Koshering a Kitchen
Utensils Used for Cold
- Any vessel that is usually used for cold can be koshered with cleaning it well with cold water. Ashkenazim initially are strict for the opinion that it was used for non-kosher cold it should be koshered with hagalah.
- A pot that is usually used for cooking and sometimes used without liquids, according to Sephardim can be koshered with hagalah since we follow the majority of its use. Initially Ashkenazim are concerned even for a minority of uses.
- Usually it is necessary to kosher a pot on the inside and the rim but if it is used on the outside of the pot such as a ladle it needs to be koshered on both sides.
- If one has a big pot and can't put it into another pot it should have a rim added to it and when the pot boils up the water will splash onto the rim and kosher it or one can boil a pot and drop a rock in so that the pot boils over the rim.
- If a pot is used on the inside but became non-kosher on the outside of the pot it needs to be koshered on both sides.
- If one is doing hagalah of iruy on a large item through pouring one should do so bottom up.
- It is possible to do Libun on Pesach but not hagalah.
- Libun Chamur can be accomplished with a minimum temperature of 752 degrees fahrenheit, because metal would become visibly red hot in the dark at that temperature. Self-clean of an typical oven is 850 degrees and is certainly libun chamur. 
- Libun Kal is certainly achieved at fahrenheit 451 because paper burns at that temperature.
- Libun chamur with a blow torch should be done on each spot for approximately 9 seconds. The could be dangerous and damage the oven thermostat.
Koshering a Knife
- A knife should be koshered with hagalah.
- The minhag is to have designated meat and milk knives. Preferably one should have three knives, one for meat, one for milk, and one for parve.
- It is forbidden to use a dirty meat knife to cut cheese or even bread which will be eaten with cheese.  The same is true vice versa.
Using a non-Kosher Knife
- It is permitted to use a non-kosher knife on a one-time basis for cold if you first stick it into hard earth ten times.  But in order to use the knife for hot one even once one needs to do a proper hechsher.
- Similarly, to use a meat knife to one time cut cold bread that will be eaten with cheese it is sufficient to stick it in hard earth ten times. However, in order to use a meat knife to cut cheese one should do a proper hechsher. 
- Some poskim say that if one doesn't have a knife, cleaning the knife with soap is considered the equivalent of sticking it in the ground ten times.
- If one wants to use on a consistent basis a meat knife for cold dairy one must do a hechsher of the knife.
Using Non-Kosher Utensils
- It is permitted to use a clean cold non-Kosher utensil to eat cold kosher food on an irregular basis for a one-time use. See footnote regarding earthenware. However, knives have another requirement that they first be stuck into the ground ten times. See section on using non-kosher knives.
- It is initially forbidden to place cold kosher food into a cold pot or container that was used for non-Kosher if the container wasn’t washed since the kosher food that goes into the container will have some non-kosher on it and one might forget to wash off the kosher food. If the kosher food is usually washed before being eaten it is permitted to initially place it in a cold pot used for non-kosher. 
- It is initially permitted to place kosher food into a cold pot or container that was used for non-kosher if the container was washed. However, one shouldn’t use non-kosher earthenware utensils even for cold.
- It is permitted to own a non-kosher utensil and not use it as there’s no concern that you’ll come to use it for a forbidden use.
- A frying pan that became non-kosher can only be koshered with libun chamur.
- A frying pan that was used for chametz can be koshered for pesach with libun kal or hagalah. See Kashering_the_Kitchen_for_Pesach#Pans for Ashkenazic minhag and fuller discussion.
- A frying pan that is milk and one wants to make it parve or the opposite, one can kosher it with hagalah or libun kal. However, if it is used for meat and milk it needs to be koshered with libun. If it is used for meat and 24 hours later is used for milk it can be koshered with hagalah or libun kal. The opposite is true of the opposite (unless it is used for a sharp food - Dvar Charif).
Not Switching Between Meat and Milk
- The minhag is not to switch over utensils from meat to milk except before Pesach when one is koshering the utensils for Pesach anyway. 
