There is considerable confusion in the observant community whether glass may be “koshered” (made Halachically usable after it was forbidden) and whether it may be used for both milk and meat. We will survey the many opinions regarding these issues, emphasize the difference between Sepharadim and Ashkenazim in this regard and attempt to outline the Halachic consensus.
II Talmudic Sources
The Babylonian Talmud does not contain sources which unambiguously state the Halachic status of glass regarding koshering. However, the Talmud does contain sources from which the commentaries infer the Talmud’s position. How these sources are to be evaluated and what inferences are to be drawn from them is a matter of debate among the commentaries, which we will outline in the next section.
In Shabbat 15b, the Talmud states that Chazal declared that glass can become impure. The basis for this declaration is that glass’s material composition is similar to pottery since glass is produced from sand. The Torah rules that pottery may become ritually impure, and because of their similar composition Chazal assigned glass the Halachic status of pottery.
On the other hand, the Talmud (Avoda Zara 75b) states that glass utensils which were once owned by a non-Jew must be dipped in a mikvah prior to use with food. This rabbinic decree stems from the Torah’s requirement that metal utensils once owned by a non-Jew must be immersed in a mikvah. Here the Talmud compares glass to metal. Like metal, glass can be repaired after it breaks (Rashi notes that the repairing process is identical; both metal and glass are melted down and fashioned).
The Meiri (Shabbat 15b) explains that these two Talmudic passages do not contradict each other. Both metal and pottery are viable analogs for glass, and the Talmud chooses the analog which leads to a stringent ruling in each of these cases.
The third source is Avot deRabi Natan (41:6), which states that glass utensils do not absorb or exude.” According to this statement, it would appear that glass cannot be placed into any of the Halachic categories since it is qualitatively different than earthenware and metal.
Four categories of opinion appear in the Rishonim concerning our issue. Tosafot (Avodah Zarah 33b s.v. Koonya), Ra’avya (chapter 464) and Ran (Pesachim 9a in the pages of the Rif), are among the Rishonim who rule that glass utensils are smooth and nonporous. (Rashba (Teshuvot, number 233) states that glass does not absorb even if hot food or drink is placed in it. These authorities base their ruling on empirical evidence (“Hachush Meid”) and on the passage in Avot deRabi Natan which explicitly states that glass utensils do not absorb. Accordingly, these authorities rule that one is not required to kosher glass that came in contact with non-kosher food and that glass utensils may be used for both milk and meat, even for cooking.
The second category includes Rabbeinu Yechiel from Paris (cited in Mordechai, Pesachim, Chapter 3, section 574) and Semag (cited in Terumot HaDeshen no.132) who adopt an entirely different approach.
They rule that Cazal assigned glass utensils the status of earthenware utensils (Shabbat 15b). This comparison is asserted generally and is not limited to the issue of ritual impurity. Just as earthenware cannot be koshered, so, too, glass utensils cannot be koshered, and certainly may not be used for both meat and milk. In generalizing the equation between glass and earthenware in Shabbat 15b, these authorities contradict the position of Avot deRabi Natan which defines glass as non-absorbent. One may resolve this problem in a number of ways. First, they might consider the passages in Avot deRabi Natan as Aggadic and not Halachically binding. Second, perhaps Avot deRabi Natan simply states a property of glass but does not draw any Halachic implications. Indeed, this passage is part of a list of phenomena which are characterized in this chapter of Avot deRabi Natan in general terms, without mention of Halachic implications.
Alternatively, since glass does not absorb, the Torah ruled that glass cannot become impure and never requires koshering; however, Hazal declared that since its composition is similar to earthenware, it can become ritually impure and can never be koshered.
The question remains, though, why these authorities chose to follow the Talmudic comparison of glass to earthenware in Shabbat 15b over the comparison of glass to metal in Avoda Zara 75b. The answer might be that the comparison to earthenware is more compelling since it relates to the essential nature of glass- its composition - rather than to the more incidental issue of how it is repaired.
