What potential Halachic issues might there be (if any) with heating a knife on Shabbos or Yom Tov to help it cut through a frozen cake? The heating could be accomplished in a number of ways, including but not limited to:

  • heating over a burner
  • putting it in the oven for a short while
  • running it under hot water

(Another equivalent case might be heating a spoon to help scoop ice cream.)

A good answer should provide sources that discuss the background of the Halachic problem that you think might apply to this case, and explains why you think it applies here.

  • Is it glowing? How hot is this getting? Forging metal can be problematic – Double AA Apr 2 '18 at 13:28
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    @DoubleAA up to you - I would think that is the beginning of a good answer... (For example: "one issue that relates to this would be the permissibility of heating metal objects, which is discussed in bleh bleh bleh. It is only an issue if bleh bleh bleh etc. ) BTW, being that I'm not asking about a specific or exact case, if you feel it is too broad, vote to close. – רבות מחשבות Apr 2 '18 at 13:34
  • @msh210, I rolled back the retag, as there may be slight adjustments based on if it is Yom Tov or Shabbos (e.g. if there is an Issur of Bishul involved), and would therefore prefer to leave it as a Yom Tov question as well. – רבות מחשבות Apr 2 '18 at 14:35
  • bishul applys to non-foods like metals. Forging issues are worse if it is a new utensil IIRC – David Kenner Apr 2 '18 at 16:05
  • If you put it in the oven you will ruin the handle. If you heat the blade till it is red hot you will also ruin the handle. I doubt there is an issur molid on the frozen cake because the cake does not become a liquid when cut so you have not changed the state of the cake. – The GRAPKE Dec 7 '20 at 2:49

I can think of 2 problems:

  1. Heating the knife: if it could become red-hot then it would be considered a Tolda of cooking.

  2. Melting food: causing food to melt could also be a Tolda of cooking.

Both of these are codified in the Rambam Hilchos Shabbat Ch. 9 (though I cannot find their parallel in the Shulchan Aruch.)

ו: הַמַּתִּיךְ אֶחָד מִמִּינֵי מַתָּכוֹת כָּל שֶׁהוּא אוֹ הַמְחַמֵּם אֶת הַמַּתָּכוֹת עַד שֶׁתֵּעָשֶׂה גַּחֶלֶת הֲרֵי זֶה תּוֹלֶדֶת מְבַשֵּׁל.‏
וְכֵן הַמְמַסֵּס אֶת הַדּוֹנַג אוֹ אֶת הַחֵלֶב אוֹ אֶת הַזֶּפֶת וְהַכֹּפֶר וְהַגָּפְרִית וְכַיּוֹצֵא בָּהֶם הֲרֵי זֶה תּוֹלֶדֶת מְבַשֵּׁל וְחַיָּב.‏
וְכֵן הַמְבַשֵּׁל כְּלֵי אֲדָמָה עַד שֶׁיֵּעָשׂוּ חֶרֶס חַיָּב מִשּׁוּם מְבַשֵּׁל.‏
כְּלָלוֹ שֶׁל דָּבָר בֵּין שֶׁרִפָּה גּוּף קָשֶׁה בָּאֵשׁ אוֹ שֶׁהִקְשָׁה גּוּף רַךְ הֲרֵי זֶה חַיָּב מִשּׁוּם מְבַשֵּׁל:‏

  • A bit of clarification and organization would make this a good answer – רבות מחשבות Jul 20 '18 at 17:50

This question would be relevant when one wants to place a blech on a fire on Shabbos. Regarding that, there is a disagreement between Reb Moshe Feinstein, who says placing a blech on a fire is permitted on Shabbos, even though it will become Yad soledes. While the Chazon Ish is of the opinion, that is is forbidden because it is Chatzi shiur of cooking. So, there is a disagreement if chatzi shiur applies just to half the amount, or even to half the accomplishment of a full shiur. Source: חזון איש, או"ח סימן ל"ז ס"ק י"א סוף ד"ה שהי', סימן נ' ס"ק ט' ד"ה ויש, אגרות משה או"ח ח"א סימן צ"ג ד"ה ומה, שו"ת מנחת שלמה חלק א' סימן י"א הערה 2.

In short, heating metal even when it won't get red hot may be forbidden because of a chatzi shiur of bishul.

  • I don't see how this answers the question. – רבות מחשבות Jul 20 '18 at 17:50
  • @רבותמחשבות - The question was about issues with heating a knife on Shabbos. The answer given here, is that heating metal may be forbidden because of a chatzi shiur of bishul. – פרי זהב Jul 20 '18 at 18:08
  • @רבותמחשבות - My point was even if it doesn't get red hot it may be forbidden. – פרי זהב Jul 20 '18 at 18:14
  • I think I understand, but could you please write clearly what you think the Issur would be here, and how it would apply. For example, you might want to mention cutlery in your answer... – רבות מחשבות Jul 20 '18 at 18:17
  • @רבותמחשבות - Sorry about that. Does this edit make it better? – פרי זהב Jul 20 '18 at 18:26

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