Whether glass can be koshered – not only for Passover, but also whether non-kosher glassware can be koshered for year-round use – is the subject of debate among the early halachic authorities.

Many authorities1 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.2

On the other extreme, there are those who opine3 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.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 generally follow the most stringent view, and do not use chametz glassware on Passover, nor do they kosher non-kosher glassware. However, there are exceptions, so consult with your rabbi regarding accepted protocol in your community.

Yours truly,

Rabbi Menachem Posner