The third set of opinions adopts the position of the Talmud (Avodah Zarah 75b) and assigns glass to the Halachic category of metal. Consequently, these authorities consider glass to absorb food but also permit it to be koshered. Or Zarua (Pesachim no. 256) and Ritva (Pesachim 39b) citing the opinion of Ra’ah are among the proponents of this view. The Torah presents the laws of koshering and immersing utensils in the same verses (BeMidbar 31:22-23); therefore, it is reasonable to draw some parallels between the two processes. Just as Chazal assigned glass utensils the Halachic status of metal in the context of immersing of the utensils, they did so in the context of koshering utensils as well. Rabbeinu Yonah (Issur Ve’Heter 58:50) presents a fourth opinion. He concludes that it is uncertain whether glass utensils are assigned the status of earthenware or metal. He concludes that since the matter is in doubt, one must rule stringently - that glass, like pottery, cannot be koshered.
IV Shulhan Aruch - Pesach
Rav Yosef Karo (Shulchan Aruch, Orach Chaim 451:26) rules that a glass utensil does not absorb even if hot food was placed in it; his ruling follows the description of glass outlined in Avot deRabi Natan. The fact that Rav Karo’s lenient ruling appears in his laws of Passover is especially significant, in light of the overall tendency of Halachic decisors to rule more stringently regarding Passover issues than in other Halachic contexts. Prei Chadash, a premier Sephardic authority, echoes Rav Karo’s view and writes, “it is correct and this is our accepted practice”. Sdei Chemed (5:29) and Rav Ovadia Yosef (Teshuvot Yehave Daat 1:6) write that common practice among Sephardim is to follow Rav Karo’s ruling even if hot food was placed in a glass utensil and even on Passover. Darchei Avoteinou (2:11) confirms that this is the practice of Moroccan, Tunisian and Algerian Jews as well.
Rama, on the other hand, comments on Rav Karo’s ruling, “There are those who are strict and believe that glass utensils may not even be koshered and this is the custom in Germany and [Eastern European] lands.” The Vilna Gaon (Biur Hagra to Orach Caim 451:50), Mishna Berurah (451:154), and Aruch HaShulchan (451:50) all explain that the Rama is following the opinion of the Rishonim who rule that glass utensils have the status of earthenware in the context of koshering as well as ritual impurity. Rav Ovadia Yosef (Teshuvot Yabia Omer 4: Yoreh Deah 5:31 and Teshuvot Yehave Da’at 1:6), though, adopts the approach of Rav Yaakov Emden (Mor UKetziah 451) who explains that Rama follows the opinion of Ra’ah cited by Ritva, that in principle glass, like metal, may be koshered, but in practice we forbid it. Since glass is delicate we possibly may not kosher it properly. For example, we may not heat the water sufficiently or we may not cover the entire utensil with boiling water. Ra’ah, therefore, rules that we may not kosher glass lest we believe we have kosher it properly, when in fact we have not.
 There exists a debate whether the requirement for metal utensils acquired from a non-Jew to be dipped in a mikvah is of biblical or Rabbinic origin. The consensus is that it is of biblical origin; see Taz, Yoreh Deah 120:16, Bi’ur HaGera, Yoreh Deah 120:36, and Aruch HaShulchan 120:4. Yalkut Yosef (Y.D. 120:1) states “it appears from Maran Rav Yosef Karo that he believes that it is of biblical origin”.
 Rav David Zvi Hoffman (Teshuvot Melamed Leho’il 2:49) suggests that this decree pertains exclusively to glass and not to all utensils which may be repaired by melting. This suggestion seems to have been adopted by the Halachic community as evidenced by the fact that most observant Jews do not immerse plastic utensils acquired from non-Jews in a mikvah. See Teshuvot Tzitz Eliezer 7:37 and 8:26, Teshuvot Helkat Yaakov 2:163, and Teshuvot Yabia Omer 6: Yoreh Deah 68. Even Rav Yitzchak Yaakov Weisz (Teshuvot Minchat Yitzchak 3:76-78) who rules that plastic utensils must be immersed, takes Rav Hoffman’s suggestion into consideration and rules that one should not recite a blessing when immersing plastic utensils.
