Chapter 4 of Meseches Shabbos closes with its second mishnah, which also deals with the laws of hatmanah - insulating a pot to retain its heat - except that it opens specifically from the angle of the laws of muktzeh.
Two insulating materials and two rulings:
"Tomnin bishlachin, umetaltelin osan" - shelachin are animal hides. One may insulate hot pots in them before Shabbos, since they do not add heat; and one may even move them on Shabbos, since they are not muktzeh - people would regularly use them as a utensil, as a rug to lean back and relax on.
"B'gizei tzemer, v'ein metaltelin osan" - fleeces of sheep's wool also do not add heat, and therefore one may insulate with them; but one may not move them on Shabbos, since they are muktzeh - they stand designated for spinning and weaving, to make cloth from them.
"Keitzad hu oseh? Notel es hakisui v'hein noflos":
A person who wishes to insulate with fleeces of wool, even though they are muktzeh, insulates with them before Shabbos - for there is no muktzeh before Shabbos begins. On Shabbos, when he wants to take out the food, he removes the pot's cover, and the fleeces fall away to the sides on their own.
Two novel points are learned from here:
Indirect moving (tiltul min hatzad): Even though the fleeces are moved indirectly, in the manner called 'tiltul min hatzad', it is permitted.
A base for a forbidden object: We do not say that the pot's cover, which is covered by fleeces of wool that are muktzeh, becomes forbidden to move because it is a 'basis l'davar ha'asur' - an object serving as a base for something that may not be moved, as was explained in chapter 3. That concept is not relevant here, since the fleeces are not placed on the cover in order for it to hold them up; rather, the cover is placed there for the purpose of covering the pot - which is exactly the reverse.
How to remove the pot from the basket:
The pot is set in the middle of a kupah - a box - filled with fleeces of wool on all sides, and in this the Tanna'im disagreed:
Rabbi Elazar ben Azaryah says: "Kupah - mateh al tzidah v'notel" - after removing the cover, he tips the basket onto its side and takes some of the food, and the rest he may leave in place for a later meal. But he should not remove the pot from the fleeces, "shema yitol v'eino yachol l'hachazir" - for once the pot is taken out, the fleeces are liable to collapse into the empty space, and he will not be able to put it back without inserting his hand and shifting the fleeces, directly moving muktzeh.
And the Chachamim say: "Notel umachzir" - we are not concerned for this, and one may take the pot out. And even the Chachamim agree that if the fleeces did collapse into the space where the pot had been resting, it may no longer be returned, since one may not move muktzeh directly.
"Lo kisahu mib'od yom - lo y'chasenu mishtechshach":
This is the general rule about performing hatmanah on Shabbos: one who did not cover the pot before Shabbos may not cover it after dark, once Shabbos has begun. This prohibition applies even with a material that does not add heat, out of concern that when he comes to insulate it he will find that the pot has cooled off, and he will come to heat it on Shabbos. All this is where he did not cover it while it was still day; but one who returns the pot to the insulated state it had been in, as was explained in the previous mishnah, may do so.
"Kisahu v'nisgalah - mutar l'chasoso":
One who covered the pot while it was still day, and afterward it became uncovered, may cover it again.
Insulating to preserve the cold:
In closing, the mishnah mentions wrapping something that is not hot, whose entire purpose is to keep it cool: "Memaleh es hakiton v'nosen l'sachas hakar o tachas hakeses" - he fills a vessel with water and places it under the pillow or the blanket so that it will stay cold, and there is no prohibition of hatmanah in this, for the Chachamim did not enact a decree in such a case.
In summary: In this mishnah we learned the distinction between hides, which may be used for insulating and may also be moved, and fleeces of wool, which may be used for insulating but may not be moved because they are muktzeh; the permission to remove them by way of 'tiltul min hatzad' and the reason the cover does not become a 'basis l'davar ha'asur'; the dispute between Rabbi Elazar ben Azaryah and the Chachamim about removing the pot from the basket; the prohibition to insulate in the first place once it is dark, as opposed to the permission to cover it again; and finally, that the laws of hatmanah were not stated regarding a wrapping whose entire purpose is to preserve the cold. With this, chapter 4 is complete.