Is it possible to kasher pyrex dishes for use on Pesach? Rav Yosef begins his answer by presenting three views on whether or not one can kasher glass. According to Avot D'Rabi Natan, glass neither absorbs nor releases absorbed material, and therefore it only needs to be washed out in order to be kashered. According to the Or Zarua, glass can never be kashered, as it is comparable to earthenware. And according to Rabbeinu Yonah, we are unsure if glass is to be considered like earthenware, since it is also made from sand, or like metal utensils, since it can also be fixed when broken, and thus we apply the stringencies of both to it and we do not allow hagala, but we require tevila.
From there, Rav Yosef moves to note that the Shulchan Aruch follows the view that glass can be kashered by mere washing, although the Terumat HaDeshen and the Ramo hold that one may not use previously used glass dishes on Pesach. As such, Rav Yosef rules that Sephardim may follow the lenient view and use pyrex if it has been washed well (he suggests three times). For Ashkenazim, he suggests that the reason that glass cannot be kashered via hagala is that it will break in the heat, but pyrex is meant to withstand such heat, and thus perhaps even Ashkenazim can use pyrex if hagala is done to it.