May I wrap the challa on the urn with a towel?

Many are accustomed to place challa above the urn, on Shabbos or before and indeed there is nothing wrong in doing so. As explained in previous shiurim, challa is a baked solid and may be reheated above yad soledes bo in the permitted manner, i.e. above an urn or other container, on the hotplate or blech. [1]

The problem is that one may not do hatmana, even before Shabbos.

What is hatmana?

Hatmana is the wrapping of food in order to store or contain heat. There are two types of hatmana - hatmana that adds heat - îåñéó äáì and hatmana that contains heat -àéðå îåñéó äáì. [2]

Enveloping a food item with towels, sheets or cloth near or on a heat source is hatmana that adds heat. [3] Although the wrappings do not add heat, the entire arrangement does, as we view the wrappings together with the heat source as a method that adds heat.

But I am wrapping the challa before Shabbos!

Chazal prohibited this even before Shabbos. This does not mean that you may not place a towel over challos; it means that you may not do hatmana. Because the normal manner to contain heat is by completely enveloping the food item or container, wrapping a towel in a way that the challa is not totally enveloped is permitted. Leaving a noticeable section uncovered is not hatmana.

I would suggest that you leave the entire end of the challa unwrapped.

Does the same halacha apply to containers on the blech or hotplate?

Yes it does. One may not use a towel to completely envelope any single container. If the towel is placed so that a noticeable section of the container remains uncovered it is not hatmana. [4] Consequently one may place a towel over a couple of containers on a heat source because the towel only wraps a portion of each container and not an entire container. It is also accepted that surrounding on all sides except the lid is not hatmana. [5]

What about an urn surrounded by an insulating sleeve?

This would seem to be a perfect case of prohibited hatmana. The sleeve is intended to ‘add’ heat to the electric urn and since it almost completely surrounds it. It is hatmana that may not be done even before Shabbos.

Which hatmana is permitted?

Hatmana before Shabbos in the absence of a heat source: I have seen people remove the soup from the heat source close to Shabbos for the Friday night meal and store it in feather downs. This is perfect hatmana, as the down completely envelops the container of soup. However, since this is performed before Shabbos and there is no heat source it is permitted. This is different to the challa on the urn that is close to a heat source.

And on Shabbos?

Such wrapping, even in the absence of a heat source, is prohibited on Shabbos. [6] For example, the cholent container was removed from the heat source. It then became apparent that the d’var torah was taking longer than expected. The container had been placed on the counter without the intention of returning and thus cannot be returned back to the heat source. To save the day, the host then wished to wrap the container in a towel to conserve heat, until ready for serving. This is forbidden, being a classical case of hatmana. The container of cholent may however be partially wrapped in a way that a noticeable section of the container is exposed.

May one fill a thermos with water from the urn on Shabbos?

The point is that on the one hand a thermos bottle contains and preserves heat because of the wrapping of the inner container, which would appear to be hatmana. On the other hand it is a single k’li and hatmana is usually the wrapping of a k’li with other items. For the same reason food wrapped by the container itself is permitted. Common custom is to use the thermos bottle on Shabbos for two reasons. [7] A) It is a single k’li. B) The prohibition of hatmana only applies to the k’li that is or was on the heat source. If its contents are transferred to another k’li, it may be wrapped.

May I place a pulka inside the cholent container to heat it up?

Obviously the cholent container is off the fire, because you would be doing prohibited chazora by ‘returning’ the pulka to the heat source. The halachic issue is whether hatmana applies to food within food.

The answer is that hatmana only applies to food in a container and not directly to food. [8] Accordingly the pulka may be put inside the cholent off the fire. [9] An egg in its shell might be subject to the constraints of hatmana, because it is viewed as food within a k’li. [10] One may therefore not place a cooked egg inside a cholent to reheat unless a noticeable portion of the egg emerges from the cholent sea.