We bought from eBay glass fish-shaped plates, manufactured by Arocroc (France) in 1970 or so. The listing had "unused" in the description but that is not certified. How does one kasher glass plates?
The Sephardic opinion is that glass never absorbs, so it never has kosher issues.
Ashkenazic opinions vary; a lenient (albeit very authoritative) one is that of Rabbi Moshe Heinemann of the Star-K:
Q: Can one purchase and utilize used or antique crystal bowls or glasses?
MEAT / DAIRY MIX-UPS Q: If someone poured hot milk on a cold meat glass/pyrex utensil or hot meat on a cold dairy glass/pyrex utensil, what should be done? A: The dishes should be washed off and not used for 24 hours. The glass/pyrex utensil can then be used as it was originally designated.
Q: If someone poured hot milk on a hot meat glass/pyrex utensil or hot meat on a hot dairy glass/pyrex utensil, what should be done? A: Ask a Sheilah from your Rav.
But as always, ask your rabbi!