. 83]
 Maharsham 2:241; Shearim Hametzuyanim 56
 P”M 452 A”A 13
 Maharsham 2:241; Darkei Teshuvah 122:49; Shearim Hametzuyanim 56 in name of Maharsham ibid and Daas Kedoshim; Hakashrus ibid
 See Halacha D for the significance of the passing of 12 months.
 Sheileiy Tziyon Tinyana Y.D. 32; Shevet Hakehasi 1:225; Hakashrus ibid footnote 17
 Chasam Sofer ibid; M”B 451:19; See Shraga Hameir 6:22
 P”M 509 A”A 11; Maharsham 2:241; M”B 509:25; Beir Moshe 3:105
 See Hakashrus ibid
 Maharsham 2:241; Daas Kedoshim; Darkeiy Teshuvah 121:59; Shearim Hametzuyanim 56; Tzitz Eliezer 9:38
 Beir Moshe 3:105; Rivivos Efraim 5:519; Hakashrus ibid; See Lechem Hapanim Y.D. 121 in name of Shlah