A dispute arose in the time of the Rishonim regarding glass kelim. Some say that since glass is smooth and hard, and even if it held hot foods, the taste of those foods is not absorbed into it and does not adhere to it. Therefore, even if glass kelim were used with prohibited foods or ḥametz throughout the year, it is sufficient to clean them thoroughly to enable their use for kosher food or on Pesaḥ (Raavyah, Rosh, Rashba, Ran, SA 451:26).
Others disagree, saying that since glassware is made of sand, like earthenware, even if glass kelim do not actually absorb, they have the status of earthenware, which cannot be kashered. Therefore, if one used them with hot ḥametz foods, there is no way to kasher them for Pesaḥ (Rabbeinu Yeḥiel, Smag, Rabbeinu Peretz, Terumat Ha-deshen, and Rema).
Still other Rishonim adopt a middle position. In their view, glass kelim have the same status as metal kelim. If they were used with boiling hot ḥametz food, they must be kashered by means of hagala in boiling hot water (Rambam, Or Zaru’a, and Shibolei Ha-leket).
Many Sephardim follow the lenient opinion and kasher glassware by rinsing it only, and many Ashkenazim have the custom not to kasher glassware. In practice, however, it seems that the middle position, according to which glassware has the same status as metal kelim and can be kashered by means of hagala, is primary. Those whose family custom is to be lenient may maintain their custom, and those whose family custom is to be stringent should maintain their custom.