Kashering a Microwave and General Kashering Summary

Kashering a Microwave for meat/dairy/Pesach:[1]
Does it need to be Kashered? Some Rabbanim[2] say that a microwave does not need to be Kashered at all prior to use for meat/milk/Pesach, and simply needs to be wiped clean.[3] Accordingly, it may be used intermittently for meat and then dairy and Pesach, after cleaning it. The majority consensus of Poskim[4] however is that a microwave requires Kashering, and thus cannot be used with meat/dairy/Pesach in its un-Kashered state.[5] Practically, one must abide by the latter opinion, as the former opinion does not take all factors into account and is hence inaccurate.[6] According to all, a combination microwave oven, which is both an oven and a microwave, requires Kashering. We will now discuss if and how a microwave can be Kashered:
How to Kasher it: Some Poskim[7] rule that it is not possible to Kasher a microwave.[8] Other Poskim[9] rule it can be Kashered through steaming water inside it.[10] This is accomplished through the following steps: The microwave is to be cleaned, not used for 24 hours[11], and have water with bleach/soap placed in it and heated for about 10-20 minutes[12], until it steams out.[13] Practically, one is to purchase a new microwave for Pesach.[14] If this is not possible, and one is in need to use the microwave, then one is to do as stated below, and use a hermetically sealed container to cook the food, as stated next.
Cooking in a hermetically sealed container:[15] In all cases, it is permitted to cook in a microwave food [meat/dairy/Pesach] that is contained within a hermetically sealed container which does not allow any vapor to escape or enter. This applies even if the microwave has not been Kashered, and certainly if it has been Kashered in the method mentioned above.[16] Some Poskim[17] however discourage using the microwave in this method as it occurs that the hermetic sealing tears or opens during the cooking, which would then pose a Kashrus issue. It is therefore best to only use the microwave in this method after first Kashering it.

Kashering:
The laws of Kashering vessels are complex and hence it should only be done by one which is expert in these laws.
One must be aware of the following details:

1. Is the vessel made of a Kasherable material?
2. How has the vessel been used? What form of Kashering is required for it? Libun? Hagalah?
3. What must one do if the vessel is rusty?
4. How does one do Libun?
5. How does one do Hagalah?
6. May the vessel be Ben Yomo?
7. Must the vessel one is Kashering in be itself Kashered?
8. How long must the vessels remain in the water by Hagalah?
9. May many vessels be Kashered simultaneously?
10. May one Kasher on Erev Pesach or Pesach?

A. What to do with Chametz vessels that one does not plan to Kasher:
Cleaning: All Chametz vessels which one does not desire to Kasher for Pesach, or is unable to Kasher them, are to be cleaned from Chametz. One is to scrub them and slightly rinse them down from any recognizable Chametz.
Putting them away: One is to hide the vessels in an area which he is not accustomed to enter into throughout the entire Pesach. Furthermore, it is proper to place the vessels in a room [or closet] which will be locked, and then hide the keys, in order to prevent any possibility of entering there during Pesach. Those which are accustomed to place the vessels in a very high area which is visible, have upon what to rely, although one who is stringent to hide them away from sight will be blessed.
Un-cleanable Chametz vessels: All vessels that are difficult to clean from Chametz are to be sold to a gentile and stored away as written above.

B. Buying new vessels:
New vessels which are bought from a gentile do not need to be Kashered. Hence all vessels which appear new may be bought from a gentile store or company. However vessels sold by a gentile individual from his home are not to be purchased.

Do new pots and pans need to be Kashered today due to suspicion of them having been smeared with non-Kosher fats?
Many Poskim rule that there is no need to Kasher new pots or aluminum which had non-kosher fats smeared on them during manufacturing. However some are accustomed to be stringent. Practically the worldwide custom is to not require them to be Kashered. However in certain areas, such as Eretz Yisrael, many are accustomed to do so.

C. What form of Kashering does a vessel require-General rules?
*The below list only refers to the form of Kashering required for vessels made of Kasherbale materials. For a list of those materials that are Kasherable and those materials that are not Kasherable-see Halacha E! For a list of vessels and their specific Kashering Laws see Halacha F!
Chametz cooked in liquid in the vessel: All Kasherable material vessels that had Chametz cooked with liquids require Hagalah or Libun Kal.
Chametz baked without liquid in the vessel: All Kasherable material vessels that had Chametz baked in them [without liquids] require Libun Chamur.
Chametz soaked in the vessel: All Kasherable vessels which had Chametz soaked in liquid for 24 hours require Hagalah or Libun Kal.
Do pots which are used to cook in have the status of absorbing food through liquid, or the status of absorbing food directly? All pots that had food cooked inside with liquid have the status of absorbing food through liquid, even if the food burnt inside the pot, and thus it does not require Libun Chamur, as there is always some liquid found on the bottom of the pot.

