Kashering Electrical Appliances

  • Harav Baruch Gigi

Translated and adapted by Rav Eliezer Kwass

Originally appeared in Alon Shvut Bogrim, vol. 2, pp. 75-78


"Thirty days before Pesach we begin to inquire about and publicly discuss the laws of Pesach" (Pesachim 6a).


"Kashering," a process that renders a utensil fit for use ("kosher") by removing material that has been absorbed in it, has its roots in Bemidbar 31:21-24. Elazar, the Kohen Gadol, instructs the people how to treat the utensils that they acquired as booty in the war against Midian. "Anything that was used with fire should be passed through fire to be purified, but [then] needs to be immersed in a mikveh [lit., the waters of the nidda], and anything not used with fire should be passed through water." (Trans. According to the Sifre and the Ramban.)

[Kashering should not be confused with immersion. Food vessels bought from a non-Jew that were used for non-kosher food must be both "kashered" to remove the non-kosher food absorbed in them, and immersed in a mikveh. Non-kosher food that got absorbed in a vessel owned by a Jew necessitates kashering but not immersion; a new unused vessel bought from a non-Jew requires immersion but not kashering.]

Kashering applies both to utensils used with non-kosher food (as in the biblical passage) and to those used with kosher food which can later pose a halakhic problem (as in chametz, or meat and milk). See below for any differences.

The basic rule of thumb in kashering is: "Things can be removed in the same way they got absorbed (ke-vol'o kakh polto)" (Pesachim 30a). If, for example, non-kosher soup was boiled in a pot, kashering it would involve boiling. Likewise, if non-kosher meat was roasted on a spit in fire, removal of the non-kosher meat absorbed by the spit requires fire.

Three factors must be taken into account in order to determine how to kasher something: 1. What the utensil is made of; 2. What has been absorbed; and 3. How it has been absorbed.

1. Metals can always be kashered; pottery and materials like it (many plastics) never can. Glass is like pottery according to the Rama, but does not absorb at all according to the Shulchan Arukh OC 451:26. 2. Removing prohibited non-kosher foods requires a more intense type of kashering than permitted foods (e.g., removing milk from a utensil that one wants to use with meat). In other words, "heteira bala" is easier to rectify than "issura bala." Rishonim differ about whether chametz that was absorbed into a utensil before Pesach is considered a permitted material or a prohibited one. Most adopt the stricter opinion, but sometimes, when there are other reasons to be lenient, chametz can be considered "heteira bala" (Mishna Berura OC 451:28). 3. The most crucial determinant of what method to use for kashering is how the food got absorbed. This issue is dealt with extensively in Shulchan Arukh OC 451. The general rule is as stated above, that removing material requires the same process as that which caused it to get absorbed. Things absorbed through fire can only be removed through fire; things boiled, through boiling; pouring hot water over something can remove food absorbed through pouring. Sometimes we veer from this principle because of practical difficulties in applying it. It is the main use of the utensil which determines the method of its kashering.

There are three main methods of kashering: Heating a utensil until sparks fly ("libbun chamur"), which destroys any chametz that might have once been there; heating it until straw burns on the outside of the utensil ("libbun kal"); and using boiling water ("hag'ala") to cause the material absorbed inside a utensil to leave it ("pleita"). Utensils used directly in fire require "libbun chamur" for kashering; those used in boiling liquids or with hot foods require "hag'ala." "Libbun kal" is used for hard-to-reach places on things which normally need "hag'ala," and is sometimes required instead of "hag'ala" as a stringency. "Hag'ala" is sufficient for removing permissible food, even if it has been absorbed through fire.


The racks of the oven are kashered differently than the body of the oven itself.


Since the racks have absorbed actual chametz through fire, they require "libbun chamur" for kashering, i.e., heating them until sparks fly. Though, as stated above, some consider chametz during the rest of the year as a permissible food that can be removed through "hag'ala" even if it was absorbed through fire, the Shulchan Arukh (OC 451:4) rules like the majority who require "libbun chamur." Because of the difficulty in executing "libbun chamur," most people keep separate racks for Pesach. (See Mishna Berura 451:32 and Chazon Ovadia p. 71, note 2, for a discussion of what to do afterwards ("bedi'avad") if they were used without "libbun chamur.")


