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Can one Kasher glass vessels for Pesach:
It is disputed amongst the Poskim as to whether glass vessels need to be Kashered even if used for hot Chametz foods or if it has the same status as earthenware and cannot be Kashered. Practically, the custom amongst Ashkenazi Jewry is not to Kasher for Pesach any glass vessels which have absorbed Chametz. Thus, if the glass vessel had hot Chametz poured into it, or had Chametz soak in it for 24 hours, it is not to be Kashered. This applies even if the majority usage of the vessel is for cold foods, and the vessel is not Ben Yomo. These vessels are not to be used for Pesach, and are rather to be put away with the Chametz vessels. [Sephardim, however, are lenient to allow using glass vessels even without Hagala, so long as the glass has been washed and cleaned well. It is a dispute amongst Poskim if Pyrex material glass may be Kashered.]
Glass coated vessels: If the vessel is coated with glass on its interior, in the area where the food is placed, then it may not be Kashered. If it is coated with glass only by its exterior, then if it is never commonly placed directly over a fire to cook in, such as silver vessels coated externally with glass, then it may be Kashered through Hagala. If, however, it is not uncommon to cook with it over a fire, or even to occasionally heat food in it over a fire, then it may not be Kashered.
Story Alter Rebbe and Maggid and his glass Kiddush cup:
This story was related by Chassidim in name of the Tzemach Tzedek: It was customary of the students of the Great and Holy Maggid to remain by him for the Pesach Seder, and so was followed by the Alter Rebbe in one of his early years by the Maggid. The arrangement with the students was that they provide the utensils for the meal, while the Maggid provides the wine and Matzah. In his younger days, the Alter Rebbe was very poor and could not afford to even purchase a new Kiddush cup for the Seder night. He was in a dilemma; he either had to go begging for money, or be lenient to Kasher the glass cup through Miluiy Veiruiy. After a long inner debate, he concluded to Kasher the vessel and not throw himself until the public. When the night of the Seder arrived, all the students sat around the table with their cups prepared with wine to begin Kadesh. When the Maggid lifted his cup to begin the Kiddush he suddenly exclaimed “I smell Chametz.” He instructed his student, Rav Menachem Mendel of Vitebsk to search around the table to see the source of the Chametz smell. The student circled the table three times and came up empty handed. When the Maggid saw that the smell was yet to be uncovered, he exclaimed “What can I do, the Rama is standing opposite me with a very angry expression and will not allow me to make Kiddush.” When the Alter Rebbe heard this he immediately understood that he, and his Kashered glass cup, were the source of the commotion, as the Rama rules that glass cannot be Kashered. The Alter Rebbe proceeded to confess to the Maggid, explaining to him his predicament that led to the Kashering. In return, the Maggid got up and kissed the Alter Rebbe on his head, saying “You power is very great as you were able to bring the Rama to come down.” The Maggid then instructed the Alter Rebbe that he will drink the four cups of wine from his personal cup.
 Admur 451:73; Michaber 451:26
 Some Poskim rule that glass is a hard and smooth material which cannot absorb the taste of foods and thus does not require Hagala. [1st and Stam opinion in Admur ibid; Michaber ibid; Ravayah 464; Rashba 1:233; Ran 2:9] Rather, one is to simply clean and wash them from any possible Chametz and they may then be used for Pesach. This applies even if they were used for cooking hot Chametz during the year or had Chametz, such as bear, stored in them for long periods of time. Other Poskim, however, rule that glass has the same status as earthenware, and can never be Kashered. Thus, if a glass vessel was used for hot Chametz, or had Chametz soak in it for over 24 hours, it may not be used during Pesach. [2nd opinion in Admur; Rama ibid; Mordechai 574; Semag L.S. 78; Iggur 737 in name of Yechiel Miparish] Some Poskim rule that Bedieved the vessel is Kasherable through Hagalah just like metal. [Admur ibid and M”A 451:49; See P”M 451 A”A 49 that the M”A ibid holds glass is Kasherable]
Are Ashkenazim stringent regarding glass also by other Issurim? This matter is disputed in Poskim.
 Admur ibid; Rama ibid; Iggur ibid
 Bedieved: If one placed hot Pesach food in a glass Chametz vessel, then if the vessel was most commonly used for cold Chametz that did not soak in it for periods of 24 hours, while hot Chametz and soaking Chametz for 24 hours was its minority usage, the food remains Kosher [if the vessel was not Ben Yomo]. If, however, the vessel was majority used for hot Chametz foods, or for soaking cold Chametz for past 24 hours, then the Pesach food may not be eaten, if the vessel did not have Hagala or Iruiy Vemiluiy done to it. If the vessel was Kashered, then the food remains Kosher in all cases. [Admur ibid; M”A 451:49; M”B 451:155; See however Taz 451:30 who is lenient even if the vessel was not Kashered at all, as explained in P”M 451 A”A 49] If the hot Pesach food was poured from a Keli Sheiyni onto the vessel, then seemingly the food likewise remains permitted, even if the vessel was majority used for hot Chametz, and even if the vessel was not Kashered. Vetzaruch Iyun. If the vessel is not Ben Yomo at the time of use, some Poskim are lenient in a case of great loss. [M”B 451:155]
Time of need: Some Poskim rule that in a time of need, if there are no other cups or vessels available, one may be lenient to Kasher the vessel with Hagalah or Miluy Veiruiy. [Chayeh Adam; M”B 451:156 and Shaar Hatziyon 451:201]
 As so rules Michaber ibid; Yechaveh Daas 1:12; However, some Sefaradim are stringent to Kasher the vessel. [Rav Poalim 3:29 that so is custom of Bagdad] Others are stringent not to use it at all as rules the Rama. [Sdei Chemed Mareches Hei 29
 See Piskeiy Teshuvos 451:54; The following Poskim say its Kasherable: Tuv Taam Vedaas 2:25; Mahram Shick Yoreh Deah 141; Yaskil Avdi 4:13; Tzitz Eliezer 8:21; 9:26; Yechaveh Daas 1:6; Yabia Omer 4:41 [Rav Ovadia holds that for Sephardim they do not need to be Kashered just like glass] ; Shearim Hametzuyanim 126:11; The following Poskim say it may not be Kashered for Pesach but may be Kashered the rest of the year: Minchas Yitzchak 1:86; Beis Avi 1:115; Yesod Yeshurun 6:170; Shraga Hameir 7:143; Shevet Halevi 2:43
 Admur 451:75
 The reason: As the custom is today to never Kasher glass through Hagala just like an earthenware vessel. [ibid]
 The reason: As although the Chametz which the glass absorbed will not come out through Hagala, and there is thus suspicion it may enter into a hot Pesach food that one places in it, nevertheless, it is allowed, as the heat of a food which is off the fire does not have ability to remove the absorbed taste of an outer coating and then bring it back into its own food. This applies even if the material of the vessel is metallic in which case we say that when part of it becomes hot it makes the entire vessel hot, as although it has the ability to heat up the entire vessel, it does not have the ability to remove the absorbed taste from the outer coating. [Admur ibid]
 The reason: As when food is heated in it over a fire, the heat of the fire has the ability to remove the Chametz taste that is still absorbed in the glass coating and never became removed through Hagala. [Admur ibid] Thus, we do not allow it to be Kashered even if one plans to never use it over the fire on Pesach, as we suspect one may come to do so. [So is implied from Admur ibid]