Using a Thermos on Shabbos

Since most of the laws of Shabbos are derived from
the construction of the Mishkan, it is an appropriate week to discuss:

Question #1: Using a Thermos

“May I pour hot water from an urn or a kettle that is on the
blech into a thermos on Shabbos?”

Question #2: Wrapping a Thermos

“May I wrap a thermos bottle, containing hot water, with
towels on Shabbos to keep the water hot?”


Explaining the background behind both of these questions
involves an in-depth analysis of the rabbinic injunctions instituted by our
Sages to safeguard the Shabbos. The laws of Shabbos include many
Torah prohibitions, such as not to cook or stir a fire, and also many rabbinic
prohibitions to guarantee that people not violate Torah laws. We will begin our
explanation of this topic with an extensive glossary, but bear in mind that
this is a brief overview of these concepts and not to be used for practical halacha.

Shehiyah – leaving food on the fire

Chazal prohibited shehiyah, which is leaving
food on a fire or in an oven when Shabbos begins, because of concern that someone might mistakenly stir the
coals. However, they permitted leaving food this way when one fulfills any
of the following three requirements:

1. Covering the fire

One may leave food cooking or warming as Shabbos
begins, if he covers the fire in a way that lessens its heat and also reminds
one not to stir the fire on Shabbos (see Shabbos 36b with
and Ran). The most common method used today to accomplish this
is to place a blech on top of the stove. It is preferable that the blech
also cover the dials, to avoid inadvertently adjusting the flame (Shu”t
Igros Moshe, Orach Chayim

2. Adding raw meat

A second method to permit cooking or warming food when Shabbos
begins is to place raw meat into the pot immediately before Shabbos (Shabbos
18b). By doing so, one knows that the food will certainly not be ready to
eat for the Friday night meal, and it will be ready for the Shabbos day
meal, so there is no need to be concerned about turning up the fire (Rashi ad

Several late poskim are reluctant to rely on this heter
today, for reasons beyond the scope of this article (Chazon Ish, Orach
37:22; Teshuvos Ivra in Kisvei Hagaon Rav Yosef Eliyahu
, Volume 2, page 19).

3. Cooked before Shabbos

A third approach is to have the food cooked before Shabbos
begins. According to Ashkenazic practice, one may leave the food even on
an open fire, as long as it is considered edible when Shabbos begins. Sefardim
follow a more stringent approach, allowing this heter only if the food
is fully cooked and only for heating water and similar foods that do not improve
by remaining longer on the fire. To prepare chamin shel Shabbat, what
call cholent, a Sefardi must rely on one of the
other two heterim mentioned above, whereas an Ashkenazi may leave
his food even on an open flame, if it is edible when Shabbos begins.

Chazarah – warming food on Shabbos

A second prohibition that Chazal instituted is called
chazarah, which includes placing food, even if fully cooked, on a heat
source on Shabbos to warm it up. The details of this prohibition
are complicated, but for our purposes we will mention that it is permitted to
return a pot or food to the fire on Shabbos, even if the food is fully
cooked, only in two general ways:

A. The food is still hot, one removed it from the blech
intending to return it to remain hot or warm, provided he kept his hand on the
handle of  the pot the entire time that it was off the fire. Many
are lenient, maintaining that one does not need to observe the
last two requirements, provided the pot of food was not placed on the ground; Ashkenazim
can be lenient about returning the food to the fire, if someone mistakenly
forgot these two requirements. Concerning how hot the food must be, Sefardim
are stricter than Ashkenazim, contending that the food must be too
hot to hold directly in one’s hand in order to permit returning. Ashkenazim
rule that one may return the food as long as it is still warm enough to eat.

B. Under certain circumstances, Chazal permitted
warming dry food on Shabbos in a way that is different from the way one
normally cooks food. For example: One may place a fully-baked kugel on
top of a pot that is on the fire.

Hatmanah – insulating

A third prohibition that Chazal instituted, one very
relevant to our topic, is called hatmanah, wrapping or insulating food to keep it hot. This
includes two different sets of rules – one for someone who wraps the food before
and one for someone who wants to wrap his food on Shabbos.

Before Shabbos

Chazal prohibited hatmanah
before Shabbos in a way that increases the heat,
such as with hot ash, fertilizer, or the remaining crushed-out pulp of olives
or sesame seeds. These materials are called davar
hamosif hevel,
items that increase heat. This is prohibited because of a
concern that someone might mistakenly stir coals on Shabbos (Shabbos 34b). However, it is permitted to
insulate foods before Shabbos with materials that do not increase heat,
called davar she’eino mosif hevel, such as clothing, blankets, towels, or
sawdust. (In the case of sawdust, one may also have to deal with the laws of muktzah,
but that is not today’s subject.)

