[Avodah] Materials valid for a Keli Sheni

[Akiva Miller]

.
As I prepare my tea and coffee on Shabbos, I sometimes marvel at the idea
that when I pour the water from the urn to a cup and then to another cup,
it has cooled down enough to be incapable of Bishul (according to Rav Moshe
Feinstein). And yet, my insulated mug keeps it above yad soledes bo for
close to an hour - no exaggeration!

Skimming through some random issues of the OU's "Daf Hakashrus", I came
across this psak from Rav Yisroel Belsky z"l: (
https://oukosher.org/content/uploads/2017/10/2017.09-10-Daf25-10g.pdf -
page 19 of the pdf file, marked as page 57 in the publication)

> Thermoses and Styrofoam cups are designed to *preserve* heat.
> Since the leniencies associated with a keli sheni stem from
> the *cooling* effect of the walls of the vessel, Rav Belsky has
> stated that Thermoses and Styrofoam cups do not have the status
> of a keli sheni when receiving food and beverages from a keli
> rishon.

It's not clear whether Rav Belsky was talking about Keli Sheni in Shabbos
or in Kashrus. But even if he was referring to the kashrus implications, it
would certainly apply to Shabbos as well. According to this, when one takes
hot water from the urn, it should go into an ordinary keli, not one which
is designed to preserve heat, so that it would be a keli sheni. I would
presume that if one wants to end up with a keli shlishi, that one too,
would need to be an ordinary keli, not specifically designed to preserve
heat. Then - and *only* then according to Rav Belsky - is it cool enough to
be considered a keli shlishi.

This psak is appealing (as I explained in my first paragraph), even if it
is inconvenient. But it turns out that Rav Shimon Eider (page 384) brings
numerous sources that a "Thermos" or "insulated cup" DO become a keli
sheni. Shemirat Shabbat K'Hilchata (1:70 in the 5739 edition, 1:2 in the
5770) also says that a Thermos bottle becomes a keli sheni.

Akiva Miller
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