- Some say that it is permitted to kosher a utensil that used to be Parve and now became dairy to become parve again. 
- It is permitted to switch over utensils after 12 months have passed.
- Some have the practice that if one has a utensil which one wants to switch from meat to milk that one intentionally makes that utensil non-kosher and then it is fine to kosher it and use it for the other type.
- According to most Sephardim, glass utensils don't absorb any taste and therefore, do not become non-kosher, between meat and milk or chametz and pesach. However, the common practice is to have two sets of dishes, one for milk and one for meat.
- Shulchan Aruch YD 121:1. Rashba Teshuva 1:372, 817, 3:279, and Ran Pesachim 8b s.v. aval hold that we follow the majority of the uses of a utensil to determine how it should be koshered.
- Rama 451:25, Mishna Brurah 451:149
- Shulchan Aruch 451:6.
- The Gemara Pesachim 30b concludes that knives need hagalah for pesach even though it discussed possibly needing libun. What was the reasoning of the gemara? The Ran Chidushim 30b s.v. vhilchata writes that the knife is sometimes used over the fire yet it can be koshered with hagalah. Either because Rashi and Raavad hold that chametz is hetera baala. Or according to the Ramban that it is isura baala we only need hagalah because we follow the majority of uses of the knife. Meiri Pesachim 30b s.v. hasakinim agrees with the second approach.
- The Rashba responsa 1:372 writes that we don’t need to be concerned for a minority of uses because otherwise how could the Torah establish different categories of utensils that some are used for cold, some for hot liquids, and some directly with the fire. Rather we only concern ourselves with the way that the utensil is normally used. This is reiterated in 1:817 and 3:279 and Ran Pesachim 8b. The Bet Yosef understands that this is also the opinion of the Rif Pesachim 8b and Rambam Machalot Asurot 5.
- However, the Ravyah holds that we’re concerned even for a minority of uses of a utensil. Tosfot Avoda Zara 74b s.v. darash, Hagahot Maimoniyot (Hilchot Hagalah), and Tur 451:6 agree. Shulchan Aruch 451:6 follows the Rashba and Rama 451:6 is concerned for the Ravyah.
- How does Rov Tashmisho work? 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. How can that be explained?
- The Rashba responsa 1:372 writes that the reason that we can follow the majority of uses is even though it is known that it was used for another type of use but after 24 hours the absorptions are negative tasting and the entire need for koshering is rabbinic. The rabbis established to follow the majority of uses. This explanation is reiterated by the Rama Mpano 96.
- Chazon Ish OC 119:15 points out that according to this answer it can’t be used for a spice grinder since a dvar charif extracts tastes even though they weren’t used within 24 hours. However, the Mishna Brurah 451:80 seems to apply the rule of rov tashmisho even to a grinder. Chazon Ish answers that it is referring to a concern that it was used for chametz but we don’t know that it was ever used for chametz otherwise we couldn’t use the majority of uses.
- However, there is a small group of rishonim that imply this even for ben yomo utensils biblically. The Machzor Vitri Hilchot Pesach ch. 4 writes that the torah established the categories of koshered based on majority uses. Rav Aryeh Idnason in Haotzer v. 15 p. 189 explains that either the Machzor Vitri holds that taam kikar is derabbanan or that the Torah categorized utensils based on the majority.
- Rama 451:6, Mishna Brurah 451:45
- Magen Avraham 452:11 quoting the Maharil
- Shulchan Aruch 452:6
- Pitchei Teshuva YD 96:3
- Shoel Umeishiv 5:4 writes that if one is pouring hot water on a large item one should do the hagalah bottom up. He explains that if one does the opposite the zeyia from the bottom could rise and infuse non-kosher taste into the top that was already koshered.
