סימן עב סעיף יח
הַתַּבְשִׁילִין שֶׁמַּטְמִינִין בַּתַּנּוּר לְיוֹם הַשַׁבָּת כַּנָּהוּג, אַף עַל פִּי שֶׁהַפֶּתַח אֵינוֹ טוּחַ בַּטִּיט, מֻתָּר. וְאָסוּר לִפְתֹּחַ אֶת הַתַּנּוּר בַּלַּיְלָה, כִּי שֶׁמָּא יֵשׁ קְדֵרוֹת שֶׁעֲדַיִן לֹא נִתְבַּשְּׁלוּ כָּל צָרְכָּן. וְעַל יְדֵי שֶׁיִּסְתֹּם אַחַר כָּךְ אֶת הַתּנּוּר, יִגְרֹם בִּשּׁוּל.
Cooked food [may be] placed in the oven [on Friday] to be eaten on Shabbos, as is our custom.92Cholent, etc. Even though the door is not sealed with mortar, it is permitted.93The Halachos of hatmanah are numerous. The following is a brief introduction that will enable you to understand these important laws. There are two concepts, one called שִׁהְיֶה, shehiyah which means leaving food [on the fire, or in an oven, etc.], and הַטְמָנָה, hatmanah, which means “hiding,” in other words, covering the pot to keep it warm. There is a Rabbinical prohibition against leaving certain food on open coals because then you might stir the coals on Shabbos. Tosafos and Rosh rule that this prohibition is applicable even on Erev Shabbos and thus food may not be left on coals over Shabbos unless it is completely cooked. The Shulchan Aruch 253:1 seems to agree, but we are accustomed to follow Rashi’s ruling and permit this on Erev Shabbos if it is half cooked, (or one third if need be), which makes it fit to be eaten with difficulty. The Biur Halachah says that it is preferable to be stringent in this matter. In our times we use gas or electric heat, but the prohibition remains, for you are liable to adjust the flame. In the case of a range, you are allowed to put up the meat cholent on Erev Shabbos even if it will be only slightly cooked when Shabbos arrives, if you remove all the coals (which is called גְרֵיפָה, gerifah) or you cover the coals (which is called קְטִימָה, ketimah). For this reason we use a בְּלֶע (blech) to cover both the flame and the knobs, (see Igros Moshe [Orach Chaim, Responsum 93]) since this makes the range קְטוּמָה (Ketumah). However, Rav Akiva Eiger ruled, that ketumah is not sufficient if the food is not at least half-cooked before Shabbos. See Biur Halachah 253. Also see Sha’arei Teshuvah 254: 5 that a metal sheet (blech) does not make it ketumah. If the meat is totally raw, you may put it up to cook immediately before Shabbos and we are not concerned that you will stir the coals. Since the meat was totally raw at the onset of Shabbos, it will be left untouched all night. [Rav Henkin z” l ruled that in our times it is best not to rely on this as our ovens are more efficient and that meat will be partially cooked by the time it is actually Shabbos according to Rabbeinu Tam who rules that Shabbos begins 72 minutes after sunset.] Even putting in one piece of totally raw meat immediately before Shabbos, qualifies the rest of the pot (at whatever stage of cooking it may be) [Shulchan Aruch Harav]. If you put the pot into the oven on Erev Shabbos, and seal the oven door with mortar, there is no concern that you will stir the coals because it is sealed. [Rema and most Poskim and Kitzur.] Even if food is totally cooked on Erev Shabbos, but will improve with shrinking in prolonged heat, (which is called מִצְטַמֵק וְיָפֶה לוֹ —it shrinks and is improved thereby), according to Tosafos, you may not leave it on a range that is not גְרוּפֶה or קְטוּמָה. As mentioned, it is preferable to be stringent in this matter if you can. Also, see Mishnah Berurah 253: 70–73. However, it is forbidden to open the oven at night, for there may be food which is not thoroughly cooked, and by closing the oven afterwards you will cause the cooking to be accelerated.
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.)