Alongside the four questions we ask on Seder night, perhaps we can add a fifth that needs to be asked a few days earlier: “Can we kasher our ovens for Pesach?” Every year as the familiar routine of packing away the chametz dishes and taping up the cupboards unfolds, one appliance sits conspicuously in the center of the kitchen, and many of us wonder if it too has to be sealed up, or perhaps there’s a possibility that the oven can be a part of our Pesach preparations as well. In this article we will provide an overview of the major issues involved in kashering (or not kashering) an oven for Pesach.
The source of the laws of kashering utensils is found in Parshat Matot after the war with Midian. Regarding the utensils that were captured as spoils of war the Torah states:
כל דבר אשר יבא באש תעבירו באש וטהר אך במי נדה יתחטא וכל אשר לא יבא באש תעבירו במים
Everything that comes into fire, you shall pass through fire and it will be purified, but it must be purified with the water of sprinkling; and everything that would not come in the fire, you shall pass through water.
From here Chazal derived the basic principle of kashering – “כבולעו כך פולטו” – forbidden taste is removed from a utensil in the same way in which it was absorbed. Therefore, utensils which were used directly with fire can only be kashered by direct contact with fire, while utensils in which food was boiled can be kashered by being placed in boiling water. These two methods are known as libun and hagala.
The process of libun involves destroying any absorbed non-kosher or chametz taste absorbed in the utensil, through the heat of the fire. There is a dispute among the Rishonim as to the degree of heat needed to achieve this requirement. The prevalent opinion, quoted by the Shulchan Aruch, is the heat must be at the level which sparks would fly off the utensil. However, a minority opinion states that the heat necessary is far less – such that straw placed on the outside of the utensil would burn. This view is quoted by the Rema, who states that the practice is to follow the stringent opinion, however in a case where hagala is required, this form of heat would nonetheless suffice to make the utensil kosher. Placing a utensil through this lower degree of heat is known as libun kal, whereas placing it through the stronger fire is known as libun chamur.
Hagala is the process of placing the utensil in boiling water, whereby the absorbed taste will be released into the water, thereby rendering the utensil fit for kosher use. Utensils requiring hagala may also be kashered by libun, however the reverse is not true. Hagala would not suffice for anything with came into direct contact with fire, such as a skewer or grill used for roasting.
Ovens – The Problem
Cooking done in an oven is a classic example of where the food and utensil come into direct contact with the fire or heat source. It then stands to reason that the oven itself should require libun chamur. However, with the exception of self-cleaning ovens which reach extremely high temperatures, the highest heat reached by a standard oven does not suffice for the heat required for libun chamur (but only enough for libun kal). The problem is further compounded by the fact that whenever it is questionable whether a utensil could withstand the extreme heat required for libun without sustaining damage, the concern exists that one will remove it from the fire before libun has been achieved, and thus it cannot be kashered. For this reason, the trays in which food is cooked or baked in the oven cannot be kashered either. Accordingly, it would seem that the oven in which one cooks all year long cannot be used for Pesach.
One crucial difference between our ovens and those used during the time of Chazal may pave the way towards a solution. In the times of the Gemara, cooking was done directly on the walls of the oven itself. In such a case there would be a concern that the taste of the food would be absorbed directly through fire into the walls and floor of the oven, and food later cooked directly on those surfaces would absorb that taste. However, in our ovens, food is rarely, if ever, placed on the inner walls of the oven itself. Generally, the cooking is done on oven racks or within trays. Since the food makes no direct contact with the oven itself, perhaps the need for libun has been eliminated.
Nevertheless, the poskim indicate a number of concerns.
Reasons to be Stringent
The primary Kashrut concern generated by the oven itself is the issue of zei’a (steam). The Shulchan Aruch rules based on a teshuva of the Rosh that it is forbidden to place a pot of meat in an oven above a pan in which milk is being cooked, due to the zei’a being emitted from the milk, and even bedi’eved the pot would be forbidden. Similarly, the food cooked in our ovens generates zei’a which is absorbed in the walls and ceiling of the oven. Subsequently when Pesach foods are to be cooked in the oven, the concern is that previously absorbed zei’a will be released and enter the foods, thereby rendering them chametz.
