A major question that arises in every observant Jewish home is how to kasher one's conventional oven for Pesach. No consensus has emerged on this topic and different families and communities have differing practices in this regard. In this essay, we will explore the basis for the different opinions rendered by the outstanding halachic authorities of this generation. We will begin by outlining the basis rules of kashering.
Introduction To Kashering
In Parshat Matot (במדבר לא: כא-כג) the Torah presents the basic rules of kashering. "כל דבר אשר יבא באש תעבירו באש וטהר", every non kosher utensil that was used with fire must be kashered by fire, is the Torah's first rule. Rashi explains (based on Gemara Pesachim 03 and Avoda Zara 57) that this phrase is referring to two methods of kashering utensils used to cook non-kosher food items. If the non-kosher food was cooked in hot water, then the utensil may be kashered by using boiling hot water (הגעלה). If the non-kosher food was cooked directly on the fire, then fire must be used to kasher the utensil (ליבון).
The Torah, on the other hand, teaches כל אשר לא יבא באש תעבירו במים, if only cold non-kosher food was placed in this utensil, then all that is required is to clean the dish and it may be used (It must be placed in a mikvah if it is metal or glass).
The general principle that emerges from this section of the Torah is articulated by Rashi in his commentary to the Torah - כדרך תשמישו הגעלתו - the manner in which a utensil was used for non-kosher foods is the manner in which the utensil should be kashered. The Gemara (Pesachim 03b) formulates this rule similarly: כבולעו כך פולטו, the manner in which the utensil absorbed non-kosher food is the way in which the non-kosher food is extracted from it.
The Theory of Kashering
Two questions are commonly raised concerning kashering. First, if kashering extracts the non-kosher food absorbed into a utensil, then why don't we say that the food that is extracted reenters the utensil after it is extracted. Second, why is kashering not a violation of the rule of אין מבטלין איסור לכתחילה - that one is forbidden to intentionally mix non-kosher foods with kosher food, even if the amount of non-kosher food is small enough that there is sufficient kosher food to nullify the non-kosher food?
Tosafot (Avoda Zara 67a s.v. מכאן) offers an answer to the first question. They write that if one wishes to kasher a utensil that has been used in the previous twenty four hours )בן יומן( in which the absorbed non-kosher food re-emits a good taste into food cooked subsequently in that utensil (נותן טעם לשבח), then in order to kasher the utensil, at least sixty times as much water as the volume of the utensil must be used. In this manner, the non-kosher taste emitted into the kashering water does not become reabsorbed into the utensil, because the non-kosher taste is nullified by the large volume of water.
Tosafot clearly indicates that if the utensil is an אינו בן יומו, For example, it has not been used in the past twenty-four hours, then the great volume of water is not necessary. This follows from the rule of נותן טעם לפגם, that after twenty four hours of being on a utensil, the non-kosher food absorbed in the utensil develops a bad taste and does not render food that absorbed from this utensil non-kosher. This is because poor-tasting food that is extracted does not render food it is absorbed into non-kosher; non-kosher food with no taste or poor taste cannot render kosher food non-kosher (see Avoda Zara 86a)
Accordingly, the non-kosher taste extracted from the אינו בן יומו utensil does not render the newly kashered utensil non-kosher, because the taste that is reabsorbed is poor. However, one may ask why kashering isn't an unending cycle in which the extracted taste is reabsorbed into the utensil, returning the utensil to its pre-kashered status. The Meiri (Avoda Zara 67a) explains כל שתחלת בליעתו מן הפגם מותר that the only time it is forbidden to use an אינו בן יומו utensil is when it contains non-kosher food that had a good taste at the time it was absorbed in the utensil. However, if the non-kosher taste was poor when it was initially absorbed into the utensil, then it is completely permissible to use that utensil. Accordingly, even though kashering constitutes a repeating cycle, it renders a utensil permissible by changing the status of the non-kosher taste absorbed into the utensil (the Rosh, Avoda Zara 5:63, explains that the reabsorbed taste is permitted because it is נ"ט בר נ"ט().
The Rema (Yoreh Deah 121:2) records that the practice is to kasher only a utensil that has not been used in the past twenty four hours. The Chazon Ish (Yoreh Deah 32:1) offers a method of kashering utensils that have been used in the past twenty four hours without requiring a huge volume of water. The Chazon Ish rules that if the water used for kashering has a bitter substance placed in it then the extracted taste from the utensil becomes poor tasting. Thus, when the taste is reabsorbed into the utensil it has a poor taste, and the utensil is then completely permitted for kosher use. This author has seen this ruling of the Chazon Ish implemented by reliable Kashrut agencies in a case of great need such as when Kashering a hospital's kitchen.
Regarding the issue of אין מבטלין איסור לכתחלה, most Rishonim and Acharonim adopt the explanation that this prohibition applies only when one intends to benefit from the non-kosher food that is nullified in the kosher food (see, for example, Smag negative commandments, number 87).