D. What materials may be Kashered?
Earthenware pottery: Can only be koshered through placing it in an oven and heating it to the point that it can be reformed. Even if an earthenware dish was used for only cold Chametz, one should not place even cold Pesach foods on it. An earthenware oven can be Kashered through Libun Gamur. Examples of earthenware vessels that cannot be Kashered: Crockpot; Mugs.
Sundried clay vessels: Vessels made of sun dried, is Kasherable
Wood vessels: Is Kasherable so long as it does not contain cracks and the like. The custom is to Lechatchilah never use any wooden vessels which were used for flour consistently, even if one cleaned it and performed Hagalah. All wooden vessels may be sanded down and koshered.
Metal vessels: Are Kasherable. If the vessel absorbed the food through cooking in water, then it suffices for it to be heated to the point that its outside reaches the point of “Yad Soledes Bo”. This can be done by either using a torch [Libun Kal] or dipping it in boiling water [Hagalah]. If the vessel absorbed the food directly, without any liquid then it requires “Libun Gamur, which means that it must be heated until sparks begin to fly off from it, or until a layer of it peels off. This applies even Bedieved.
Glass vessels: The custom amongst Ashkenazic Jewry is not to Kasher for Pesach any glass vessels which have suspicion they may have absorbed Chametz. These vessels are not to be used for Pesach, and are rather to be put away with the Chametz vessels. [Sefardim however are lenient to allow using glass vessels even without Hagalah, so long as they have been washed and cleaned.]
Glass coated vessels: If the vessel is coated with glass on its inside, in the area where the food is placed, then it may not be Kashered. If it is coated with glass only on its outside, then if it is never commonly placed directly over a fire to cook in, such as silver vessels coated externally with glass, then it may be Kashered through Hagallah. If however it is not uncommon to use it to cook with over a fire, or even to occasionally heat food in it over a fire, then it may not be Kashered.
A vessel placed together using glue: Hagalah is invalid for such a vessel as the heat can easily ruin it and there is thus suspicion that to prevent this one will not heat the water enough for the Hagalah.
See the chart below for all of the following materials: Ceramic; Enamel; Marble; Plastic; Porcelain; Pyrex; Teflon.

E. Practical list of items:
List of vessels and their Kashering status

Pots, Cutlery and Kitchenware

Forks/Spoons:
All forks, spoons and other cutlery made of Kasherable material, such as silver or stainless steel, is to be Kashered through Hagalah.

Knives:
It is a Mitzvah Min Hamuvchar for all those which have the capability of doing so, to buy new knives for Pesach. However from the letter of the law doing Hagalah to them does suffice [and one may certainly rely on this if it is not so feasible for him to get new knives.] One must sharpen the blade prior to doing Hagalah in order to remove any rust he blade may contain. If the knife contains a handle and the blade is inserted into the handle then it cannot be koshered due to the inability to remove any Chametz from in between the crevices. Likewise if the blade is attached to the handle with glue it cannot be Kashered.

Cups:
The custom is to Kasher cups through Hagalah. If one used the cup for a hot Pesach drink without previously Kashering it, the drink remains Kosher.
Glass cups: Are not Kasherable.
May one Kasher a Kiddush cup that contains an upper lip? Yes, as the lip is external and there is thus no worry that Chametz entered inside.

Pots:
Whether or not a pot may be Kashered is dependent on the material that it contains of-See previous Halacha E! If the pot is made of a Kasherable material, such as metal without a Teflon coating, then if it is used for cooking with liquid it requires Hagalah.
Teflon/Enamel: If coated with enamel/Teflon then it may not be Kashered.
Do the handles of pots and pans need to be Kashered? From the letter of the law they do not need to be Kashered and certainly one need not worry of the cracks that they contain [which may have food on them]. Nevertheless one should clean it and do Hagalah to it, or do Iruiy Keli Rishon without needing a stone.
Do pot covers need Hagalah? Yes.

Frying pans and all pots used for frying:
If one used this pot to fry the food with a nice amount of oil, than by Kashering for Pesach the pan needs Hagalah. If however one fried food in it using very little oil, just enough so the food does not stick, then the pot needs Libun.
Teflon/Enamel: If coated with enamel/Teflon then it may not be Kashered.