One could argue that since the zei’a is itself liquid, the kashering required would be hag’ala, and by extension libun kal would suffice. However, a number of prominent poskim seem to indicate that zei’a of solid foods is judged as the food itself and therefore requires libun. The Tur writes that the lid used to cover a solid food cooked in the oven requires libun just like the utensil itself since:
- a) Many times, contact is made between the lid and the food
- b) Because of the zei’a emanating from the food
Significantly, the Magen Avraham quotes both reasons given by the Tur as to why the lid would require libun. The Bach goes further and interprets the Tur’s words to explain that zei’a is the sole concern, and libun is required nonetheless.
A second reason to be stringent is the concern that actual pieces of chametz may remain in the oven. Crumbs or pieces of food may have spilled or fallen and remain lodged in the oven. A particular cause for concern is the oven door and the crevices which are very difficult to clean. As a result, the worry exists that either during cooking or after, one of these crumbs may fall into the food, rendering it chametz.
First we will examine two possibilities which may alleviate the above concerns, followed by several other principles whose application may ease the kashering process.
No Zei’a with Solids
Although the concern for zei’a in ovens is a legitimate one, it should be noted that it is by no means accepted by all the poskim. The Mishna Berura, in explaining the ruling of the Shulchan Aruch regarding a lid used to cover solid food, entirely omits any mention of zei’a, implying that he may reject this concern. The Pri Megadim explicitly quotes the Shut Mas’at Moshe who rules that is no concern of zei’a with solid foods (as opposed to liquids). This view is also shared by the Rema in Torat Chatat and Rav Shlomo Kluger.,
A separate leniency may be the position of the Aruch HaShulchan, who holds that zei’a would only be of a concern in a small, confined space. He holds that our ovens, due to their size, would not fit this definition. Rav Moshe Feinstein argues and holds that even if an oven has a vent through which steam can exit, zei’a still reaches the walls and ceiling and would present a problem. It seems that the common practice is not to rely on leniency of the Aruch HaShulchan during the year (regarding meat and milk) and so it would be difficult to invoke it regarding Pesach.
Regarding the difficulty of removing every crumb of tangible chametz, a solution can be found based on the ruling of inedible chametz. The Shulchan Aruch states that chametz which became disqualified for animal consumption before the time of its prohibition began is permitted for one to derive benefit from for the duration of Pesach. The Rishonim disputed whether this allowance would apply to eating as well, and the halacha follows the stringent view. However, this would only apply in a case where one intended to eat the (inedible) chametz in question, and not in a situation where it unintentionally made its way into one’s food.
If the oven is thoroughly cleaned with a chemical cleansing agent, any possible remnants of chametz will certainly be rendered inedible, posing no further problem.
As stated above, any utensil in which forbidden food was cooked by direct contact with fire requires libun. The Gemara makes a distinction between different types of absorbed flavors, and the requirement of libun applies only when the absorbed flavor was of a forbidden food. However, if the absorbed taste was of a permitted food then only hagala is required, even if there was direct contact with fire.
Is the absorbed taste of chametz throughout the year considered a permitted (“Hetera Bala”) or prohibited taste (“Issura Bala”)? The matter is the subject of a major dispute among the Rishonim. Many, including the Ramban, Rif, Ran and Rabbenu Yerucham, hold that chametz is considered “issura bala”. Thus, the Shulchan Aruch rules that skewers on which chametz foods were roasted require libun chamur.
However, the Mishna Berura points out that the issue is far from clear-cut, and whenever other doubts or leniencies are involved, the opinion that chametz is considered “hetera bala” may be invoked, since several major Rishonim subscribe to this view. Furthermore, the Shulchan Aruch himself seems to rely on this view under certain circumstances.
Since performing libun chamur on an oven that is not self-clean is all but impossible (except for those who are knowledgeable on how to blowtorch it safely), there is great room to rely on this opinion, especially when the oven hasn’t been used for 24 hours and the entire requirement to kasher is thus rabbinic in nature. Therefore the view that chametz is “hetera bala” can be relied on, meaning the oven would be kashered by the process of libun kal.
“Kebol’o Kach Polto” – Also by the Degree of Heat?
A final factor to consider revolves around the meaning of the basic principle of kashering – “kebol’o kach polto”. Does this principle apply simply to the method of cooking used (roasting on fire, boiling in water) or can it be applied to the degree of heat as well? This would allow us to perform libun by bringing the oven to the highest temperature to which it could possibly be raised, rather than requiring the heat at which sparks would fly off.
