Koshering a Kitchen
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Koshering a Knife
- A knife should be koshered with hagalah.
- The minhag is to have a designated meat and milk knife. 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.
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 121:7
- 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.
- 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. Badei Hashulchan 89:112 agrees. However, the Aruch Hashulchan 89:17 argues with the Magen Avraham that we can't invent gezerot nowadays.
- 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.