- Rama 452:1
- Rabbi Ribiat in Halachos of Pesach p. 315
- Rabbi Ribiat in Halachos of Pesach p. 315
- Halachos of Pesach by Rabbi Ribiat p. 353
- Shulchan Aruch YD 121:7
- Rama 89:4
- Maharshal (Yam Shel Shlomo Chullin 8:8), Badei Hashulchan 89:111
- Rashba (responsa 1:76), Shulchan Aruch YD 89:4
- Rama YD 89:4
- The Gemara Avoda Zara 76b states that in order to kosher a non-kosher knife one should just stick it in the ground ten times. Tosfot 76b s.v. hasakin says that even though the Yerushalmi says three times one should be strict to stick it in the ground ten times. Tosfot chullin 8b s.v. vehilchata says that sticking it in the ground cleans the knife from non-kosher fat that got stuck to it. Shulchan Aruch YD 121:7 rules accordingly that in order to use a non-Kosher knife once for cold it should be stuck into earth ten times. The Rama carefully adds that if one wants to use it on a consistent basis one must do a proper hechsher.
- Rav Huna in Gemara Avoda Zara 76b, Shulchan Aruch YD 121:7. Shulchan Aruch explains that this procedure is sufficient even to cut a cold sharp food (such as an onion).
- The Rama YD 89:4 writes that to kosher a meat knife to be used for cold dairy it is sufficient to stick it in the ground ten times. The Taz 89:6 explains that if one is just going to cut bread for dairy use the knife just needs to be cleaned, however, for cheese it should be stuck in the ground ten times. However, the Shach 89:22 says that it should be stuck into the ground ten times even to cut bread that will be used for dairy. The Badei Hashulchan 89:108 is strict for the Shach and explains that it is forbidden to cut cheese with a meat knife unless one did a proper hechsher.
- Maadenei HaShulchan (M'taamei Hashulchan YD 89:17 p. 62)
- Badei Hashulchan 89:108 based on Rama YD 121:5
- *The Ran (Chullin 40b s.v. imlich) asks why a non-kosher earthenware utensil had to be broken if it could just have been used for cold uses for kosher food. He answers that it must be that there is a rabbinic prohibition not to use the earthenware utensil for cold because one might come to use it for hot. However, that's only true by earthenware which can't be fixed. On the other hand, the Mordechai (Pesachim no. 565) uses this logic to say that one shouldn't use any material non-kosher utensil for cold lest one come to use it for hot. The Rama YD 121:5 rules that on an irregular basis one may use non-kosher utensils for cold kosher food but not consistently to be concerned for the opinion of the Mordechai. This is also the opinion of Shulchan Aruch YD 94:3, Badei Hashulchan 91:15, and Kaf Hachaim 94:40.
- Chelkat Binyamin 121:42 says based on the Pri Chadash that one shouldn't use earthenware utensils even for a one-time use unless it is a non-Jew's house and there's no possibility to do a hechsher.
- Shach 121:9
- Shulchan Aruch YD 91:2 based on the Baal HaItur and Tur
- Shach 91:3, Badei Hashulchan 91:15, Kaf HaChaim 91:5. Is there a concern of beliyot when using a utensil for cold?
- The Orchot Chaim states that it is permitted to store spices in a non-kosher utensil since there are no beliyot of non-kosher being transferred by storing the spices in the utensil. The Bet Yosef 105 cites this. The Tur 451 permits storing cold matzah in a chametz utensil.
- However, the Tur 91:2 cites the Baal Haitur who writes that it is forbidden to place meat in a dairy utensil since one might forget to wash it off afterwards.
- Question: The Rama (Torat Chatat 17:2 and Darkei Moshe 91) is bothered with this contradiction. He answers 4 answers:
- We are concerned with meat since it is moist but not concerned about spices or matzah which are dry.
- We are concerned for other prohibitions but not chametz.
- We are concerned when the utensil was originally used for hot non-kosher but not if it was used for cold non-kosher (or meat in pot that was originally used for cold dairy).
- We are concerned when the utensil wasn't cleaned well.
- Rama 91:2 implies that he accepts answers 1 and 3 and requires both but Taz 91:3 explains that either is sufficient. Shach 91:3 only accepts the fourth answer.
- Shach 91:3, Pri Chadash 91:3, Badei Hashulchan 91:15. Pri Chadash 91:3 explains that the reason to be strict is that we're concerned that a person is going to use it for hot. However, for a metal utensil we're not concerned for a short usage that one will use it with hot since one would first do hagalah. See Kaf HaChaim 91:10 who permits using non-kosher earthenware utensils that belong to a non-Jew for cold.
