V Rama - Prohibitions Other Than Passover
Rama records his stringent ruling concerning glass (i.e. that it cannot be koshered) in the context of the laws of Passover. Some authorities (Knesset HaGedolah, Yoreh Deah 121; Kehal Yehuda, Yoreh Deah 121; and Zera Emet 2:Yoreh Deah 43) believe that Rama’s stringent ruling applies exclusively to Passover. These authorities point out that Halachic authorities generally rule more stringently on Passover issues relative to most other areas of Halachah. They also believe that this distinction may be inferred from Rama’s glosses to Shulchan Aruch. In the laws of non-kosher wine (Yoreh Deah 135:8), Rav Karo rules that glass utensils used for storing non-Jewish wine may be used to store kosher wine, and Rama makes no comment. These authorities interpret Rama’s silence as signaling agreement with the ruling that glass does not absorb in Halachic contexts, other than Passover.
Magen Avraham (451:49) disagrees. He writes that Rama’s silence in the context of the laws of wine should be understood as the exception, rather than the rule, since drinking non-Jewish wine is one of the less stringent Rabbinic prohibitions. Thus, in kashrut issues other than wine, Rama would rule that glass utensils cannot be koshered. This also appears to be the opinion of Taz (Orach Chaim 87:2).
Pri Megadim (Orach Chaim, Mishbetzot no. 30), and Aruch Hashulchan (Orach Chaim 451:50) distinguish, in the context of Hilchot Pesach, between utensils which have absorbed hot food and those which have absorbed only cold food. Biblically, absorption takes place only with hot items; the food is then assumed to remain absorbed in the container for twenty four consecutive hours. Pri Megadim and Aruch Hashulchan rule that one can be lenient regarding “cold absorption” in the case of glass, even regarding Pesach. According to this approach, one could argue Rama did not comment on Rav Karo’s ruling in the laws of wine because Rav Karo is not speaking of glass that absorbed hot non-kosher food. However, Mishnah Berurah (451:156) rules regarding Pesach that one may not kosherize glass even if it absorbed only cold non-kosher food, except for exceptional circumstances.
VI Contemporary Poskim
It is generally accepted that Sepharadim follow Rav Karo’s ruling that glass does not absorb even hot food and need not be koshered even for Passover. Similarly, it is generally accepted that Ashkenazim do not kosher glass for Passover, save perhaps for exceptional circumstances. However, there is an active debate whether one may kosher glass utensils for use other than for Passover. Rav Waldenberg (Tzitz Eliezer 9:26) cites Rav Yehudah Leib Zirelson who believes that the accepted practice is never to permit glass utensils to be koshered. This is also the opinion of Rav Shmuel Wosner (Teshuvot Sheivet HaLeivi Yoreh Dei’ah 1:43). On the other hand, Rav Aharon Felder (Oholei Yeshurun p. 87 n. 82) cites Rav Moshe Feinstein, who accepts the opinion that glass utensils do not absorb even hot foods and need not be koshered for non-Passover use.
Many authorities adopt a compromise position regarding non-Pesach use. They rule that glass absorbs hot foods and but may be koshered. Adherents of this view include Rav Waldenburg (Teshuvot Tzitz Eliezer 6:21), Chacham Ovadia Yosef (Teshuvot Yechaveh Da’at 1:6 - regarding Ashkenazim), Rav Yechiel Yaakov Weinberg (Teshuvot Seridei Eish 2:36), and Rav Weisz (Teshuvot Minchat Yitzchak 1:86). These authorities, generally speaking, do not base their ruling on the Talmud’s comparison of glass to metal. Rather, it is a compromise between the opinions which rule that glass may not be koshered and those who rule that glass need not be koshered because it does not absorb.
Rav Menachem Genack, Rabbinic Administrator of the Kashruth Division of the Union of Orthodox Jewish Congregations of America, reported (in 1989) that his organization inquired of Rav Moshe Feinstein whether one may wash glass utensils on a non-kosher dishwasher. Rav Feinstein ruled that one is permitted to do so. When his organization posed this question to Rav Yosef Dov Soloveitchik, however, the Rav ruled that it is forbidden to do so.
Accordingly, it appears that the majority of the outstanding contemporary authorities rule for Ashkenazic Jews that glass is not considered to be nonporous even for non-Passover use, but may be koshered. Rav Hershel Schachter, Rosh Kollel of Yeshiva University, rules that glass may be koshered; however, he requires it to be koshered three times. Similarly, one should not use glass utensils for both meat and milk if either type of food is hot.
 After cleaning the glass utensils, of course. Koshering, though, is not needed.
 Also see the distinction made by Aruch HaShulchan (Orach Chaim 451:50).
 A number of prominent Rabbanim have expressed their opinion to this author that common practice reflects this opinion (see Rav Shimon Eider, Halachos of Pesah, p. 139).
 A similar question may be raised whether one may wash dairy glass utensils in a “meat” dishwasher and vice versa. The answer depends on how one resolves the issue of whether Halacha regards glass as nonporous regarding non-Passover issues.
 Rav Schachter requires this to accommodate the opinion of Baal Ha’Ittur (Sha’ar Hechsher HaBasar) that earthenware may be rendered kosher by placing it in boiling water three times. Although Baal Ha’Ittur’s opinion is not accepted as normative Halachic practice, it is used as a consideration in rendering Halachic opinions; see Aruch HaShulchan, Yoreh Dei’ah 121:26-27; Teshuvot MeLameid LeHo’il 2:52; and Teshuvot Iggerot Moshe 3:26-29). Accordingly, Rav Schachter wishes to use the opinion of the Baal HaI’ttur as a consideration (“Senif Lehakeil”) to rule that one may kosher glass.
 The halachic standard for determining whether something is hot is Yad Soledet Bo, the temperature which causes one’s hand to be withdrawn spontaneously for fear of being burnt. Contemporary equivalents range from 110 degrees to 120 degrees fahrenheit (see Rav Shlomo Zalman Auerbach, Teshuvot Minchat Shlomo 91:8, and Rav Shimon Eider, Halachos of Shabbos p.243 n.19).