סימן עב סעיף כא
הַמַּטְמִין קַאפֶע בְּעֶרֶב שַׁבָּת בְּתוֹךְ הַגּוּמָה לְצֹרֶךְ שַׁבָּת וּמְכַסֵּהוּ בְּכָרִים וְכַדּוֹמֶה, שֶׁיְהֵא חַם, אִם הוּא מַטְמִינוֹ בְּתוֹךְ הַחוֹל, אָסוּר לְהַטְמִין כָּל הַכְּלִי בְּתוֹךְ הַחוֹל. וַאֲפִלּוּ אֵינוֹ מַטְמִין כָּל הַכְּלִי אֶלָּא מִקְּצָתוֹ וְעַל מִקְּצָתוֹ מְכַסֶּה בִּבְגָדִים וְכַדּוֹמֶה בְּאֹפֶן שֶׁכָּל הַכְּלִי מְכֻסֶּה מִכָּל הַצְּדָדִים, גַּם זֶה אָסוּר, אֶלָּא צָרִיךְ לַעֲשׂוֹת בְאֹפֶן זֶה, שֶׁהַכְּלִי אֲשֶׁר בְּתוֹכוֹ הַקַּאפֶע יַטְמִינוֹ רַק עַד חֶצְיוֹ אוֹ עַד שְׁלִישִיתוֹ בְּתוֹךְ הַחוֹל, וְהַמּוֹתָר יְהֵא בָּאֲוִיר, וְיִתֵּן עַל הַגוּמָּה דַּף אוֹ יִכְפֶּה עָלָיו כְּלִי, שֶׁיְהֵא אֲוִיר בֵּינוֹ וּבֵין הַכְּלִי שֶׁבְּתוֹכוֹ הַקַּאפֶע, וְאָז יָכוֹל לִתֵּן עֲלֵיהֶם בְּגָדִים וְכָרִים וְכַדּוֹמֶה (היי"א). (יֶתֶר דִּינֵי הַטְמָנָה בְּשֻׁלְחָן עָרוּךְ סִימָן רנ"ז, רנ"ח, רנ"ט.)
Erev Shabbos, when you put a pot of coffee, into a hole [in the ground], [to retain its warmth,] so that it can be used on Shabbos, and cover it with pillows or similar things to keep it warm; if you put it into sand,98Sand is called מוֹסִיף הֶבֶל, mosif hevel—a substance that adds heat. Such substances are prohibited for purposes of hatmanah even on Erev Shabbos, lest you place it in hot ash and coal and stir it [Rashi, Shulchan Aruch, etc.]. However, substances which do not add but merely retain heat are permitted on Erev Shabbos for Shabbos. But on Shabbos you may not make hatmanah even in such substances [Rashi. See Rosh on Maseches Shabbos 3, 10]. it is forbidden to insert the whole vessel into the sand.99The author maintains that only total covering is considered hatmanah when not on coals but if the pot is not completely covered, even if it adds heat it may be used on Shabbos to keep food warm, once it is thoroughly cooked. Even if you don't insert the whole vessel but only part of it, and you cover the remainder with clothing or similar things, so that the whole vessel is covered on all sides,100It is not considered hatmanah unless all the sides are covered. Even according to the Shulchan Aruch which rules that setting a pot on coals is considered hatmanah, it applies only when the bottom of the pot actually touches the coals; but if it does not, the whole pot must be covered to be considered hatmanah. See Shulchan Aruch 257: 8. this too is forbidden. You must do it in such a way that the pot of coffee is inserted so that only one half or one third is in the sand, and the remainder exposed to the air (in the hole). You may then place a board or inverted vessel to cover the hole so that there is air space101See Shulchan Aruch 257: 8. between it (the cover) and the pot of coffee. After doing this you may put over them clothing, pillows or similar items. (For more laws pertaining to hatmanah see Shulchan Aruch, Chapters 257, 258, 259.)
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)