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May one wrap a towel around one’s pot, or water urn, before Shabbos, in order to help keep it warm?
It is Rabbinically forbidden for one to insulate before Shabbos a pot of food or liquid in a material that continuously increases heat. This applies even if the food or liquid is fully cooked before Shabbos. Accordingly, it is forbidden to wrap a pot of food, or a water urn, in a towel before Shabbos, and leave the pot or water urn on a source of heat, such as on a Blech or electric plate, or on top of pots that are found on a Blech or electric plate. It goes without saying that one may not wrap an electric water urn in a towel before Shabbos. Many Poskim rule, however, that this prohibition only applies if the towel is wrapped around the entire area of the pot [i.e. top and walls]. Other Poskim however rule that if the towel is wrapped around even majority of the walls of the pot, it is forbidden, even if the top is left uncovered. Practically, one is to be stringent like the latter opinion, and not wrap a towel around the majority of the walls of a pot that is sitting on a source of heat, such as a Blech, or electric plate. Likewise, one is not to wrap a towel around majority of the walls of an electric water Urn [i.e. Shabbos urn] or crockpot. However, one may rest a towel on top of the cover of the pot, so long as it does not droop down and cover majority of the pots walls. The above prohibition is only applicable when the pot is sitting on a source of heat, and thus gradually increases the heat of the pot. However, it is permitted to fully insulate a pot on Erev Shabbos, and completely wrap it in a towel, if it is not sitting on a source of heat. Thus, one may wrap a pot of soup in towels and blankets if it will be left on one’s counter and not on the Blech.
It is forbidden to wrap a pot of food or liquid in a towel before Shabbos, if the pot is sitting on a source of heat. This applies even if the pot is not fully wrapped in the towel, as so long as majority of the walls are insulated, it is forbidden.
If one transgressed and wrapped a pot or urn which is sitting on a source of heat, is the food permitted to be eaten?
If the pot/urn was not fully wrapped in the towel, such as the top, or part of the walls [and not just the bottom] was left uncovered, then one may be lenient to eat the food on Shabbos. If, however, one covered the entire top and walls [even if the bottom was left uncovered] then one may only be lenient in a pressing situation, if the food was already half cooked before Shabbos, and one does not repeat this mistake on a steady basis.
 Admur 257/1; Michaber 257:1; Shabbos 34b
The reason: This restriction is due to a decree that [if this were to be permitted] one may come to insulate the pot in ember, which is ashes which has coals mixed into it, and then afterwards on Shabbos one [may] forget and stoke the coals. [Admur ibid; Taz 253/1; M”A 253/3; M”B 253/5] Alternatively, some Poskim rule that the reason why insulating on Erev Shabbos with Mosif Hevel is forbidden is because one may come to do so on Shabbos. [Rambam Hilchos Shabbos 4/2] See Kesef Mishneh ibid that explains the source of the Rambam from the above Gemara and how he interprets it differently.
 Admur ibid; Michaber 257/7
The reason for why it is allowed to leave half cooked food over a fire from before Shabbos, but is not allowed to insulate a food with a material that increases heat: [The above scenario of insulation] is not similar to leaving a pot on a flame over Shabbos, which is permitted to be done when left on top of a Kirah that is not swept or covered, as long as the food has been cooked to the point of the food of Ben Drusaiy from before Shabbos, as usually insulation is done for the need of [food which will be eaten] the next day. As one insulates the food and covers it so it not cool down by the next day, and therefore it requires to be stoked more [than food left on a flame], in order so the food not cool off over the entire night. However, by leaving food on a flame, usually this is done for the [food to be eaten by the] night meal, as one leaves the pot there without insulation, it therefore does not need that much to be stoked if the food has already cooked to the point of the food of Ben Drusaiy from before Shabbos. [Furthermore] even if one were to leave this food [on the flame until] tomorrow, there is [still] no suspicion that one will come to stoke the coals, as the little bit of stoking that can be done will not be of help for it to stay hot until the next day being that the pot is not insulated. However, when the pot is insulated, a little bit of stoking can help [it retain heat until the next day] and therefore we are worried that one may come to stoke the coals if the pot is insulated in ember. [Now although the above suspicion only applies to ember, being that it can be stoked, as opposed to other materials which add heat, nevertheless] the [Sages] decreed [against using] any material that increases heat because of [the suspicion involved with] ember. [Furthermore, this prohibition applies] even if one insulates [the pot] for the need of the night [meal], being that the Sages did not differentiate in their decree.. [Admur ibid; M”A 257/16; M”B 257/7]
Other Opinions by foods cooked to the point of Ben Drusaiy: There are opinions which argue on this and say that even to insulate from before Shabbos in materials which add heat is permitted as long as the food has cooked to the point of the food of Ben Drusaiy [from before Shabbos] just as it is permitted to leave it on a Kirah which is not swept or covered, as explained in chapter 253. [2nd opinion in Admur ibid; Rashbam brought in Tosafus 47b; Rama 257/7 in name of Yeish Omrim]
The Final Ruling: In a community that it is the custom to be lenient like this [latter] opinion, one should not protest against them doing so. However, this custom should not be followed in other places.
