In the previous Halacha we have written that according to Maran Ha’Shulchan Aruch, glassware does not absorb any flavor from foods placed in it and thus, there is no prohibition to use a glass vessel for meat and then after it is washed well, to use it for dairy (although the Rama does rule stringently that glass shares the same law as pottery; this is indeed the prevalent custom among many Ashkenazi Jews).
Maran Rabbeinu Ovadia Yosef zt”l ruled in his works and Torah lectures in accordance with the opinion of Maran Ha’Shulchan Aruch that glassware does not require Hag’ala and it may be used for both meat and dairy. There were those who challenged the ruling of Maran zt”l by claiming that although Maran Ha’Shulchan Aruch rules that glass does not absorb, nevertheless, the glass produced today is different than the glass that existed in previous generations. They supported this claim with an experiment which showed that a used glass weighed slightly more than a new glass and they claimed that this was a result of the fact that the glass absorbed some of the liquid stored in it in addition to other such proofs that glassware absorbs.
Nevertheless, nothing has really changed, for Rabbeinu Nissim writes that although glassware does absorb, however, since its absorption is much less than other forms of dishes, this is not enough of a reason to prohibit using it for both meat and dairy. Thus, all of the above experiments proving that glass does absorb are meaningless, for the absorption is so miniscule that it is insufficient to create a prohibition.
Maran Rabbeinu Ovadia Yosef zt”l writes that just as it is permissible to use glass dishes for both meat and dairy or for both Chametz and Pesach according to the Sephardic custom, the same leniency applies to dishes made of “Pyrex” which is heat-resistant, for this material is also considered glass and does not absorb. He writes furthermore that even according to the Ashkenazi custom that glassware shares the same law as pottery and cannot be koshered even through Hag’ala, nevertheless, Pyrex and other heat-resistant glassware (such as “Corelle”) may be koshered by means of Hag’ala, for the reason why the Poskim write that glass cannot be koshered through Hag’ala is only because of the concern that one would not perform the koshering process correctly out of fear that vessel will explode because of the intense heat of the boiling water and not because they are like pottery which cannot release the flavor it has absorbed through Hag’ala. However, there is no such concern regarding heat-resistant glass and even according to the Ashkenazi custom, such dishes may be used for meat and then dairy when Hag’ala is performed in between them. This is indeed the opinion of Hagaon Rabbeinu Yosef Shalom Elyashiv zt”l. The Responsa Tzitz Eliezer rules that there is room for leniency in this regard when Hag’ala is performed three times. According to the Sephardic custom, however, a thorough washing is sufficient, as we have explained.