Question: Must one designate two different sets of glassware for dairy and meat as one would with other utensils?
Answer: We have already established in the previous Halacha that one is obligated to designate two separate sets of dishes and flatware for dairy and meat, for dishes used with either (hot) dairy or meat will absorb some of the food’s flavor and will then release some of this flavor into another food cooked in them.
The great Rishonim disagree regarding whether or not glass vessels absorb flavor from foods like other utensils. There are three opinions regarding this matter: The Ohr Zarua, Samag (Sefer Mitzvot Gadol authored by Rabbeinu Moshe of Coucy), Rabbeinu Yonah, and others maintain that glassware retains the same law as earthenware and pottery in that it absorbs flavor from foods stored in them but it cannot be koshered even by a process of Hag’ala in boiling water (for these kinds of dishes can never fully release the flavors stored in them and will always release small amounts of that flavor, be it dairy or any other flavor of a forbidden food and for this reason, Hag’ala does not suffice). The reason why these Poskim rule that glass retains the same law as pottery is because glass is formed from sand and is therefore similar to pottery.
The Rambam rules that although glass absorbs flavor like other forms of dishes, they are not comparable to pottery which cannot be koshered through Hag’ala; rather, glass can be koshered through Hag’ala like other metals.
On the other hand, the Tosafot, Rabbeinu Tam, Rashba, Rosh, and many other great Rishonim rule that glass does not absorb flavor at all and may be used for both dairy and meat. Maran Ha’Shulchan Aruch (Chapter 451) rules in accordance with this view and this is the prevalent Sephardic custom as attested to by the greatest Sephardic sages who write that one need not designate two sets of glassware for both dairy and meat. In the words of Maran Ha’Shulchan Aruch: “Glassware, even when used with hot food requires no method of koshering since it does not absorb and a thorough washing is sufficient.”
Nevertheless, the Rama rules stringently in accordance with the opinion that glassware shares the same law as pottery in that it absorbs flavor but can never be koshered even through Hag’ala. He writes that this is the prevalent Ashkenazi custom. Some Ashkenazi Poskim write that the Rama only meant to rule stringently on this matter regarding Pesach, for there are many stringencies adopted then because of concern of Chametz which is a Karet-bearing prohibition, however, regarding the laws of milk and meat, even the Rama would rule leniently. Nonetheless, Hagaon Harav Eliezer Yehuda Waldenberg zt”l rules stringently on this issue even with regards to the laws of milk and meat in his Responsa Tzitz Eliezer.
Summary: The Sephardic custom is to use glassware for both meat and dairy. Nevertheless, the Ashkenazi custom is to require two sets of glassware for meat and dairy.