Heating cutlery to help it function better?

Question

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.

Answer 1

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.

Answer 2

Another angle to discuss is the possible cooking of the food if the utensil is more than yad soledes.

Here is what I found on dinonline;

On Shabbat is it permitted to put a spoon or ice cream scoop in a mug of hot water (From an urn with water heated prior to Shabbat) in order to make it easier to scoop hard frozen ice cream or sherbert for dessert at the end of the seudah? Answer One can be lenient in this matter. Sources: The problem of melting down the ice cream is not an issue (of nolad), for there is clearly no intention of changing the state of the solid ice cream, and only to make it easier to scoop out. See Mishnah Berurah (318:91). The question, however, is whether or not there is a concern of bishul for the actual touching of the hot scoop with the ice cream: Is this a problem of “cooking” the ice cream. I believe that this will not be a problem, because there is no interest in any “cooking,” and the actual “cooking” that takes place is entirely negligible. This is somewhat similar to the question of pouring hot water into a cup that is not entirely dry, for which Rav Shlomo Zalman zt”l was lenient, because the cooking is negligible, and one has no interest in it. An added factor is that ice cream is general pasteurized, so that there are no major ingredients that have not already been cooked, and therefore the issue will only be bishul achar bishul. This is especially the case for ice cream, which is solid, though the issue of something that becomes liquid as it cooks might apply.

Answer 3

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:‏