610. Insulating Food

Shabbos 3:22

If tar, sulfur, oily things, wax, straw, or stubble are used as fuel for a fire, it is not necessary that the majority be lit before Shabbos begins because these things will catch fire quickly.

Shabbos 4:1

There are some substances that, if used to insulate food, will actually cause the temperature to increase and add to its cooking, just like fire. These include olive refuse, manure, salt, lime and sand. One may likewise not use grape skins, unprocessed wool or grass when these things are moist, even if that’s their natural state. Other substances, if used as insulation, will only preserve heat and will not add to the food cooking. These include grape skins, unprocessed fabric and grass when they are dry, articles of clothing, produce, feathers, chips from combing flax, sawdust, animal pelts and wool shearing.