Translation:Shulchan Aruch/Orach Chaim/257
257ː Laws of Insulating the Hot Food. Contains 8 seifim.
1 Insulating 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 dark, insulating is done. And insulating is not done in a substance that adds heat, even while it is yet day. And if one did insulate in a substance that adds heat, the food is forbidden even if it was done. And this specifically refers only to cold food that was heated or that it improves by further shrinking. But if it retains its warmth as it was at the first time, it is permitted. Glossː There are those who say that if one forgot and insulated accidentally with a substance that adds heat, it is permissible to eat. [Hagahos Mordechai]. And there are those who say, that all this is only forbidden when he does for the need of the night. But if he insulates for the need of tomorrow, it is permissible to insulate while it is still day in a substance which adds heat. [Mordechai, beginning of Chapter 'Kira'; and Beis Yosef, end of Siman 253, in the name of Shibolei Leket]. And if it was done, one can rely on this, and only provided that one does not become accustomed to doing so.
2 It's forbidden to insulate on Shabbos even for food that was cooked as much as needed, even with a substance that does not add heat. And nevertheless, one is permitted to place objects over the food to protect it from the mice or so that it shouldn't become filthy from dust, since this is not like one who insulates for heating; rather it is like one who protects and puts a cover on the pot. [And see above, Siman 253].
3 These are the substances that add heat: waste of olives or of sesame, and manure, and salt, and lime, and sand, – whether they are wet or dry, and straw, and grape refuse, and soft material, and grasses, at the time when the three of them are wet. And these are the substances that do not add heat: clothing, and fruits, and dove downs, (glossː or other feathers. Rabbi A. in Notes), and worn linen fibers, and carpenters' shavings (meaningː the fine scraps that fall from the wood when they are planed by a plane). Glossː There are those who say that it is permitted to insulate with rocks, even though they add heat, since the rabbis did not decree in unusual circumstances. [Tosefos and Mordechai, beginning of Chapter Lo Yachpor].
4 Even though insulating is not done on Shabbos even with a substance that does not add heat, if one insulated while it was still daytime and it became uncovered once it darkened, it is permitted to cover it again. And similarly, if one wanted to add to it on Shabbos, he adds. And similarly, if one wanted to remove it entirely and to place another one in its place, whether the first one is warmer than the second, or whether the second one is warmer than the first, even if it was only covered with a sheet, he could remove it and cover it with thick woolen cloaks. And that is when the food was cooked as much as needed. But if it is not cooked as much as needed, even to add to the covering is forbidden, since this addition causes it to get cooked.
5
If one transferred the food on Shabbos from the pot in which it was cooked to another pot, it's permissible to insulate it with a substance that does not add heat.
6
It is permitted to insulate a cold thing on Shabbos in something that does not add heat so that it doesn't get colder or in order that its chill should fade away. But in something that adds heat, and even to insulate something completely cold, and even while it's still daytime, is also forbidden.
7 Wherever we forbid insulating, we forbid even if the food is cooked as much as needed and even if it's shrinking to its detriment. Glossː And so is the main opinion. And there are those who are lenient and say that it is permissible to insulate as long as it is completely raw or cooked as much as needed, just like regarding leaving it, and as was explained above in Siman 253. And in a place where their custom is to be lenient in accordance with this opinion, they should not be prevented, but other places should not follow this custom.
8 Even though it is permitted to leave a pot standing on a kirah that contains coals according to the conditions that were explained in Siman 253, if it is covered with cloths, even though the cloths do not add heat because of themselves, they nevertheless add heat because of the fire that's underneath them [and it is forbidden]. And nonetheless, as long as it's in a way that the cloths do not touch the pot, even though there is fire underneath them, it is permitted since he is not doing it in the normal manner of insulating. Therefore, where one places a pot on a kirah or kupach that contains coals, and the bottom of the pot does not touch the coals, it is called leaving and permitted according to the rules that were explained in Siman 253. And if one placed over the pot a wide utensil that doesn't touch the sides of the pot and put cloths over that wide utensil, it is permissible, for since the cloths are only placed over that wide utensil that does not touch the sides of the pot, there is no insulating. And similarly, it is permitted to place the pot in our ovens by putting a raw piece into it, and that is provided that the pot should not touch the coals. And even if he covers the mouth of the oven with cloths, since the cloths are not touching the pot, it is not insulating, and it is permissible. Gloss: And the insulating which is done in these regions, whereby it is stored in the oven and the mouth of the oven is sealed with clay, is permitted according to all opinions (Or Zarua; and Terumat HaDeshen, No. 69; and Agur), and as was explained above in Siman 254, and will be explained below, end of Siman 259. And it is a mitzvah to wrap food for Shabbos so that he will eat hot food on Shabbos, for this is an honor and delight of Shabbos. And whoever does not believe in the words of the sages and forbids eating hot food on Shabbos, we suspect that he might be a heretic. (Baal HaMaor, Chapter 'With What Do We Insulate'; and Kol Bo).