Appliances and furniture

The stove:
The grates: Need Libun Chamur. If one cannot do Libbun Chamor, then clean very well and wrap the grates in thick aluminum that will last throughout Pesach.
Burners: Clean the burners and use toothpicks or needles to remove any dirt or food from within the gas holes of the burner. Afterwards, turn on the fire for some time to accomplish Libun Kal.
Stove top surface: One is to clean the stove surface well and then do Iruiy Keli Rishon to it. If the surface is made of non-Kasherable material, such as enamel, that it must be covered with aluminum.
Knobs: One is to clean the stove knobs very well, cover them or attach clean replacement knobs.
Covering all items: Practically the custom is that even after Kashering all the above items of the stove, one covers all the surfaces with aluminum.

Oven:
From the letter of the law it requires Libun Chamur. If ones oven does not have self clean oven it is very difficult to accomplish Libun Chamur through using a blow torch, as the oven can break in the process. One is thus to buy a Pesach oven or alternatively Kasher it in the following way:

1. Clean the oven well using a Chametz killing agency such as bleach or oven stain remover.
2. Wait 24 hours prior to Kashering.
3. Turn the oven on for a period of at least one hour to its highest temperature or blow torch the oven from the inside.
4. After the Kashering process is complete one should cover the walls and floors with aluminum foil.

Self cleaning oven:
An oven with self cleaning mode reaches a temperature of 900° F and is equivalent to Libun Chamur. An oven with a “Continuous cleaning” cycle is not equivalent to Libbun Chamur, and hence the above mentioned method must be used.

Counters/tables:
If one is accustomed to place hot pots on his counter or table then the custom is to do Iruiy Keli Rishon with Even Meluban. However other surfaces on which one is not accustomed to place hot Chametz pots do not need to be koshered.
Covering the counter or table: If one covers his table or counter then from the letter of the law it does not need to be Kashered. Likewise if the counter or table has been Kashered it does not need to be covered. However the custom is to do both, to Kasher and cover the areas.
The walls of the counter: They are to have Iruiy Keli Rishon performed and then covered.

Must one cover all kitchen surfaces such as tables, counters, cabinets, refrigerator shelves and the like?
From the letter of the law, once these areas have been properly cleaned and Kashered they may be used for all foods without any cover. However some Poskim rule that one is to cover the surfaces even after they are Kashered due to suspicion that perhaps they still contain actual Chametz that was not properly removed. Practically the widespread custom is to cover all items that contact food even after they have been cleaned and Kashered.

Sink:
The actual sink: The Kashering of a sink is dependent on the material that it is made of. A metal sink can be Kashered through Iruiy Keli Rishon. Most sinks are made of Porcelain or enamel which are non-Kasherable materials and thus cannot be Kashered. Nevertheless, the custom is to do Iruiy Keli Rishon on such material sinks. One is then to insert a sink insert which will be used throughout Pesach.
The spout: The custom is to clean and wash the spouts of the sink as throughout the year they have been used with hands that are dirty from Chametz. [One is to pour boiling water of the spout, and leave it open with the hot water running.]
Knobs: Wash and clean.
The drain: Pour boiling water that contains bleach or Drano down the drain.
Metal strainer: Iruiy Keli Rishon.
Using the hot water on Pesach: It is advised not to use hot water that is over Yad Soledes [110° F] on Pesach, in a sink that is not Kasherable, as one can possibly Treif up the vessels in the sink through doing so. Thus one should not turn on the hot water to the point of Yad Soledes and is likewise not to pour hot water into the sink. If the water is warm, below Yad Soledes, it may be poured into the sink. Likewise if the hot water of a pot was placed in a Keli Sheiyni, it may be poured into the sink even if it is still very hot.

