Kashering a Microwave Oven for Pesach by Rabbi Chaim Jachter


This week we will focus our attention on a very important Pesach issue, the question of whether a microwave oven may be kashered.  We will present the relevant points of debate and encourage our readers to consult their rabbis for guidance concerning this issue.

Two Important Considerations

We have previously mentioned that there are two important issues concerning kashering a microwave for Pesach.  The first is whether the oven can be thoroughly cleaned.  Some rabbis express reservations about kashering microwave ovens due to the many vent holes as well as the crevices which may be nearly impossible to completely clean.  The second is that the interior of the oven is coated with plastic, and Rav Moshe Feinstein has ruled that plastic should not be kashered for Pesach (although many authorities permit kashering plastic for Pesach).

Kashering Glass

Many microwaves contain a glass plate and hence we must review the halachic literature concerning the kashering of glass.  There are three primary opinions regarding glass among the Rishonim:  1. That it need not be kashered as it does not absorb "taste particles" 2. That it cannot be kashered as it has the status of earthenware 3. That it has the status of metal and we are required to kasher it.

The Mechaber (Shulchan Aruch Orach Chaim 451:26) rules in accordance with the views of Tosafot (Avoda Zara 33b s.v. Koonya), Raavya (chapter 464), and Ran (Pesachim 9a in the pages of the Rif) that glass is   , smooth and non-absorbent.  Thus according to Rav Yosef Karo, glass need not be kashered even for Pesach.

The Rema, on the other hand, adopts (at least as far as the strict laws of Pesach are concerned) the opinion of Rabbeinu Yechiel of Paris (cited in Mordechai Pesachim 3:574) and Smag (cited in Trumat Hadeshen 132) who rule that glass utensils may not be kashered.  They believe that since Chazal (Shabbat 15b) assigned glass utensils the status of earthenware utensils regarding Tumah, so too glass utensils have the status of earthenware with respect to kashering and thus cannot be kashered.

Accordingly, Sephardim need only clean the glass plate in a microwave oven and Ashkenazim would have to remove or replace the glass plate for Pesach (for a full review of the halachic literature concerning kashering glass, see this author's essay in the Fall 1993 issue of the Journal of Halacha and Contemporary Society).

Rav Moshe Feinstein's Ruling Concerning Kashering Microwaves

We will now present Rav Moshe Feinstein's approach to the question of how to kasher a microwave oven.  We will also discuss criticisms of this approach and suggest a defense of his ruling.

Rav Moshe Feinstein

By now, Rav Moshe's ruling concerning kashering a microwave oven (cited by Rav Eider in his Halachos of Pesach p.182 note 166) is well known.  He rules that one must thoroughly clean the oven, wait twenty four hours since its last Chametz use, and then put water in the microwave and have the steam generated by the water kasher the microwave.  Even though Rav Moshe (Igrot Moshe Yoreh Deah I:60) has also ruled that steam cannot be used for kashering, this situation is different.  When a utensil absorbs Chametz or non-kosher "taste particles" through a liquid medium it cannot be kashered through steam, according to Rav Moshe.  The Torah teaches that water can serve as a Matir (an agent to render kosher) to kasher a utensil, but no provision is made for steam to serve as such a Matir.  Even though steam is hotter than water, this is irrelevant because kashering (as Rav Soloveitchik asserted many times) serves as a Matir, and only that agent which the Torah has designated to serve as a Matir can do so (there has been a great debate whether steam can be used for kashering see Hagalat Keilim 10:4 for a review of the literature regarding this issue).

Nevertheless, argues Rav Moshe, the situation is entirely different in the case of microwave ovens.  This is because in Rav Moshe's view, the action of steam is the only relevant concern as far as a microwave oven is concerned.  The steam rising from Chametz cooked in the microwave is the only means by which Chametz was absorbed in the walls of the microwave.  Since we say    (the way something absorbs non-kosher or Chametz is the way it is kashered), so too just as a microwave absorbed Chametz taste particles through steam, it may be kashered through steam.