 For a summary of these opinions see Rav Gedalia Felder, Yesodei Yeshurun 6:166-168. Rav Felder and Rav Ovadia Yosef (Teshuvot Yehave Da’at 1:6) write that the majority of Rishonim subscribes to the view that glass utensils do not absorb.
 A precedent for this ruling from the Babylonian Talmud is the Talmud’s (Pesachim 74b) assertion that “the heart is smooth and does not absorb.” We see that the Babylonian Talmud believes that some objects do not absorb. See, however, Tosafot ad. locum s.v. Shani.
 Pesachim 30b based on Vayikra 6:21.
 For a discussion of whether Aggadic statements are Halachically binding, see Jerusalem Talmud Peah 2:4 and Pitchei Teshuvah, Even HaEzer 119:5.
 In addition, see Biur HaGera 451:2 and Teshuvot Minchat Yitzchak 1:86 for alternative explanations of Avot deRabi Natan.
 According to the authorities even if one demonstrates that glass is nonporous, glass would still retain the halachic status of “earthenware” which may not be koshered. This is similar to the debate concerning porcelain which Rav Eliezer Waldenberg (Teshuvot Tzitz Eliezer 4:6) summarizes succinctly as follows: “Look at all the ink spilled in an attempt to rule that since porcelain is nonporous it need not be koshered. Nevertheless, the consensus of halachic authorities and the accepted practice among the observant community is to treat porcelain as earthenware which may not be koshered.” The fact that porcelain is nonporous is seen by most authorities as irrelevant.
 Ra’ah rules, however, that since glass may break when placed in boiling water, one may not kosher glass since it is too likely that one will not kosher glass properly. Ra’ah fears that one may believe that he has koshered glass properly when he truly has not. Shaar HaTziyun (451:196) notes that most authorities have not adopted this position since one is required to kosher a utensil only at the temperature at which it absorbed forbidden food. Since one does not place glass in exceedingly hot water one is not required to kosher glass in exceedingly hot water one is not required to kosher glass with water that is so hot that one would fear that the glass would break.
 For a discussion of the linkage of the laws of koshering and immersion, see Ritva Avoda Zara 75b, Teshuvot Chatam Sofer, Yoreh Deah no. 2120, and Rav J. David Bleich, Contemporary Halachic Problems, II pp. 46-47.
 Rav Shlomo Kluger (Teshuvot Tuv Taam V’Daat 3:2:25) and Maharam Shick (Teshuvot Yoreh Deah no. 141) write that Rav Karo’s ruling does not apply if a glass utensil absorbed Hametz via fire. Both Sdei Chemed and Rav Ovadia Yosef reject this interpretation of Rav Karo’s ruling.
 Yalkut Yosef (Orach Chaim 451:39) notes that some Sephardic communities outside of Israel (such as Iraqi Jews) adopt a strict approach in regards to usage of glass on Pesah. Mr. Jack Varon of Congregation Shaarei Orah, the Sephardic Congregation of Teaneck, confirms that this is the custom of the Turkish Jewish community in Seattle, Washington. Yalkut Yosef rules that when they move to Israel they may follow the ruling of Rav Karo, the Mara D’atra of Eretz Yisrael. One wonders what Rav Yosef would rule regarding Iraqi Jews who moved to Israel but later moved to North America, whether they should resume the strict practice their grandparents observed in Iraq.
 Regarding Ashkenazic practice. Rav Yosef follows in the path of Rav Yehuda Leib Graubart (Teshuvot Havalim Be’ni’imim 4:6) in his ruling for Ashkenazim.