Hatmana - Restriction Prior To Shabbos

QUESTION: In a previous halacha it was noted that hatmana is forbidden even before Shabbos. Please explain under what circumstances this halacha applies.

ANSWER: One may not insulate hot food before Shabbos if the food is on a fire or other heat source. Chazal were concerned that if it were permitted to do so, one might insulate the pot by surrounding it with ashes that contain hot coals, and this may lead to stoking the coals on Shabbos, which is forbidden. If one insulated food on the fire on Shabbos, even if done innocently without realizing that it is forbidden, the food may not be eaten on Shabbos if this caused any improvement to the food. If one insulated food on the fire before Shabbos, although it was forbidden to do so, the food would be permitted, as long as the food was already cooked at the beginning of Shabbos.

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