Using a non-Kosher glass vessel of a gentile:
The status of glass is disputed amongst the Poskim, with some ruling that it does not need to be Kashered even if used for non-Kosher hot foods, and others ruling that it has the same status as earthenware and cannot be Kashered. Practically, although the custom amongst Ashkenazi Jewry is to be stringent and not Kasher Chametz glass vessels for Pesach, regarding other Issurim [non-Kosher foods] they are lenient to use them without even requiring Kashering. Thus, they purchase old glass vessels from gentiles and use them after simply washing them clean. Nevertheless, some Poskim rule that Ashkenazim are to be stringent with non-Kosher glass vessels and Kasher them before using.
Wine glasses-Glass vessels of a gentile used to drink and store Yayin Nesech: It is permitted to drink from clean wine glasses, and other clean wine vessels of a gentile without Kashering them in between even if they were used to drink and store Yayin Nesech. This applies even for Ashkenazim. [This applies even if one lent the wine glasses to a gentile.] Thus, one may use wine glasses of a non-Kosher bar, hotel lobby, or restaurant to drink, so long as they are cleaned well beforehand.
Glass cookware of a gentile-Heat resistant glass such as Pyrex/Dulerex: Tempered glass which is manufactured to have a high heat resistance and is used to cook with on a fire or in an oven is debated amongst the Poskim as to its status, and whether it has the same laws as the glass discussed above. Some Poskim rule it is more strict than regular glass and hence the non-Kosher glass cookware of a gentile is forbidden to be used unless it is Kashered. Other Poskim, however, rule it has the same status as all glass and may hence be used by Sefaradim so long as the cookware is cleaned well after purchasing from the gentile.
 See Admur 451:73; Michaber 451:26; Darkei Teshuvah Y.D. 121; Sdei Chemed Mareches Hei 29; Hakashrus 3:52-54; Koveitz Hearos 770 Yud Shevat 5771
 The three opinions: 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 and final ruling of Beis Yosef 451:26; 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 and Hagahos Maimanis 5 in name of Yechiel Miparish; Terumos Hadeshen 1:132] Some Poskim rule that the vessel is Kasherable through Hagalah just like metal. [Admur ibid and M”A 451:49 regarding Bedieved; See P”M 451 A”A 49 that the M”A ibid holds glass is Kasherable; Or Zarua Pesachim p. 58  in Pirush of Rav Shmuel Miplaish to Piyut of Elokei Haruchos “Glass..if it was used with hot it suffices with Hagala as they are only similar to earthenware regarding Tuma, however regaridng Hagalah they are like metal.”; See also Shibulei Haleket; Ritva Pesachim in name of Reah]
 Admur ibid; Rama ibid; Iggur ibid
 See Kneses Hagedola Y.D. 121:25 [that even the Machmirim are only stringent regarding Pesach], recorded in P”M 451 M”Z 31 [see Minchas Yitzchak 1:86]; Kehal Yehuda Y.D. 121 “Regarding other Issurim the custom is not to be stringent at all”; Yad Yehuda Y.D. 69 Aruch 89 Katzar 17; Shevilei David Y.D. 121:6 [lenient to use for cold foods by opposite meal]; Aruch Hashulchan Y.D. 121:2 “So is the widespread custom to buy all forms of glass vessels from gentiles and use them without Kashering.”; Poskim in Darkei Teshuvah Y.D. 121:2; Mishneh Halachos 9:168 regarding using glass cups/dishes for meat and milk; .”; See Hakashrus 3:53 that the Ashkenazi custom is to be lenient to Kasher them; See Admur 87:2 regarding a glass potty and in Kuntrus Achron 87:2 “In 451 it is only an initial stringency”; See M”A 451:48 who contrasts the stringency of Chametz to the leniency by Yayin Nesech “Although we are stringent by Chametz, do not compare it to Yayin Nesech, as many leniencies apply by Yayin Nesech and not by Chametz”
 Aruch Hashulchan ibid
 Maharam Shick Yoreh Deah 141 regarding glass cups of Issur; Sdei Chemed ibid 30; Shevilei David Y.D. 121:6 [stringent against using for hot foods by opposite meal]; See all Poskim below regarding Pyrex who rule that by other Issurim we may only be lenient to Kasher the glass, but not to use without Kashering [However, it is unclear if these Poskim would apply their stringency only to glass cookware or even to glass cups]; See M”A 451:48; Minchas Yitzchak 1:86; Chelkas Yaakov 2:163 [not to allow an egg with blood to soak in a glass vessel]
 Michaber Y.D. 135:1 [Not a problem by any vessel unless wine is stored in it for a while]; 135:8 [By glass is not a problem even if stored in it]
 M”A 451:48 “Although we are stringent by Chametz, do not compare it to Yayin Nesech, as many leniencies apply by Yayin Nesech”; Rama ibid omits writing any gloss on this statement of Michaber ibid, and so is implied from Shach 135:23 and Taz 135:11 who both comment on this ruling and explain that glass is smooth and does not absorb.
 See Michaber and Rama 136:1 that vessels designated for wine require a Chosem and otherwise need to be Kashered; However, see Shach 136:3 that the entire stringency here is because it’s as if he entered it for storage, and hence since by glass we are lenient even if it is entered for storage, therefore it does not need to be Kashered.
 See Piskeiy Teshuvos 451:54
 The reason for differentiation: The glass that was around in the times of the Talmud and Poskim was not heat resistant and hence would never be placed on a fire for cooking. Accordingly, perhaps the leniencies stated in the Poskim regarding glass only applied towards glass that was not used over a fire, while heat resistant glass would be more stringent and require Kashering according to all. In addition, some Poskim raise the fact that in the manufacturing of Pyrex company heat resistant glass, a small percentage of aluminum is added, thus changing the equation of the entire leniency mentioned in Poskim relevant to pure glass.
 See Tuv Taam Vedaas 2:25 [regarding glass lamps of Cheilev]; Mahram Shick Yoreh Deah 141; Yaskil Avdi 4:13; Tzitz Eliezer 8:21; 9:26; Shearim Hametzuyanim 126:11; Minchas Yitzchak 1:86; Beis Avi 1:115; Yesod Yeshurun 6:170; Shraga Hameir 7:143; Shevet Halevi 2:43; Mishneh Halachos ibid “Since they cook on the fire Chas Veshalom to be lenient and meat and milk even one after the other”; Siddur Pesach Kehilchaso 9 footnote 27 in name of Rav Elyashiv; Rav Mordechai Eliyahu, brought in Sefer Hakashrus ibid footnote 148; See all Poskim regarding Bedieved; Piskeiy Teshuvos 451:54
Other opinions: See article of Rav Braun in Koveitz Hearos ibid who concludes Ashkenazim are stringent even by other Issurim that it cannot be used even in Kashered.
 The reason: As cooking on glass is more severe than the case discussed in the Shulchan Aruch which only involved using glass cups and plates, while here the food is actually cooked in the glass on the fire, and is a question of a Biblical prohibition. [See Poskim ibid; Mishneh Halachos ibid] Alternatively, as Pyrex and Duralex glass contains metal additives to the formula and hence it does not have the same status and leniency as regular metal. [Tzitz Eliezer ibid]
 Yechaveh Daas 1:6; Yabia Omer Y.D. 4:5