The laws of koshering utensils differ, depending on the material the utensil is made out of, as well as how it was used. The Torah does not address the question of glass, which was not used in Moses’ times.

So how is it to be classified?

On the one hand, similar to clay pottery, glass is essentially made from sand. Thus, the Talmud states that glass can become ritually impure with a similar halachic status as pottery.1

On the other hand, since glass can be repaired through melting it down and reforming it, it needs to be immersed before its initial use,2 similar to metal utensils.3

What about koshering?

The Torah tells us that materials like metal can generally be purged through exposure to high temperatures. Pottery, on the other hand, cannot be koshered in this way, primarily because it is very porous.

Kashering Glass

The question of whether glassware can be koshered, year-round and especially for Passover, is the subject of debate among the early halachic authorities.

Many authorities maintain that since glass is made of sand, glassware is deemed to be earthenware. As such, hag’alah (immersion in boiling water) is not effective in kashering glass utensils.4

Other authorities are of the opinion that glass is a completely non-porous and non-absorbent substance.5 As such, it never absorbs any traces of non-kosher foods or chametz, and a thorough washing of its surfaces would suffice to make it kosher.6

And yet others are of the opinion that it is similar to metal, which can absorb the non-kosher food but can also be koshered.7

Practical Applications

In general, Sephardic communities follow the more lenient opinion that glass is a completely non-porous and non-absorbent substance.8

In principle, Ashkenazi Jewry may agree with the third opinion that glass absorbs but can be kashered. However, in practice, most are stringent not to kasher glass.9 Some argue that this stringency only applies to glass which absorbed chametz, and one wishes to kosher it for Passover.10 Others, however, are stringent all year round.11

At the same time, the way glass usually comes into contact with food may be different than other sorts of materials. For example, it is most likely that one’s glassware will consist of cups or plates, which are generally not used with very hot food.

In light of the above, in cases of need, one should consult a rabbi regarding the accepted protocol in one’s situation and community.