III The Distinction Between "Hagalah" and "Libun"
We mentioned that sometimes kashering is effected by boiling water (הגעלה) and other times fire is necessary. The generally adopted explanation is that kasherng with boiling water extracts absorbed taste (מפליט), whereas kasheirng with fire (ליבון) involves burning the absorbed taste causing thetaste to lose its integrity and thus its halachic status as food (see Taz, Yoreh Deah 121:7 and the Shach, Y.D., 121:71).
IV The Problem of Kashering Ovens
Kashering a conventional oven is problematic because on one hand, Hagalah is obviously not relevant or possible. It seems that Libun would be required because absorption of Chametz in the oven walls took place in the absence of a liquid medium. Rav Yosef Karo (Shulchan Aruch Orach Chaim 154:4) rules that Libun (based on the Yerushalmi at the conclusion of Masechet Avoda Zara) is accomplished when sparks fly (ניצוצות ניתזין) from the object being kashered. Rav Hershel Schachter and Rav Mordechai Willig (Roshei Yeshiva at Yeshiva University) have told this author that the consensus opinion is that ניצוצות ניתזין is accomplished at 007 degrees Fahrenheit, sparks fly from wrought iron at 007 Fahrenheit, see however Rav Feivel Cohen, Badei Hashulchan p. 512, for a dissenting opinion). Since conventional ovens can be heated only to 055 degrees Fahrenheit it would seem that Libun cannot be accomplished in them. Rav Moshe Feinstein זצ"ל and Rav Yosef Dov Soloveitchik זצ"ל disagree about this issue.
The Strict Opinion - Rav Moshe Feinstein
Both Rav Shimon Eider (Halachos of Pesach p. 971) and Rav Aharon Felder (Ohalei Yeshurun p. 77) cite Rav Moshe that an oven requires Libun that cannot be accomplished by setting the oven at its highest temperature. Rather, Libun can be accomplished by using a blow torch for seven minutes on an area of eight square inches. Since this is difficult to accomplish, many families place an insert into the oven after it has been cleaned thoroughly. In this case no Chametz can be extracted from the oven walls, because extraction does not take place through two utensils-the insert and the oven wall (see Rema to Yoreh Deah 29:8).
The Lenient View - Rav Yosef Dov Soloveitchik
Many families follow the lenient view of Rav Soloveitchik and other Poskim (Rav Shimon Eider cites that Rav Aharon Kotler זצ"ל also ruled leniently) who rule that a conventional oven can be Kashered for Pesach by setting the oven to its highest setting for a period of an hour or two. This view is based on an application of the basic rule of kashering articulated by Rashi in his commentary to the Torah-"כדרך תשמישו הגעלתו"-the way an object is used is the way it is kashered. Since an oven is never used above 055 degrees Fahrenheit, it can be kashered at 055 degrees Fahrenheit. Rav Ahron Lichtenstein relates that Rav Soloveitchik argues that the rule כדרך תשמישו הגעלתו determines how to kasher a specific item. The strict view believes that the rule כדרך תשמישו הגעלתו merely determines which method of kashering should be used, Hagalah or Libun. However, once it is determined that Libun is required, then the general rule of Libun applies, namely, that the object must have fire applied to it until sparks fly. For a thorough discussion of this issue see Teshuvot Minchat Yitzchak 3:66, Sefer Hagalat Keilim, Introduction to Chapter Four, and Rav Mordechai Willig, Mesorah 4:38-69.
Another lenient consideration is that of the Rema (O.C. 154:4) cites the view of some Rishonim who believe that Libun is accomplished when it reaches the temperature at which straw burns (קש נשרף). Many authorities believe this is accomplished at 005 degrees Fahrenheit. Hence, Libun can be accomplished in our ovens according to these authorities. This is an especially cogent leniency, since the Rema notes that these opinions are followed when Hagalah is what is halachically required to kasher an item, but boiling water is impractical to use. The Gemara (Avoda Zara 67a) presents the rule that if food was absorbed into an object when the food was still kosher (היתירא בלע), but only subsequently became not kosher then even if it was absorbed through fire without a liquid medium, only Hagalah is required. Since many Rishonim rule that Chametz is considered היתירא בלע since it was permitted at the time it was absorbed into the oven (before Pesach), then Hagalah would be sufficient to kasher even objects that absorbed Chametz through fire (see Shulchan Aruch Yoreh Deah 121:4 and the Biur Ha-Gra 121:9). Hence according to these Rishonim, following the Rema's rule an oven can be kashered by heating it merely to 500 degrees Fahrenheit. Accordingly, the lenient approach of Rav Soloveitchik is bolstered by the many Rishonim who believe that Chametz is considered היתירא בלע. It should be noted that the aforementioned Shulchan Aruch and Gra use these Rishonim as a consideration for a lenient ruling even for Pesach.
Accordingly, one can see that there are cogent arguments for both the strict and lenient approach for kashering a conventional oven for Pesach. One should ask his Rav for guidance regarding how to kashering a conventional oven as well as the various types of ovens such as a self-cleaning oven and a microwave oven. A Chag Kasher Ve-Sameach to all of Klal Yisrael, ונסו יגון ואנחה!!