Shabbat 48 - Wrap the food
On Friday afternoon before Shabbat, when one takes hot food off the stove, he may want to wrap, or insulate it, in order to keep it warm. That is allowed, as long as the material he uses does not add heat on its own. The reason is that if he were allowed to put his pot into heating substance, he might come to put it into hot ash mixed with live coals, and that in turn could lead him to rake the coals.
Therefore, one should not put the hot pot into olive pulp, manure, salt, lime, or sand, whether wet or dry, because all these add heat. He also may not use straw or grape skins if they are moist, but the dry ones are safe. He may use dry flocking (tufts of unprocessed cotton or soft wool).
Rav Adda bar Matnah asked, "Granted that one may insulate his food with flocking, but can he then move that flocking? Is it not set aside, and not fit for Shabbat use?" Abaye answered with a question, "Just because he does not have a box of straw and uses expensive material, does he renounce his use if it for later? - Of course not, he will do something with it later, and therefore he should not move it on Shabbat!" The Talmud answers, "It could be that this logic only applies if he did not use flocking for insulation, and he is not going to lie down on it, so indeed it has no use on Shabbat, but if he used it as insulation, that's exactly its use!" The question thus remains not completely resolved.
Art: Gustave Caillebotte - Interior Of A Studio With Stove