Usually no cooking is done on the walls of the oven itself. However, steam from chametz frequently reaches the walls of the oven, and periodically crumbs of chametz fall on them during cooking.

At first glance, the chametz that has entered into the walls of the oven through steam, a liquid, should be removed through boiling water, hag'ala," according to the principle "ke-vol'o kakh polto." Since this is practically impossible, boiling can be replaced by "libbun kal," heating the utensil until it is so hot that straw burns on its outside (see Rama OC 451:4 and Mishna Berura 451:30).

The pieces of chametz that sometimes fall on the walls of the oven pose another challenge. It should, ostensibly, require "libbun chamur" for kashering. However, because these crumbs do not constitute the main use of the oven, one might follow the Shulchan Arukh's lenient ruling (OC 451:10) that the method of kashering an object is determined by its predominant use. Here, since the main absorption of chametz into the oven is through vapors that hit its walls, only the type of kashering needed to remove that - "libbun kal" - is required.

This is difficult for two reasons: 1. The Rama, in his comments on that passage in the Shulchan Arukh, follows the stringent opinions that demand taking even secondary use of an object into account when kashering. 2. Even the Shulchan Arukh himself might only be referring to an object that is used in two different ways at two different times. A table fork that is used occasionally to hold a hot- dog in the fire at a barbecue requires only "hag'ala" according to the Shulchan Arukh because it was normally used with cooked foods, but "libbun chamur" according to the Rama because it is sometimes used in the fire itself. Our oven, though, is always used in the same manner, but through that single use absorbs in two different ways. Maybe the Shulchan Arukh would also rule stringently in this case.

It is possible that an oven is similar to a frying pan: Rishonim differ about whether a frying pan requires only "hag'ala" or rather "libbun" (see Bi'ur Halakha 451:11 for a summary of their positions). Usually foods are cooked in a frying pan with oil and therefore only "hag'ala" is required to kasher it. Some Rishonim, though, require "libbun" because of the likelihood of the oil periodically drying out which means that the chametz is then cooked directly on the metal without any liquid as a medium. The Shulchan Arukh 451:11 rules that only "hag'ala" is required, while the Rama records the custom to kasher through "libbun." Our case, an oven that through its normal use periodically absorbs chametz through crumbs falling directly on its walls, should be similar. However, the only reason that the Rosh (quoted in the Mishna Berura) is lenient is because, as he sees it, some oil is always present, even though mosof it has dried out. In our situation, where there is definitely no liquid between the crumbs and the walls of the oven, "libbun" should be required.

There are, however, still two reasons to be lenient and require only "libbun kal" for an oven and not "libbun chamur":

A. Maybe chametz never requires "libbun chamur" for kashering. Some Rishonim rule that chametz is considered a "permissible material" ("heteira bala"), requiring only "hag'ala" (and in our case "libbun kal" because hag'ala is impossible to accomplish) for kashering even if absorbed through fire, not the usual "libbun chamur." Though the Shulchan Arukh rules that chametz is considered "a prohibited thing" ("issura bala"), he is lenient when there are other contributing lenient factors involved. In our case, because the chametz has only been absorbed through occasional use, one could rule leniently regarding to what kind of kashering chametz requires.

B. Maybe heating the oven to its highest setting is considered "libbun chamur." Acharonim differ (see Yechaveh Da'at 2:63 and the book Hag'alat Keilim) about what mechanism is involved in kashering through "libbun chamur": Does that level of heat destroy the chametz it comes in contact with, or does it cause the chametz inside to leave the vessel (i.e., a more intense version of hag'ala, needed for materials which entered the utensil through the medium of fire)? One of the practical differences between these two approaches is how hot of a fire is required for "libbun chamur." If the "libbun" kashers through destroying, nothing less than "hot-enough-to-make-sparks-fly" suffices. If, however, the fire removes what it absorbed, only the level of heat which caused it to be absorbed in the first place (the highest setting on the oven) is required.

