In Chapter Three, the halakhot of cooking were discussed extensively. Although the Torah prohibited cooking on Shabbat, it is permitted to leave food on the fire on Shabbat eve so that its cooking will be completed on Shabbat, as explained in Chapter One and Chapter Three of this tractate. A series of related halakhot about insulating on Shabbat are discussed in Chapter Four.
It was customary to place hot food and liquids into various substances with the capacity to preserve and even add heat. This act of insulating is comparable to placing a cooked dish in coals and embers, which is prohibited by rabbinic decree, lest one come to stoke the coals and ignite them. In this case, although there is an explicit principle that a decree is not issued due to concern lest one come to violate another rabbinic decree, the Sages prohibited insulating food for Shabbat in those cases most similar to placing a cooked dish in coals. In cases of this sort, the Gemara explains that the different decrees were issued contemporaneously. Therefore, it is not a case of a decree issued to prevent violation of another decree, as they are all one decree.
The general discussion in this chapter includes a list of substances with which it is permitted and those with which it is prohibited to insulate hot food on Shabbat.
Most of the substances listed in the first Mishna add heat, i.e., they spontaneously generate heat as a result of different internal chemical reactions. When the solid residue of grapes, sesame, manure, and straw are moist they undergo a process of fermentation which generates heat, to the point that they sometimes ignite. Lime and salt undergo different processes. When they absorb moisture from the air new compounds are created, in the course of which heat is released. For this reason, the Sages prohibited insulating food in those materials before Shabbat.