May glass vessels/cookware be used for both meat and dairy products?
The three opinions: 1) Some Poskim rule that glass is a hard and smooth material which cannot absorb the taste of foods and thus does not require Kashering from dairy to meat or from non-Kosher to Kosher. [Accordingly, a glass vessel may be used for both dairy and meat foods even if the foods are hot and are heated within the glass, so long as the glass is cleaned well in between uses. Likewise, a glass vessel of a gentile can be washed well and then used to cook Kosher food. Likewise, one’s glass Chametz vessels can be washed well and 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.] 2) Other Poskim, however, rule that glass has the same status as other metal vessels which absorb taste and must hence be Kashered prior to use from meat to milk or Chametz to Pesach. 3) Other Poskim, however, rule that glass has the same status as earthenware, and not only does it absorb taste of food, but can never be Kashered. Thus, if a glass vessel was used for hot Chametz it may not be used during Pesach. Likewise, a glass vessel used for hot meat may not be used for dairy, and a non-Kosher glass vessel may not be used for Kosher foods.
The final ruling-Using glass plates, cups, bowls for Meat & Milk: 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, the custom is to use glass plates, bowls, and cups for both dairy and meat foods even if the foods are hot when poured into the glass, so long as the glass is cleaned well in between uses. Nevertheless, some Poskim are stringent and rule that Ashkenazim are to be stringent with glass vessels just like on Pesach, and not switch their use between meat and dairy. Practically, the widespread custom today amongst many is to initially be stringent and designate separate glass vessels for both meat and milk, however those who are lenient may continue their custom.
Glass cookware-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 is forbidden to be used for cooking or baking of both meat and milk, and it must thus be designated for one food. Other Poskim, however, rule it has the same status as all glass and may hence be used by Sefaradim on Pesach, and may be used for cooking meat and milk one after the other so long as the cookware is cleaned well in between uses.
Ashkenazim: The Ashkenazi custom is not to use any Chametz glass vessels on Pesach even if they are Kashered. However, glass eating vessels [i.e. cups, plates, bowls] may be used for both meat and milk even if hot, so long as they are well cleaned in between uses. However, glass cookware, such as Pyrex and Duralex, may not be used for meat and milk even if it is well cleaned in between uses. If one did so then the vessel is to be Kashered. Practically, the widespread custom today amongst many is to initially be stringent and designate separate glass vessels [i.e. cups, plates, bowls] for both meat and milk, however those who are lenient may continue their custom.
Sefaradim: The Sefaradi custom is to allow using glass eating vessels [i.e. cups, plates, bowls] for both meat and milk [even if hot] so long as they are well cleaned in between uses. The same applies regarding using glass Chametz vessels for Pesach, or using the glass vessels of a gentile for Kosher food. Some Sefaradi Poskim are lenient even regarding glass cookware, such as Pyrex and Duralex, to allow cooking meat and dairy one after the other so long as they are cleaned well in between. Others, however, are stringent.
 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
 1st and Stam opinion in Admur ibid; Michaber ibid and final ruling of Beis Yosef 451:26; Michaber Y.D. 135:8 and Taz 135:11 regarding Yayin Nesech; Ravayah 464; Rashba 1:233; Ran 2:9
 Admur ibid
 Bedieved opinion in Admur ibid and M”A 451:49; 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 regarding Hagalah they are like metal.”; See also Shibulei Haleket; Ritva Pesachim in name of Reah
 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
 See Darkei Teshuvah Y.D. 121:2
 Admur ibid; Rama ibid; Iggur ibid
 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 “The world is accustomed to use it for meat and milk and since this is the case, leave them be, as if they are not prophets they are the son of prophets.”; 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”
 Mishneh Halachos ibid regarding plates and cups, however not regarding tempered glass used for cooking [duralex/pyrex], as explained next
The reason: As a) Chametz contains many stringencies that we do not apply towards other Issurim. b) Meat and milk are independently Kosher and can only become Biblically forbidden if cooked together. Thus, the issue of using glass plates and cups for both meat and milk is only a Rabbinical prohibition, and by a Rabbinical doubt we are lenient. Furthermore, the use of glass plates and cups is at its best a case of Nat Bar Nat Diheteira. [Mishneh Halachos ibid] c) See Yabia Omer Y.D. 4:5 in great length that we do not suspect that one may come to
 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]
 See Mishneh Halachos ibid “I heard from one Gadol that when his wife desired to purchase a set of glass cups for meat and a set of glass cups for milk, he protested against her doing so stating that his mother did not keep such a Chumra, and she had a house visited by the Geonei and Tzadikei Hador, and so was practiced in the homes of other Geodlei Yisrael of the previous generation, and hence I do not want to swerve from the custom of my mother’s house.”
 See Piskeiy Teshuvos 451:54
 The reason for differentiation: 1) 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. 2) 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. 3) In addition, many Poskim rule that the custom is to prohibit switching usages between meat and milk even if one Kashers the vessel in between as one may come to forget to Kasher it, and the same should apply here, as one may forget to clean the vessel.
 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
Bedieved, if glass cookware used with non-Kosher or meat and milk-May the cookware be Kashered? Yes, it may be Kashered through Hagalah. [Poskim ibid; Maharam Shick ibid; Minchas Yitzchak ibid; Tzitz Eliezer ibid] See article of Rav Braun in Koveitz Hearos ibid who concludes it cannot be Kashered even by other Issurim.
 The reason: 1) 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] 2) 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] 3) As, many Poskim rule that the custom is to prohibit switching usages between meat and milk vessels even if one Kashers the vessel in between as one may come to forget to Kasher it [M”A 509:11], and the same should apply here, as one may forget to clean the vessel. [Rabbanim brought in Yabia Omer Y.D. 4:5; The Yabia Omer discusses [only] this point throughout his Teshuvah, and after analyzing all the opinions, concludes that we do not suspect for it]
 Poskim ibid
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.
 Yechaveh Daas 1:6; Yabia Omer Y.D. 4:5 [discusses only the third issue mentioned above, and after analyzing all the opinions, concludes that we do not suspect for it]