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[1] See Piskeiy Tehsuvos 451/22; Sefer Hakashrus [Fuchs] 1/47-50 [pp. 48-49]; Nitei Gavriel 80/16; See also Hadarom Choveret 6 Nissan 5722; Kovetz Beis Ahron Yisrael 4/3

How does a microwave cook? A microwave is a rapid cooking element, which can warm and cook food much quicker than traditional cooking methods. Now, how does the microwave achieve its rapid cooking? The microwave does not use the heat of a fire or electricity to cook but rather cooks the food using radiation, or radio electromagnetic waves, which is projected from a vacuum tube and bounced off the metal lined walls of the microwave which penetrate the food from all sides. These waves hasten the movement of the water molecules in the food to atomic levels hence generating heat. [Heat is generated from movement and friction.] This form of cooking cooks the food much quicker than fire or electricity, as the radioactive waves hits the food equally in all areas and furthermore, penetrates the inside of the food molecules hence making the entire mass of the food an equal recipient of the heat. This is unlike fire or electric cooking which heats the external part of the food, and that heat then must travel to the inner part of the food in order to cook it. Likewise, this form of cooking only heats the actual food, as it does not actually send heat to the food but causes the food to heat itself up. Accordingly, all other areas and items of the microwave might remain cold, including the walls and certain plastic or glass containers which cover the food. The only way these items will become hot is if they are in contact with the food itself. The radio waves harmlessly pass through these containers into the food and do not cause any heating within them being they do not contain water molecules or other polar charge component. [See Hakashrus ibid footnote 100; See here for an educational video on how a microwave works. https://www.youtube.com/watch?v=kp33ZprO0Ck]

[2] Rav Yitzchak Yosef

[3] The reason: As the actual walls of the microwave do not heat up at all, and only the food itself heats up.

[4] See all Gedolei HaPoskim mentioned in Hakashrus ibid and Piskeiy Teshuvos ibid, which include Rav Wozner, Rav Elyashiv, Rav Sheinberg, Rav Halbershtam; Nitei Gavriel ibid footnote 26 in name of Rav Neiman of Montreal;

[5] The reason: a) Although the walls of the microwave do not heat, nevertheless the steam and spills of the food inside make the walls absorb the food and hence it must be Kashered. [Piskeiy Teshuvos ibid; See Admur 451/41, Michaber 451/14, and M”B 451/81 that a vessel which absorbed the steam of an Issur requires Hagala] b) As there is a vent duct in the microwave that contains actual steam of food, and that area is not Kasherable. [Rav Neiman ibid

[6] As although the walls don’t heat up the microwave receives steam from the foods and hence must be Kashered.

[7] See Piskeiy Teshuvos ibid; Siddur Pesach Kehilchaso that 8/3 that it can only be Kashered through Iruiy Keli Rishon and Even Meluban

[8] The reason: a) Although the walls of the microwave do not heat, nevertheless the steam and spills of the food inside make the walls absorb the food and hence it must be Kashered. Now, it is not possible to Kasher the microwave as one cannot blow torch it, place it in boiling water, and many Poskim rule an item cannot be Kashered through steaming water inside it. [See Shoel Umeishiv Telisa 3/125; Sdei Chemed Mareches Hei 24; Chmaetz Umatzah 17/12 that it is not possible to Kasher through vapor] It therefore has no viable path for Kashering. A second reason is b) As there is a vent duct in the microwave that contains actual steam of food, and that area is not cleanable or Kasherable. [Rav Neiman ibid]

[9] Teshuvos Vehanhagos 2/212; Yalkut Yosef Pesach p. 360; Piskeiy Teshuvos ibid regarding Shaas Hadechak

[10] The reason: As we rule that it is possible to Kasher an item through steam, and just like the microwave absorbed the food through steam, so too it can be Kashered through steam. [See Peri Chadash 121; P”M  Y.D. 94 M”Z 1; Tevuos Hasadeh 3/3]

[11] See Hakashrus ibis footnote 105 that so ruled Rav Wozner, Rav Shternbuch and Rav Mordechai Eliyahu

[12] Piskeiy Teshuvos ibid writes to leave it on for one hour; Sefer Hakashrus ibid writes [based on Mitbach Kehalacha p. 58 and Techumin 8/21] to enter a half a liter of water and leave the microwave on until it steams out; Rav Yitzchak Yosef says to leave it on for six minutes

[13] Teshuvos Vehanhagos 2/212; Koveitz Mibeis Levi 3/22; 7/25; Yalkut Yosef Pesach p. 360; Piskeiy Teshuvos ibid; Hakashrus ibid

[14] Piskeiy Teshuvos ibid; Hakashrus ibid footnote 105; Nitei Gavriel ibid concludes that due to the many opinions, one is not to use it for Pesach without directive from a Rav

[15] Hakashrus ibid; Nitei Gavriel ibid in name of Rav Neiman

[16] Pischeiy Halacha Kashrus p. 28; Piskeiy Teshuvos ibid footnote 106

[17] See Kovetz Mibeis Levi 3/22-9; Hadarom ibid; Beis Ahron Veyisrael ibid