The simple understanding, proposed by the Pri Megadim and Rav Moshe Feinstein, is that the principle of “kebol’o kach polto” does not apply to libun, and a standardized level of heat is required, no matter at what level of heat the cooking was done. The reasoning behind this view is that the mechanism of libun is not to remove the absorbed taste, but rather to destroy it by burning. While a lower level of heat may suffice to cause the taste to be absorbed into the utensil, only an extreme degree of heat will be enough to destroy all that was absorbed. Hagala by contrast, releases the absorbed taste from the utensil back into the water, and this is done in the same way that the taste was absorbed.
There are poskim who disagree. The Beit Yosef quotes the opinion of the Kol Bo that the shovel used to remove cakes from the oven may be kashered by being placed in the oven at its highest temperature, following the principle “kebol’o kach polto”. Moreover, the Minchat Yitzchak quotes the view of the Hagahot Maimoniot, who suggests that raising an oven to the highest temperature at which it was previously used may be enough to satisfy the requirement of libun. According to these views, the measurement of libun chamur would not apply to an oven, since by definition it could never reach that heat. Consequently, raising the oven to its highest possible heat would be enough to make it kosher.
A summary of the potential problems in kashering an oven and their solutions is as follows:
The oven absorbs taste directly through heat, seemingly requiring libun chamur.
Tangible remnants of chametz in the oven.
Zei’a which reaches the walls and ceiling of the oven.
Since cooking is not directly on the oven floors and walls, libun may not be necessary.
Using a chemical cleansing agent to render any chametz remnants inedible.
Some opinions hold that zei’a from solids is of no concern.
Some opinions hold that the oven’s highest temperature satisfies the requirement of libun.
Some opinions hold that chametz is “Hetera Bala” – therefore hagala or libun kal would be enough.
Consequently, the kashering process would involve cleaning the insides of the oven well with a chemical cleansing agent (including the door), leaving the oven unused for 24 hours, and then turning it up to its highest heat for an hour or longer.
Many contemporary authorities, including Rav Shlomo Zalman Auerbach and Rav Ovadia Yosef have ruled that an oven can be kashered for Pesach. And while those who wish to be stringent have a right to do so, there is certainly no obligation to prevent the use of the oven for Pesach.
Pesach Kasher v’Sameach!
 Bamidbar 31:23
 The verse also deals with the requirement of tevila – immersion of utensils acquired from a non-Jew. This is a separate requirement, not related to the act of making the utensil kosher.
 Orach Chaim 451:4. The source for this degree of heat is found in the Gemara, Avoda Zara 76a.
 The reasoning being that although this type of heat is not necessarily strong enough to remove all remnants of the absorbed taste when the utensil came into direct contact with fire, it would still be enough to purge the absorbed taste when the utensil was used for boiling.
 The Gemara Pesachim 30b gives this rationale as to why earthenware vessels cannot be kashered, and it is brought by the Shulchan Aruch (Orach Chaim 451:1).
 Even today, bakeries produce certain types of bread in a similar fashion.
 One could argue that sometimes chametz food spills over and makes direct contact with the oven walls and floor, requiring libun chamur to remove this absorbed taste. However, the fact that future cooking is not done directly on these surfaces makes it nearly impossible for this absorbed taste to make its way back into the food. Although the Rama (451:4) rules that a trivet on which hot pots are placed requires libun, the Acharonim point out that this ruling is a chumra of Pesach and only lechatchila is libun kal required, since no transfer of taste takes place between two pots that are dry, even when there is direct contact between them (Mishnah Berura 451:34). Accordingly, even if trays are placed directly on the floor of the oven, libun kal would be enough to render the oven kosher (Yechave Da’at 2:63 quoting Shut Yaskil Avdi 6, Even Ha’Ezer 85). Furthermore there is even more room to be lenient with an oven, since the trays in which the food is cooked are usually placed on racks and rarely come into direct contact with the walls and floor of the oven.
 This is also the primary concern dictating whether the same oven can be used for milk and meat foods – a topic which requires discussion in its own right and is beyond the scope of this article.
 Yoreh De’ah 92:8
 Teshuvat HaRosh 20:26
 See Igrot Moshe, Yoreh De’ah 1:40. The source of the concept of zei’a is from the Mishnah Machshirin 2:2, from where it is clearly seen that the concern extends to the walls as well.
 The Mishna Berura (451:85) quotes only the first reason given by the Tur and Magen Avraham, implying that he may not subscribe to this view, as will be discussed below.
