Dears, I am a little lost on the topic of kashering the glass. I read there is a custom not to kasher glass (for the Askhenazi), but I also read they do kasher glass. Some just by washing it, some by immersing it to the water for 72 hours and some by boiling. Also there seems to be a difference between year round use kashering and kashering for Pesach.

So if one would want to kasher glass and it was used for hot food is there a way to kasher it and which one?

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    – DonielF
    Jan 15, 2019 at 15:25

1 Answer 1


You were right the first time, when you said that Ashkenazi custom is not to kasher glass used for hot foods. This is because once glass absorbs from hot food (assuming that it is capable of absorbing, see below), the absorbed taste cannot be properly expunged from the glass.

The practice of soaking in water is when it was used extensively for cold food or liquids, and washing is when it was used cold for a short time.

That said, Ashkenazic Halachic ruling is that glass does not really absorb, according to the letter of the law (mei'ikar hadin) so the whole matter of kashering glass is somewhat of a stringency (chumrah).

Pesach has special stringencies, of course.

  • Thank you very much. Do you have something on the possibility not to kashering glass at all? (Askhenazi) And what about that boiling thing?
    – Judita
    Jan 15, 2019 at 16:10
  • I have never heard of boiling glass. As regards the possibility of not kashering glass at all, it depends on how the hot food or liquid contacted the glass. Table utensils can be treated more leniently, because it is not by fire (keli rishon). But bottom line: you should either follow an established custom in your family or community, or ask your local rabbi, because there are too many views for this question to have one definitive answer.
    – shmu
    Jan 15, 2019 at 16:12
  • yeah I know that would be the best option but my Rabbi is Sephardi...he just washes the glass dishes and that is all (I am from Prague so I would like to follow the Askhenazi custom - but the lenient one as I clearly do not want to throw away my dishes :)
    – Judita
    Jan 15, 2019 at 16:18
  • Ashkenazim do not generally kasher glass utensils such as the ones you put in the oven. That is absorption by fire (keli rishon) and the custom is to be stringent. If it is something like a tea glass, there is much more room to be lenient. I hope this is enough info to help you out.
    – shmu
    Jan 15, 2019 at 16:27
  • There are many ins and outs regarding how exactly the glass utensil absorbed the forbidden taste, and other factors as well. In order to lenient, it is often necessary to take into account all the details of the case. If I was you, I would either call up an Ashkenazic rabbi in another country, such as England or Israel or America. Or, I would ask the Sefardic rabbi, but request that he rule according to the Rema, rather than according to Sephardic custom. In this case, since the letter of the law is that glass does not absorb, you don't have to worry as much.
    – shmu
    Jan 15, 2019 at 16:35

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