 Admur 257/10; Michaber 257/8; M”A 257/18; M”B 257/43 in name of M”A ibid; Shaar Hatziyon 257/41
 Admur 253/10; 253/14; 257/4; 257/10; Kuntrus Achron 257/3; 259/Kitzur; Rama 253/1; M”A 257/18; M”B 257/69; 258/2; Shaar Hatziyon 258/9; Ketzos Hashulchan 71 footnote 29; See Piskeiy Teshuvos 257 footnote 5 and 257/11 footnote 82; Sheivet Haleivi ; In Rishonim: Or Zarua 2/8; Ravaya 197; Mordecha 299
Background: In 253:10 and 257:10 Admur defines insulation as fully covering the entire pot with insulating material. In his words “Even if burning coals surround and are attached to the walls of the pot, [nevertheless] this is not considered [the prohibition of] insulation being that the pot is open from the top, [meaning it has no coals surrounding its top].” Similarly, in the Kuntrus Achron here number 3 the Alter Rebbe reiterates and explains this ruling, that only if both the walls and the top [meaning the pot cover] is insulated with material is it considered insulation. This ruling is also repeated here in 257:4 and in the summary of the laws of insulation brought in Chapter 259, at the very end, and is the simple understanding in chapter 253 Halacha 14. [The Sheivet Haleivi rules like this opinion, and does not make mention of the Mahadurah Basra.]
 Mahadurah Basra 259; Sefer Hayashar of Rabbeinu Tam 189; Chayeh Adam 4:22; M”B 257:43 in name of Chayeh Adam ibid; Keren Ledavid 77; Toras Shabbos 253:8; Shabbos Kehalacha Vol. 2 page 14-24; Piskeiy Teshuvos 257:11; Rav Shlomo Zalman Aurbach brought in Otzros Shabbos p. 515
Background: In the Mahadurah Basra for chapter 259 the Alter Rebbe elaborates on the opinions which rule that by insulation which adds heat we do not take the top of the pot into account. Thus, possibly even if only majority of the walls are covered, it would be considered insulation. This opinion of Admur in the Mahadurah Basra is not mentioned in Ketzos Hashulchan chapter 71
Ruling of Mishneh Berurah: It is unclear according to the Mishneh Berurah, whether he holds like the ruling of Admur in the Shulchan Aruch, that it must all be covered to be forbidden, or if he rules like the Mahadurah Basra of Admur that even insulating majority of the walls is not allowed. [Shabbos Kehalacha 2 p. 18; See M”B 257:69; 258:2; Shaar Hatziyon 258:9 which implies that the entire pot must be covered by the insulation material. However see M”B 257:43 in anme of Chayeh Adam that implies if more than half of the walls are covered it is forbidden.]
 Poskim ibid, in suspicion of Admur in the Mahadurah Basra and the Poskim who rule this way
 Admur ibid; Michaber 257:7
 See Admur 253:24-25 that after the fact we only prohibit food if a prohibition was done according to all opinions, and thus here too since this matter is a dispute it should be permitted Bedieved according to Admur.
 See Admur 257:1; Shabbos Kehalacha Vol. 2 pages 12-15; Kitzur Halachos Shulchan Aruch Admur 257:7 footnote 16