Shabbat 51B

אינו נוטל ומחזיר.

he may not remove the pot and then return it to its place.

רבי יהודה אומר: נעורת של פשתן דקה הרי היא כזבל.

R' Yehuda says: The chaff of fine flax is like manure. Therefore, one may not insulate food in it even on Friday.

מניחין מיחם על גבי מיחם וקדרה על גבי קדרה, אבל לא קדרה על גבי מיחם, ומיחם על גבי קדרה. וטח את פיה בבצק, ולא בשביל שיחמו, אלא בשביל שיהיו משומרים.

One may place a copper urn upon a copper urn, and one may place an earthenware pot upon an earthenware pot because the lower utensil will not heat the upper one. However, one may not place an earthenware pot upon a copper urn, or a copper urn upon an earthenware pot, as in that case there is concern that the upper utensil will be heated by the lower one. And one may seal the mouth of a pot with dough. All of the above may not be undertaken in order to heat the water, but only so that its heat will be maintained and it will not cool down.

וכשם שאין טומנין את החמין כך אין טומנין את הצונן. רבי התיר להטמין את הצונן.

And just as one may not insulate hot food to keep it warm, so too, one may not insulate cold food to keep it cold. R' Yehuda HaNasi permitted insulating cold food on Shabbat.

ואין מרזקין לא את השלג ולא את הברד בשבת בשביל שיזובו מימיו, אבל נותן הוא לתוך הכוס או לתוך הקערה ואינו חושש.

And one may neither crush snow nor hail on Shabbat so that its water will flow and he will be able to drink it. That act involves creation of a new entity, water from ice, on Shabbat, which is prohibited. However, he may place the snow or the hail into a cup or a dish and allow it to melt on its own, and he need not be concerned.