Kashering Glass for Pesach Use by Rabbi Chaim Jachter


This week we will continue our review of some of the halachot of Pesach.  This week we will focus on the issue of whether glass may be Kashered for Pesach use.  This issue is particularly relevant today as many microwaves contain a glass tray.  We will also outline the opinions regarding Kashering pyrex and duralex.

Talmudic Background

This issue as to whether glass may be Kashered is not dealt with in the Gemara.  It is, however, a matter of dispute among the Rishonim.  In order to grasp the basis for their controversy, we need to first survey how the Talmud regards glass in areas other than Kashering.

The primary source for this topic is Avot D'Rabbi Natan 41:b which states that "glass utensils do not absorb or exude."  Although this statement does not appear in the context of Kashering, it nevertheless teaches that glass utensils differ significantly from metal and earthenware utensils.  The Torah unambiguously indicates (Vayikra 6:21 and Bamidbar 31:21-23) that metal and earthenware utensils absorb and retain food particles from the hot food items that are placed in them.

This source, however, does not simply resolve the question regarding the status of glass in the context of Kashering.  This is because Chazal assign the status of metal and earthenware to glass in the areas of "Tevilat Keilim" and "Tumah and Tahara."  Moreover, regarding the area of "Tumah and Tahara" Chazal assigned glass the status of earthenware. 

The Gemara in Avoda Zara 75b states the Chazal decreed that glass utensils need to be immersed in a Mikvah prior to use, if it had previously been owned by non-Jews.  Even though the Torah only required metal utensils to be immersed, Chazal decreed that glass utensils are also required to be immersed.  This is because, the Gemara states, glass shares a common property with metal.  Namely, that if they are broken they can be fixed.  Rashi explains that both glass and metal are repaired in an identical manner, they are both melted down and refashioned.

Regarding the area of "Tumah and Tahara," the Mishna (Keilim 30:1) states that glass can contract "Tumah" despite the fact that there is no allusion to this phenomenon in the Torah.  The Gemara (Shabbat 15b) explains that Chazal decreed that glass can become "Tameh."  The reason given is that glass shares a property with pottery, both are from the ground.

While these two rabbinic decrees appear to be contradictory, the Meiri to Shabbat 15b explains that they are not.  He explains that essentially glass is comparable both to metal and earthenware.  Hence, the Gemara imposes the stringencies of metal and earthenware upon glass.


Three opinions appear in the Rishonim regarding the status of glass as far as Kashering is concerned.  Tosafot (Avoda Zara 33b s.v. Koonya) appears to rule that glass does not absorb food particles and need not be Kashered if it absorbed non-Kosher or chametz food particles.  A number of Rishonim including the Rashba (Teshuvot, 233) agree to this ruling.  This ruling is grounded in empirical evidence and the statement regarding glass that appears in Avot D'Rabbi Natan.

The Mordechai (Pesachim 3:574) cites a ruling from Rabbeinu Yechiel of Paris that glass cannot be Kashered.  Just as the Gemara imposes a stringency of earthenware on glass in the context of Tumah, Rabbeinu Yechiel believes that Chazal similarly impose the stringency that earthenware cannot be Kashered upon glass.  The Terumat Hadeshen (132) rules in accordance with this opinion regarding Pesach.

The Ritva (Pesachim 30b s.v. Li'avid) cites the Ra'ah who adopts a middle approach.  He rules that while we do believe that glass absorbs taste particles from food, it nevertheless may be Kashered.  The Ohr Zarua (Hilchot Pesachim 25b) also rules this way even regarding Pesach laws.  He reasons that since the laws of Toveling utensils and Kashering utensils are linked by the Torah (Bamidbar 31:21-23) the rules of these two areas of halacha should be compared.  Thus, just as glass is assigned the status of metal in the context of "Toveling" utensils, so too glass is assigned the status of metal in the context of Kashering.  The Rosh, however, rules that one should not Kasher glass, due to concern that one may not Kasher it properly because of fear of breaking.

Accordingly, three opinions appear in the Rishonim whether glass may be Kashered.  One opinion is that it cannot be Kashered, a second opinion believes that it may be Kashered, and a third opinion is that it does not even absorb any food particles and thus there is never a need to Kasher glass.

Shulchan Aruch

This controversy persists in the Shulchan Aruch as well.  The Mechaber (451:26) rules that glass does not absorb and may be used on Pesach without Kashering even if hot Chametz touched it.  Rav Yosef Karo believes that thoroughly washing the glass utensil prepares the item for Passover use.

The Rama (451:26) takes the opposite approach.  He rules (at least in the context of Pesach) that glass can not be Kashered.  He notes that this is the commonly accepted practice among Ashkenazim.  Indeed, the aforementioned Terumat Hadeshen already notes that this was the accepted practice among Ashkenazim.  It should be noted that the Terumat Hadeshen is a major source for the Rama's rulings.

The Mishna Berura (451:156) cites the Chayei Adam who rules that in case of extraordinary need (where one is unable to procure new glasses for Pesach) one may Kasher glass.  Accordingly, it may is possible that Ashkenazim should remove the glass tray from a microwave oven for Pesach use, because the Ashkenazic tradition is hold that glass cannot be Kashered for Pesach use, barring extraordinary circumstances.  One should ask his Rav for guidance regarding this issue.  For a review of the opinions regarding Kashering glass for use on occasions other than Pesach, see this author's article published on the Journal of Halacha and Contemporary Society Fall 1993, pp. 77-87.

Duralex and Pyrex Dishes

Rav Eliezer Waldenburg (Teshuvot Tzitz Eliezer 9:26) and Rav Ovadia Yosef (Yabia Omer 4:41) adopt divergent views regarding the question of whether one may Kasher pyrex or duralex utensils.  Rav Waldenburg believes that since these materials are not identical to glass, one cannot assume that Rav Karo would rule that they do not absorb food particles.  Rav Yosef argues that the Rama's strict ruling applies only to glass because of concern that it might shatter- the aforementioned opinion of the Ra'ah.  Since, however, pyrex and duralex can endure exposure to heat, the Ra'ah's concern pertaining to glass do not apply to glass and thus may be Kashered for Pesach even according to Ashkenazic tradition.  Rav Ovadia reasons that the Ashkenazi tradition is limited to refraining from not Kashering glass, but not duralex or pyrex. 

The problem with this approach is that the Ashkenazi commentaries do not agree with this interpretation of the Rama.  The Vilna Gaon (Biur Hagra to 451:26), Mishna Berura (451:154), and Aruch Hashulchan (451:50) all explain that the Rama rules in accordance with the view of Rav Yechiel of Paris that glass can never be Kashered.  Thus, even pyrex and duralex would appear to be included in the Rama's strict ruling.  Nonetheless, Rav Waldenburg cites a leniency from the eminent Rav Zvi Pesach Frank in this regard.  Rav Zvi Pesach ruled that in case of great need one may Kasher pyrex even for Pesach if it is Kashered three times.  Rav Zvi Pesach would appear to be combining three lenient considerations in this ruling.  1) The Rishonim who rule that glass does not absorb food particles.  2) The opinion of the Ohr Zarua that glass may be Kashered.  3) The opinion of the Baal Ha'itur (cited by the Tur Yoreh Deah 121) that even earthenware may be rendered Kosher, if one Kashers it three times.  One should consult his Rav for guidance if one has a need to Kasher pyrex or duralex utensils for Pesach.

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