Shabbos, Chapter 4, Mishnah 1. This mishnah and the one following it are the only two mishnayos in the chapter, and both deal with the laws of hatmanah - insulating a pot by wrapping it.
Two laws of hatmanah:
Insulating on Shabbos: One may not insulate a hot pot on Shabbos, meaning to cover it entirely with any material that helps it retain its heat.
Insulating on erev Shabbos with something that adds heat: Even on erev Shabbos one may not insulate a pot, to remain that way into Shabbos, with materials that actually add heat. The reason is a decree, lest a person insulate the pot in hot ashes or glowing coals and come to stir those coals.
Our mishnah distinguishes between insulating materials that add heat and those that do not. The principle here is a rabbinic decree: even though the added heat may be minimal in the extreme, the very fact that these materials add heat renders them forbidden by rabbinic law.
The wording of the mishnah: "Bameh tomnin uvameh ein tomnin" - with which materials may one insulate on erev Shabbos, and with which may one not.
"Ein tomnin lo b'gefes v'lo b'zevel, lo b'melach v'lo b'sid v'lo b'chol, bein lach uvein yavesh":
"Gefes" - the residue of olives or sesame seeds that have been pressed for their oil. Once the oil has come out, the remaining waste is called gefes, and it adds heat.
"Zevel" - manure, which also adds heat. In all of these the heat is chemical, generated as the material decomposes.
"Melach" (salt), "sid" (lime) and "chol" (sand) - all of these add heat.
With all of these materials there is no difference between moist and dry, and it is forbidden to insulate with them either way.
Materials forbidden only when moist:
The mishnah continues and lists materials that are forbidden specifically when they are moist: "Lo b'teven v'lo b'zagin v'lo b'muchin v'lo ba'asavim, bizman shehen lachin":
"Zagin" - the residue of grapes that have been pressed in the winepress.
"Muchin" - soft material, such as cotton, leftover strands of wool, or worn out clothing, what we would call rags.
All of these - straw, grape residue, soft fibers and grasses - are forbidden only when they are moist, "aval tomnin bahen k'shehen yeveshin" - but one may insulate with them when they are dry.
The Gemara explains that they are forbidden when moist specifically when the moisture is natural to them and not moisture that was added afterwards, that is, the liquid naturally found in the grass, the straw or the grapes. Regarding muchin, the reference is to wool that was moist of its own, moist from the animal itself - this is how most of the commentators understand it. The Rambam, however, rules that if it is moist of its own that is a problem, and all the more so if it is moist from an external source.
With what one may insulate:
The mishnah continues: "Tomnin bik'sus uvfeiros, b'chanfei yonah uvin'soress shel charashim uvin'oress shel pishtan dakah":
"K'sus" - a garment, not small soft rags, but rather taking a coat and wrapping the pot in it.
"Peiros" - produce, with which one is permitted to insulate.
"Chanfei yonah" - the feathers of a dove.
"N'soress shel charashim" - the sawdust of carpenters.
"N'oress shel pishtan dakah" - the waste material that falls off flax when it is combed, the fine particles.
All of these are permitted to be used as insulating materials. "Rabbi Yehudah oser b'dakah umatir b'gasah" - in his view the very fine pieces that come off the flax are forbidden for insulating, while the coarse pieces are permitted.
To summarize: In this mishnah we learned the parameters of hatmanah: the prohibition of insulating on Shabbos, and the prohibition of insulating on erev Shabbos with something that adds heat, because of the decree lest one stir the coals. We listed the materials forbidden whether moist or dry (olive residue, manure, salt, lime and sand), the materials forbidden only when they have their natural moisture (straw, grape residue, soft fibers and grasses), and the materials permitted for insulating (a garment, produce, dove feathers, carpenters' sawdust and flax waste), regarding which Rabbi Yehudah disagrees and distinguishes between fine and coarse.
In the next mishnah we will continue with the laws of hatmanah.