It seems to me that this issue was previously dealt with by the Rishonim. Rabbeinu Tam and the Rosh (see Rosh Avoda Zara 5:34) differ over how to kasher a vessel that absorbed prohibited food through use directly in the fire if one now only wants to use it not in the fire. Rabbeinu Tam rules that only "hag'ala" is required if the vessel will subsequently be used only with hot foods and not directly on the fire, for any prohibited food that will ever leave the vessel has already left it through "hag'ala" the first time it comes into contact with boiling water. The Rosh differs:

"This does not seem correct. Since it was used in the fire, [the absorbed prohibited food] cannot leave the walls of the vessel without "libbun;" "hag'ala" alone is not sufficient to remove everything. Every time anything is boiled in it, it lets out some of the prohibited food ... It constantly expels the prohibited food little by little."

It seems that they have different conceptions about how "libbun" works. According to Rabbeinu Tam, "libbun" DESTROYS the chametz which cannot be fully removed through "hag'ala". Whatever can be removed will leave through the first "hag'ala." If a person only wants to use a vessel with hot foods, he need only kasher it through "hag'ala." The Rosh, though, believes that "libbun" operates similarly to "hag'ala" and REMOVES the absorbed material. According to him, through "libbun" everything leaves at once, but through "hag'ala" it leaves little by little. He maintains, therefore, that something that was absorbed through fire can only be kashered through fire, even if it will later be used only with hot foods.

[There is a proof that this is the issue behind the Rosh and Rabbeinu Tam's argument: The Rosh quotes the law allowing one to kasher utensils that absorbed permissible things with "hag'ala" even if it was absorbed through fire. From here he reasons that something absorbed through fire will not leave through "hag'ala;" otherwise, we would not have permitted kashering something that absorbed through fire with mere "hag'ala." At first glance this seems to be a difficult thing for him to say; he also believes that "hag'ala" does not totally remove material that has been absorbed through fire. Following his approach, why are we lenient with regards to "permitted things" that were absorbed?

It must be that the Rosh holds that something absorbed through fire can be removed through "hag'ala" little by little. If it could only be removed through fire, we would never have been lenient and allowed "hag'ala." The leniency is because such a weak remnant of the food cannot create any prohibition (for example, any milk that comes out through "hag'ala" would be too dilute, when mixed with meat, to make "basar be-chalav"). According to Rabbeinu Tam, though, something absorbed through fire cannot ever be removed completely. Whatever is able to come out comes out through the first "hag'ala". Prohibited material ("issura") must be destroyed through fire; permitted materials ("heteira"), however, is allowed to be just partially removed, through "hag'ala."]

The Rishonim seem to have ruled like the Rosh; so does the Shulchan Arukh (YD 121). If so, "libbun" also works through removing the absorbed material through the principle of "ke-vol'o kakh polto." An oven could then be kashered by raising the heat to its highest setting. Even for those who are stringent and require "libbun" for permitted things, given that chametz is considered by some a permitted thing, this type of "libbun" could suffice. Acharonim brought a variety of other reasons to be lenient here.


Instructions for kashering an oven for Pesach: It should be cleaned well; every crumb of chametz should be removed because even the smallest amount of chametz is prohibited. It should be left unused twenty-four hours before kashering. Then the heat should be turned up to its highest setting (a self-cleaning oven should be on the "clean" setting) for about an hour or two. Of course, new racks should be used for Pesach.

Many people do not use their ovens at all on Pesach, either because of the opinion that "libbun" to the degree that sparks fly from the walls of the oven is required or because of the difficulty of removing every crumb of chametz. Though there is a basis for being lenient, the utmost care should be taken.

 To continue reading part 2 of this shiur click here.