Whether glass can be koshered is the subject of debate among the early halachic authorities.

There are those who opine1 that glass is a completely non-porous and non-absorbent substance. As such, it never absorbs any traces of chametz or non-kosher foods, and a thorough washing of its surfaces is sufficient to make it kosher.2

On the other extreme, many authorities3 maintain that glass is to be treated like earthenware, a substance which is completely un-kosherable. According to them, there is no way to kosher glass.4

Taking the "middle ground," there are those5 who see glass as a material similar to metal,6 which can be koshered through exposure to high levels of heat.7

Ashkenazi Jewry follow the most stringent view, and do not use chametz glassware on Passover, nor do they kosher non-kosher glassware. If you are of Sephardic ancestry, consult with your rabbi regarding accepted protocol in your community.

Yours truly,

Rabbi Menachem Posner