- Kaf Hachaim 91:9. See Chashukei Chemed Yoma 66a who cites the Panim Meirot 1:23 who says that there's no concern that if one owns a non-kosher utensil one will come to use it. However, the Ketav Sofer YD 28 holds that it is a concern. It is similar to the Gemara Yoma 66a and Pesachim 20b where chazal are concerned about holding onto something forbidden because you might use it.
- The Rosh (Pesachim Kol Shaah 7) records a dispute between the Ravyah and his grandfather the Raavan whether a frying pan needs libun. The Raavan held it needed libun and is comparable to baking but the Ravyah held it needed hagalah and is comparabale to cooking. The Rosh comments that he agrees with the Ravyah since the oil serves to intervene between the food and the pot. The Shulchan Aruch YD 121:4 is strict like the Raavan.
- Shulchan Aruch YD 121:4 writes that even though for other isurim a frying pan needs libun chamur, for koshering from chametz to pesach it only needs hagalah. The Biur Hagra YD 121:9 explains that the Shulchan Aruch really holds like the Rosh that a frying pan only needs hagalah, however in general we're strict to require libun chamur. Yet, for pesach since anyway some hold that chametz is hetera baala and certainly hagalah is sufficient for this case we can rely upon that opinion. Yabia Omer YD 10:58:18 and Yalkut Yosef YD 121:3 agree.
- The Shulchan Aruch YD 121:4 writes that one can kosher a frying pan from meat to milk or parve with hagalah since absorbing meat or milk are permitted and since it is hetera baala (a permitted absorption) it can be removed with hagalah (Avoda Zara 75a). The Shach YD 121:8 quotes the Rama Mpano 96 who says that a frying pan used for non-kosher needs libun since the non-kosher came in contact with the frying pan itself unlike chametz which is cooked in the frying pan with a non-chametz liquid such as water or oil. The Shach concludes that the Rama Mpano implies that if one used a frying pan for milk or meat it would need libun to be koshered. However, Rabbi Akiva Eiger 121:1 asks that either way milk and meat separately are permitted and as such should never require libun. Chelkat Binyamin 121:4 s.v. linyan explains that in fact the Rama Mpano never said that a meat or milk pan needed libun. He only meant a frying pan that was used for meat and milk needs libun. He adds that this could also be the intent of the Shach.
- Shulchan Aruch YD 121:4, Shach 121:8
- Chatom Sofer YD 110 cited by Pitchei Teshuva 121:7
- Magen Avraham 509:11 writes that the minhag is not to switch over utensils from meat to milk since one might come to make a mistake and forget whether currently it is meat or meat. Pri Megadim E"A 509:11 seems to say that the minhag is to make a utensil non-kosher so that it needs to be koshered and then switch it over from meat to milk. Pri Megadim E"A 451:30 writes that when koshering utensils for Pesach it is permitted to switch them over from meat to milk. Chatom Sofer YD 110 (cited by Pitchei Teshuva YD 121:7) and Badei Hashulchan 89:112 agree. However, the Aruch Hashulchan 89:17 argues with the Magen Avraham that we can't invent gezerot nowadays and there's no issue to kosher from meat to milk.
- Maharsham (responsa 2:241) explains that there was never a minhag in such a case to be strict not to change it over and also there's other factors to be lenient. Badei Hashulchan 89:4 s.v. shnei questions this maharsham.
- Badei Hashulchan 89:4 s.v. shnei citing Maharsham 2:241
- Pri Megadim EA 509:11 cited by Badei Hashulchan 89:4 s.v. shnei. He questions the Pri Megadim since one might forget to do the koshering altogether.
- Rabbi Mansour on dailyhalacha.com writes that Syrains are lenient but still have two sets of dishes. Yalkut Yosef (Kitzur S"A 451:39) writes that most Sephardim are lenient but some Persians are strict about this for Pesach but not milk and meat.