The process of koshering utensils is intended to clean the vessel, not of the residue left on the cutlery, pot or pan, but rather of the taste that gets absorbed in the vessel. Halachically, when one cooks food, the flavor of the food gets absorbed (or is “swallowed”, to use the Halachic term) within the walls of the utensil. Therefore, after cooking non-Kosher food we must kosher our vessels, to cleanse them of the taste absorbed within them.

As a result of its unique non-porous structure, glass will never soak up any substance within its walls

However, an additional halachic opinion is that glass, much like metal, need be koshered via intense heat exposure.

There is a third halachic opinion which maintains that glass is indeed absorbant (as is the case with other forms of earthenware vessels). Generally speaking, with regards to the holiday of Pesach, Ashkenazim are stringent, and follow this opinion. Therefore on Pesach glass utensils that were used for Chametz are unacceptable for Passover use. And, as is the case with other earthenware vessels, they cannot be koshered through boiling or blowtorching.

For the most part, many Sephardim do not follow this stringency, and even on Pesach will use glass utensils which had been used for chametz.

Please consult your own Orthodox Rabbi to decide if and how glass may be koshered year-round and on Passover.