Hatmana (definition: insulating a pot of hot food) may not be done on Shabbos even with a substance that does not increase heat. However, during the time-frame that is questionable if it’s night (Bein Hashemashos), Hatmana may be done. Hatmana may not be done in a substance that adds heat, even while it is yet day (before Shabbos). If one did Hatmana in a substance that adds heat, the food is forbidden even after the fact. But this specifically refers only to cold food heated (by Hatmana) or (cooked food) improved by further cooking, but if it only retained its original warmth, it is permitted (to eat the food). RAMA: There are those who say that if one forgot and did Hatmana accidentally with a substance that adds heat, it is permissible to eat. [Ha'gahos Mordechai]. And there are those who say, that all this is only forbidden when it is done for the night (meal), but if the Hatmana is done [on Friday] for the following day, while it is still day, it is permissible to do Hatmana in a substance which adds heat. [Mordechai, beginning of the chapter 'Kira'; Beis Yosef, end of Siman 253 in the name of Shbolei Leket]. After the fact one can rely on this opinion provided one does not become accustomed to doing so.