 Unlike other forbidden foods which are nullified in a mixture in a ratio of 60:1, chametz on Pesach is never nullified and even the smallest crumb would render the food chametz – Pesachim 29b – 30a, Shulchan Aruch Orach Chaim 447:1.
 Eshel Avraham 451:30, Mishbetzot Zahav, O.C. 447:9
 Shut Tuv Ta’am V’da’at, Mahadura Gimmel 10:76
 An extreme machloket exists between those who hold that there is no zei’a from solid foods, and those who hold that this zei’a is enough to require libun. See Hagalat Keilim, עמ’ שצח, for a summary of the various opinions. There may exist a middle approach, that there is zei’a from solid foods, but it would only require hagala. According to this, an oven could simply be kashered by libun kal.
 Aruch HaShulchan, Y.D. 92:55
 According to this view, zei’a woud be more relevant in the case of a microwave.
 Igrot Moshe, Yoreh De’ah 1:40
 Another possible solution may be to cover all food cooked in the oven on Pesach. In this way, even if we hold that zei’a presents a concern, it is prevented from entering the food.
 Orach Chaim 442:9
 Mishna Berura 442:43.
 Avoda Zara 76a
 Such as non-kosher meat, or meat and milk simultaneously.
 Such as a pot which was used for kosher meat, and one intends to kasher it for later use with milk.
 The reasoning (based on the Ramban’s explanation of the Gemara) is as follows: When the taste was absorbed through direct contact with fire, hagala is still enough to remove most of the absorbed flavor, but the final remnants will remain until libun is done. If the absorbed flavor is of a permitted food, the minutest remnants will not be large enough for a prohibiton to take effect on them and they need not be removed. However, if the absorbed flavor was already forbidden to begin with, it needs to be removed entirely.
 Once the time of the prohibition of chametz begins, all agree that the chametz would be considered a prohibited taste (“Issura bala”). Therefore, the entire discussion of kashering the oven is limited to the days before Pesach, and not the days of the chag itself.
 Orach Chaim 451:4
 See Yechaveh Da’at 2:63 for a summary of the Rishonim who take this view including the Ra’avad, Rabbenu Tam, Rashba and Or Zarua.
 The Acharonim point out that the Shulchan Aruch writes in Orach Chaim that a frying pan may be kashered by hagala, whereas in Yoreh De’ah he writes that libun is required. Many expain this discrepancy by saying that in Orach Chaim the Shulchan Aruch invoked the view of those Rishonim who hold that chametz is considered “hetera bala”.
 Forbidden taste within a utensil is only forbidden by the Torah within 24 hours. Beyond that time the taste is considered “pagum” (blemished) and no longer carries a Torah prohibition. However, midrabanan it is required to kasher the utensil, lest one subsequently come to use it within 24 hours of absorbing a forbidden taste, and transgress a Torah prohibition.
 Other reasons why one should be able to rely on the opinion that chametz is considered “hetera bala” include:
- Having to kasher the oven by libun is considered a great monetary loss, since otherwise one would need to be purchase a new oven – a considerable expense.
- The option of not being able to use an oven on Pesach would diminish from simchat Yom Tov.
- One could also include the opinion of the She’iltot, who unlike the majority of poskim rules that chametz on Pesach is nullified in a ratio of 60:1. Although we do not accept this view as halacha, the Mishna Berura (447:2) writes that where there are other reasons to be lenient, this view may be factored in as well.
 And thus satisfy even those opinions that chametz is considered “issura bala”.
 Eshel Avraham 451:30
 Igrot Moshe, Orach Chaim 1:60
 This is also the source for the method of libun kal.
 It is important to emphasize that each of these factors alone may not be enough to allow us to kasher the oven. However, when all the factors are considered together, the result is considerable room for leniency.
 A substantial period of time is needed in order for the entire inside of the oven to reach the necessary degree of heat. An hour is the amount of time given by Rav Ovadia Yosef, although some poskim require more or less time.
 Minchat Shlomo, Tanina 51
 Yechaveh Da’at 2:63
 This is also known to be the position of some American poskim such as Rav Aharon Kotler (as cited by Rav Shimon Eider in his English work on the laws of Pesach) and Rav Yosef Dov Soloveitchik.
 The leniencies listed above would apply to the oven itself but not to the trays in which the food is placed, which would require libun chamur according to many opinions. Also excluded from the discussion are self-cleaning ovens which reach the extreme temperatures of libun chamur, where there may be even more room to be lenient.