Perhaps one of the lesser known laws of Shabbat is the prohibition of insulating a pot on Shabbat. Included in this would be putting a cover over food on a hot plate. Whilst is this is done to increase the heat, and thereby the cooking, of the food on the hot plate, it may be permissible in a situation where the cover is placed over food in order to protect the food.
Today’s daf teaches three laws and gives the reason for each one. There are different opinions in the Rishonim how to understand these three laws, based on different texts of the Gemara.
1. On Shabbat one is not permitted to do Hatmanah (to insulate a pot) in material that does not add heat, because perhaps he will find that the food has cooled off and he will re-heat it (“Shema Yarti’ach”), thereby transgressing a Melachah on Shabbat.
2. During Bein ha’Shemashot one is permitted to do Hatmanah in material that doe snot add heat, because the pot is still very hot at that moment and one certainly will not feel the need to reheat it.
3. On Friday, before Shabbat, one is not permitted to do Hatmanah in material thatadds heat, because we are concerned that he might do Hatmanah in ashes mixed with coals. We are concerned that one who does Hatmanah in such a material might stoke the coals on Shabbat. (According to Rashi’s explanation, this apparently is a Gezeirah l’Gezeirah, a decree for another decree. It must be that Rashi maintains that all of these edicts were included in one general decree.)
(b) RAMBAM (Hilchot Shabbat 4:2):
1. On Friday before Shabbat one is not permitted to do Hatmanah in material that adds heat, because perhaps the pot might boil over (“Shema Yarti’ach”) and he will open and close the pot to keep the boiling down and to prevent the dish from overcooking. Closing an open pot that is resting on a source of heat constitutes the Melachah of cooking. (it may be assumed that since the Hatmanah was done while it was still day, the pot probably did not yet reach a boil before the Hatmanah was done, and he did not have a chance to adjust the heat to the desired temperature.)
2. During Bein ha’Shemashot one is permitted to do Hatmanah in material that adds heat, because by that time the pot presumably has already reached a boil and the fire has been adjusted accordingly. Since there is no concern that the pot will boil or foam over on Shabbos, there is no concern that one might open and close the pot.
3. On Shabbat one is not allowed to do Hatmanah even in material that does notadd heat, because of the concern that one might do Hatmanah with ashes containing an ember that is still burning and he will stoke that ember and make it burn more.
1. Although the Ra’avad favors Rashi’s explanation, he gives an alternate explanation based on the text of the Talmud that the Rambam had (which was also the Rif’s text of the Talmud). The Ra’avad explains that on Friday, before Shabbat, one may not do Hatmanah in a material that adds heat, because perhaps on Shabbat he will find that the food has cooled off and he will re-heat it (“Shema Yarti’ach”). Since one is showing (by doing Hatmanah) that he wants his food to be very hot, there is a concern that on Shabbat he will forget and heat it.
2. During Bein ha’Shemashot one is permitted to do Hatmanah even in material that adds heat, because usually pots that are left on the fire until Bein ha’Shemashot become so hot that they will remain hot until the night, and thus there is no concern that one will re-heat it. (It might be thought that since the person indicates his desire for the pot to be very hot by waiting until the last moment to take it off of the fire and do Hatmanah, we should be concerned that he might re-heat it on Shabbos. In truth this is not a concern, because since the pot is so hot, it will not lose any heat.)
3. On Shabbat one is not allowed to do Hatmanah even in material that does not add heat, because of the concern that he might do Hatmanah with ashes that contain an ember that is still burning and he will stoke that ember and make it burn more. (This is the same as the Rambam’s view.)
For an interesting practical application of this law see here for a discussion as to whether a thermos flask is considered Hatmanah.