Can one kasher glass vessels? If so, could one constantly use a glass dish that can be used for both milk and meat? What are the sources?
The Rema (451:26) writes that glass vessels cannot be kashered by hag’alah.
If the glass vessels were used for cold liquids alone, it is sufficient to fill the glass with water three times, each time leaving the water inside for a 24 hour period.
Some are lenient, based on the ruling of the Shulchan Aruch, to use glass for both meat and dairy, because of the assumption that glass does not absorb. From the Rambam (Maachalos Asuros 17:3), however, we see that glass does absorb, and the Ben Ish Chai (Tzav 14) is stringent in this matter.
In general, it is recommended to have different glasses for meaty and dairy, even if the glass is generally used for cold liquids, to avoid complication.