Question:
can you kasher a glass for peasach
Answer:
If the glass has absorbed something inside it with heat the Ashkenazi minhag according to most poskim is not to kasher it for pesach.
Sources:
Rema 451-26, Chayei Adam 125-22, Shulchan Aruch Horav ibid 73, Kitzur Shulchan Aruch 116-13, Aruch Hashulchan 451-50, Igros Moshe O:CH 5-32(1).