Partial hatmanah before Shabbos

The Rishonim dispute what constitutes hatmanah.
Does leaving food on a fire to continue warming when Shabbos arrives
constitute hatmanah? Although this does not fulfill our usual definition
of insulating, it warms the food on Shabbos by maintaining physical
contact with a source of heat. According to many Rishonim, placing food
so that it touches the fire is included in the prohibition of hatmanah (Ba’al
and Ran, beginning of Shabbos, Chapter 3). In their
opinion, if one heats food on a wood fire and intends to leave the food that
way into Shabbos, one must place the food atop a tripod or other device
that raises it above the burning wood and coals. Placing the pot of food on the
tripod avoids the prohibition of hatmanah (but may still involve the
prohibition of shehiyah), since the food is no longer touching any heat
source. Failing to distance the food from direct contact to the source of heat
violates the prohibition of hatmanah, and the food may not be eaten on Shabbos.

According to other Rishonim, hatmanah is
prohibited only when the pot of food is covered completely or mostly (see Tosafos,
36b s.v. Lo; Sefer Hayashar, Cheilek Hachiddushim Chapter
235). The Shulchan Aruch (Orach Chayim 253:1) follows the first
opinion that one may not have food lying directly on a flame or hot coals when Shabbos
begins. Thus, Sefardim, who follow the Shulchan Aruch’s
decisions, may not leave food for Shabbos touching the heat directly,
even if it is otherwise exposed to the air. The Rema permits partial hatmanah
on Shabbos, allowing placing a pot into warm coals before Shabbos,
as long as the lid is not covered by the coals.

Thus, people on a camping trip over Shabbos who
choose to keep their Friday night dinner warm by leaving it on their campfire
need to know if they are Ashkenazim or Sefardim. If they are Ashkenazim,
they may leave their food on the fire when Shabbos starts, as long as it
is already cooked to the extent that it is edible. If they are Sefardim,
they must have the food elevated above the fire when Shabbos begins,
and, in addition, they can do this only with food that is fully cooked and does
not improve when it stews longer.

Lid is not covered

If one is an Ashkenazi, how much of the pot may be
covered without violating the laws of hatmanah? The Shulchan Aruch
Harav (Kuntrus Acharon 257:3) contends that as long as the pot lid
remains uncovered, one may cover all the sides of the pot. He permits placing a
bottle into a pot of hot water before Shabbos, provided that the cover
of the bottle is above the water level.

The Pri Megadim (Mishbetzos Zahav, Orach Chayim 259:3)
discusses whether it is sufficient that the top of the pot be exposed, or
whether a larger area of the pot must be exposed. Based on a ruling of the Taz
(Orach Chayim 258:1), the Pri Megadim contends that one must
leave most of the pot exposed to avoid violating hatmanah. (We should
note that the Taz in Orach Chayim 253:14 appears to hold like the Shulchan

This dispute would affect to what extent one may drape
towels over an urn either before or on Shabbos. According to the Pri
, one may do this only if the sides of the urn are predominantly
exposed. According to the Shulchan Aruch Harav, it is sufficient if the
sides are partially exposed.

Shabbos sleeve

I once saw a woman prepare her electric hot water urn by
draping a cloth sleeve made especially for the urn and embroidered with the
words “Lichvod Shabbos.” I asked her why she did that and she said, “It
keeps it hotter.” When I told her she can’t use it because of hatmanah,
she was incredulous, and responded, “but it says ‘lichvod Shabbos!’” I
have no idea who produced this sleeve, but there was no hechsher
embossed on it. Unfortunately, the label on the cloth does not permit its use.

By the way, there is a simple solution for this problem. If
some space is left between the side of the urn and the towels or sleeve, this
is not considered hatmanah and is permitted (Chayei Odom, Hilchos
2:5). One may place a board or other item on top of the urn that is
wider that the urn and drape the towel over the item. In this instance, one may
leave the towel there all of Shabbos, and one may even place the towel
there on Shabbos itself. Since the towel is not resting flush against
the urn, this is not included in the prohibition of hatmanah.

On Shabbos

On Shabbos itself, Chazal prohibited covering
the food, even with something that does not increase heat (Shabbos 34a).
Therefore, one may not take a cholent pot or kettle and wrap it in
towels on Shabbos to keep it hot. The reason for this prohibition is
concern that someone insulating his food will discover that it is colder than
he wants and will mistakenly heat it (Shabbos 34a).

Kli rishon and sheini

The next part of our glossary involves explaining the terms
kli rishon,
kli sheini and yad soledes bo.

A kli rishon is a pot, pan or other vessel containing
food that was heated on top of a stove, inside an oven or any other way
directly from a source of heat. A kli sheini is the platter or bowl into
which food was poured from a kli rishon.

Here is a halachic example of the distinction between
kli rishon and kli sheini. The Mishnah (Shabbos
42a) teaches that if a pan or pot of food was removed from the fire on Shabbos,
one may not add spices into that pot, because this constitutes bishul. However,
one may add spices to a platter which contains the food after it has been
poured out of the original pot or pan. The second case is a kli sheini,
meaning that the platter itself was never on the fire.