Criticism of Rav Moshe's Ruling

Rav Eider (and other rabbis), however, already raises a question regarding this ruling.  The problem is that steam () is not the only way the Chametz is absorbed into the walls or floor of the microwave.  Hot food commonly spills and splatters inside the microwave (even if the microwave walls do not reach    - the point at which Halacha believes absorption occurs - since the food is hot, there is a situation of    (warm into cold) and thus some absorption ( ) takes place, see Shulchan Aruch Yoreh Deah 91:4.

Rabbi Eider accordingly advises that the places on which Chametz fell should be kashered by pouring boiling hot water on them (, this is effective because of the rule   ).  There is also a problem that the place where the cup used to generate steam for kashering is placed is not kashered.  Therefore, the location this utensil occupied should also be kashered by pouring boiling hot water on it (or, kasher the oven again with the cup in a different place).

The problem with Rav Eider's suggestion is not only the difficulty and danger involved in pouring boiling hot water into one's microwave oven.  Actually, the splattering of hot Chametz lands throughout the microwave.  Accordingly, hot water would have to be poured over every surface of the microwave in order to eliminate any concern.  In many cases this is simply not feasible.

Defense of Rav Moshe's Ruling - Three Lenient Considerations

There are three lenient considerations to defend Rav Moshe's ruling.  First, there are opinions that one may kasher with steam (see Achiezer IV:9 for a discussion of this issue; Rav Chaim Ozer there cites the lenient approach of Rav David Zvi Hoffman who permits the use of steam to kasher when there exists no alternative).  Thus, the entire microwave can be kashered with steam, provided that the microwave is enveloped in steam and that the utensil containing the water is moved to permit the area it is standing on to be kashered by the steam as well.

Second, there is the opinion of the Mechaber (Shulchan Aruch 451:25) that the method of kashering is determined by  , the way the utensil has been used in the majority of situations.  For example, if one cooks in a utensil in a liquid medium for a majority of the uses, and a fire medium in a minority of its uses, according to the Mechaber kashering with boiling water () suffices and kashering with fire () is not necessary.  According to this opinion, kashering a microwave with steam may suffice since the majority of absorption in a microwave occurs through steam as opposed to splattering of food and liquid.

The third lenient consideration is the opinion of the Chavat Daat (Biurim 92:96) who rules that  (steam) cannot extract that which is absorbed in the oven  (this also appears to be the opinion of the Mechaber and Rema 92:8).  A major concern when using a microwave oven on Pesach, is that  will extract Chametz absorbed in the oven.  Rav Yechezkel Landau (Dagul Mervava Y.D. 92:8) adopts the strict opinion that steam is able to extract "taste particles" absorbed in a utensil and so an unkashered microwave oven may not be used on Pesach.  However, for the Chavat Daat the concern of  is not relevant when considering using a microwave oven for Pesach.  Although many do not accept the Chavat Daat's opinion, perhaps it can be used as a  , in conjunction with the other two lenient considerations we have mentioned.  (For a discussion of this issue, author's essay in see the Fall 1996 issue of the Journal of Halacha and Contemporary Society, pp. 26-32.)


These three considerations can buttress Rav Moshe's lenient ruling.  However, many rabbis feel that it is inappropriate to rely on these opinions regarding Pesach, since Chazal and Poskim are traditionally very strict concerning Pesach laws.  Rav Eider even suggests that it is preferable to cover both the surfaces of the microwave and the kosher Pesach food cooked in the microwave, (but not with anything metallic or you may destroy the oven!) even after it has been kashered according to Rav Moshe's approach.  One should consult his Rav for guidance regarding if and how he should kasher his microwave for Pesach.


The author wishes to express his appreciation to the members of the 5753 Drisha Beit Midrash Program who greatly contributed to the ideas and approaches found in this essay.

Pikuach Nefesh and Medinat Yisrael - Part I by Rabbi Chaim Jachter

Kashering Dishwashers for Pesach - Part II by Rabbi Chaim Jachter