Insulation that's too effective TB Shabbat 48

Today we begin the fourth chapter of massechet Shabbat with TB Shabbat 48. Obviously, there’s nothing like a nice hot meal on Shabbat. This chapter deals with the question of insulating (הטמנה) the pot to keep the contents warm. There are two different categories of insulation. The first are those things that increase the heat, דבר שמוסיף הבל. The second are those things that only preserve the heat, דבר שאינו מוסיף הבל. Insulation that increases the heat is forbidden to use before and on Shabbat lest you stoke the coals to increase the heat because you want the cooking to continue. One is allowed to use insulation that just preserves the heat before Shabbat but not on Shabbat.

“The mishnayot that follow list those materials in which such a pot may be insulated on Shabbat eve and those materials in which it may not be insulated.

MISHNA: In what may one insulate a pot of cooked food on Shabbat eve, and in what may one not insulate it? One may neither insulate it in the solid residue of produce that has been pressed free of its oil, nor in manure, nor in salt, nor in lime, nor in sand, whether those materials are moist or whether they are dry. All of these materials spontaneously generate heat when piled for an extended period. Therefore, they add heat to a pot insulated in them. And one may neither insulate a pot in straw, nor in the residue of grapes that have been pressed for their juice, nor in soft material, e.g., from tattered clothing, nor in grass, when these materials are moist. However, one may insulate a pot in them when they are dry.” (Sefaria.org translation)

Solid oil residue, manure, straw, residue of grapes, soft material, and grass when wet ferments and goes through a chemical process that creates heat. When submerging the pot in salt, lime, and sand removes the air and creates a new compound. This process also creates heat.

The Gemara asks the question whether the Mishna limits the solid residue from olive oil or prohibits other kinds of solid residue like sesame oil.  “A dilemma was raised before the Sages: Did we learn with regard to the residue of olives in the mishna, but the residue of sesame seeds that were pressed for their oil, which produces less heat, may well be used for insulating food on Shabbat eve? Or, perhaps, we learned with regard to the residue of sesame in the mishna, and all the more so insulating food in the residue of olives is prohibited?” (Sefaria.org translation)

The conclusion is sesame oil is also forbidden to insulate the pot; however, the rabbis recognize that different oils have different heat properties. One is forbidden to put a pot on the solid residue of olives, but is permitted to put a pot on the solid residue of sesames. “Actually I can say to you that with regard to actual insulation, the residue of sesame is also prohibited. However, with regard to causing heat to rise, i.e., heating food that is not actually insulated in it, but merely resting upon it, the residue of olives causes heat to rise. Therefore, it is prohibited even to place cooked food upon it. However, the residue of sesame does not cause heat to rise to that extent. Therefore, it is permitted to place food upon it.” (Sefaria.org translation)