Why is there a halachic difference between a kli
rishon and a kli sheini? Tosafos (Shabbos 40b
s.v. Ushma) explains that when the vessel itself is on the fire
or inside the oven, the heat of the food is sustained by the hot walls of the
vessel, and that is why bishul occurs. However, when the container
itself was never directly warmed, the walls of the vessel diminish the heat of
the food placed therein. As a result, the food will not cook from the heat of
the kli sheini walls. In other words, cooking requires not only
sufficient heat, but also that the walls of the pot or vessel maintain that
heat. Therefore, cooking occurs in a kli rishon even after it was
removed from the fire, but, under most circumstances, not in a kli sheini.

Yad soledes bo

Whenever halacha discusses that something is hot, it
means that it is at least yad soledes bo, a term meaning that it is hot
enough that a person pulls his hand back instinctively when he touches it.
There is much dispute among the halachic authorities as to how we
measure this in degrees, which is a subtopic that we will leave for a different

Using a thermos

Now that we have completed our very extensive introduction,
we can address the questions that began this article:

“May I pour hot water from an urn or a kettle that is on the
blech into a thermos on Shabbos?”

“May I wrap a thermos bottle, containing hot water, with
towels on Shabbos to keep the water hot?”

The Gemara (Shabbos 51a) quotes a Tosefta
(see Shabbos 4:12) that provides the prologue to our question: “Rabban
Shimon ben Gamliel says that they prohibited (insulating on Shabbos)
only if the food is in the pot in which it was originally heated up, but if it
was moved to a different pot, one may insulate it on Shabbos.” The Gemara
explains that the prohibition to insulate food on Shabbos is out of
concern that someone might increase the heat by stirring coals (see Shabbos
34a). Rashi explains that the reason Rabban Shimon ben Gamliel permitted
wrapping up the pot of food in this case is because the person is actively
trying to cool off the water by pouring it into a cooler vessel. However a
thermos bottle that is being used to keep things hot may be different.

On the other hand, the Rambam (Hilchos Shabbos 4:5)
cites this law as follows: “If you moved the cooked food or the hot water from
one vessel to another, one is permitted to insulate the second vessel on Shabbos,
provided one uses material that does not increase heat… because they forbade
insulating food on Shabbos only in a kli rishon, in which
the food was originally cooked, but once it was moved from that vessel, it is
permitted.” Clearly, the Rambam understands that there was no decree
prohibiting hatmanah in a kli sheini on Shabbos
with devorim she’einam mosifim hevel. Following this logic, it would
appear that one may pour hot water into a thermos bottle on Shabbos,
even though one’s intent is to keep the water hot,since a thermos is
only a kli sheini. Thus, whether one may pour hot water into a
thermos on Shabbos may depend on this dispute between Rashi and
the Rambam.

In general, halachic authorities rule according to
the Rambam when he disputes with Rashi, both lechumrah and
. The Birkei Yosef (Choshen Mishpat 25:31) explains
the reason is because Rashi wrote his comments to explain the text of
the Gemara, and it is possible that he might have reconsidered had he
issued a final ruling.  Indeed, in this instance, several major
authorities appear to rule according to the Rambam (Ran; Tur;
, Orach Chayim 257:5; see also Magen Avraham 252:13).

Notwithstanding the opinions of these authorities, Rav Moshe
Feinstein writes that it is preferable to be machmir like Rashi (Shu”t
Igros Moshe, Orach Chayim
1:95). Rav Moshe concludes, however, that, even
according to Rashi, it is permitted to pour water into a thermos bottle
on Shabbos, because of a different reason. The closing of a thermos
bottle is not an act of hatmanah, but an act of closing the bottle.
However, according to Rashi, it is certainly forbidden to wrap the
thermos bottle with towels to keep it hot. According to Rambam, this
should be permitted, because there is no hatmanah in a kli sheini.

In conclusion

Rav Samson Raphael Hirsch (Shemos 20:10) notes that
people mistakenly think that work is prohibited on Shabbos in order that
it be a day of rest. He points out that the Torah does not prohibit doing avodah,
which connotes hard work, but melacha, which implies purpose and
accomplishment. Shabbos is a day on which we refrain from altering the
world for our own purposes, and the goal of Shabbos is to allow
’s rule to be the focus of creation, by refraining from our own
creative acts (Shemos 20:11).

The Gemara teaches that the rabbinic laws are dearer
to Hashem than the Torah laws. In this context, we can explain the vast halachic
literature devoted to understanding these prohibitions, created by Chazal to
protect the Jewish people from major sins. Seeing how much attention the poskim
apply to understanding the laws of Shabbos thoroughly should encourage
us to make sure we know these laws well, in all their details.