Also: are they muktzah if already heating?
February 2025
I was recently met with a heated reaction when discussing…a heated reaction.
During a recent post-Minchah halachah shiur, I was examining the halachos related to hand warmers on Shabbos. A rabbi never knows what topic may provoke a vociferous response, and the crowd grew passionate during this discussion.
“This has to be assur!” implored one.
“Well, some do see this as an issur Torah,” I responded.
“Good,” said another, and then added, “Just because we don’t know what the issur is doesn’t mean it’s muttar! I can’t understand why this is even up for discussion.”
I told the group that the day before, when I arrived at shul for Shabbos Minchah, I had noticed one of these hand warmers lying on the armrest of my chair.
“I was unsure how it got there, and then I wondered if I could move it out of the way on Shabbos. This led me to speculate about whether one would be allowed to activate such technology on Shabbos. You might ask, ‘What difference does it make why they’re assur?’ So I will start by showing that it actually does make a big difference.”
Let’s explore how these warmers work and what is problematic about using them on Shabbos.
I. First: A Muktzah Primer
Whether such gloves may be activated on Shabbos will be discused below. That issue came about as simply a byproduct to an initial sh’eilah I had: may I move them//are they muktzah?
Many mistakenly assume that the central premise behind Nechemia’s decree of muktzah is the concern that moving certain items on Shabbos may lead a person to use them for their forbidden functions. For example, a pen is muktzah in order to prevent one from writing on Shabbos.
Although the fear that one may do a forbidden melachah sometimes plays a role in muktzah (“muktzah machmas issur“, e.g. a lit candle, see below) this is not the chief concern behind muktzah (see Rambam, Hilchos Shabbos 24:12). Nor would such reasoning explain why, say, an expensive painting would be muktzah (muktzah machmas chesron kis).
In other words: muktza as an invention of a new category that was birthed together with an injunction against its movement, having nothing to do with melacha or its prevention, per se.
A Simple Demonstration:
A PEN
A pen becomes muktzah not because we fear one will forget and write with it, rather, and simply, because we assume that one is not planning to use it on Shabbos, and all items assumed to not be utilized on Shabbos are deemed muktzah.
That’s it.
Muktzah, then, simply means hukatz—to be set aside.
This means that the many categories of muktzah are, in truth, simply the various reasons for the assumption these items were set aside.
A pen is muktzah, and falls into the category of a kli shemelachto l’issur, something we assume is not planned for its main function since that act is forbidden on Shabbos.
Because a pen does have some non-abstract uses that are permitted on Shabbos – e.g. as a pointer, to open nuts, etc. – it may be used for those purposes on Shabbos if they arise, as it was never hukatz from those. Indeed, items like this that are muktzah due to their issur status are the weakest type of muktzah, and may be moved if they are but in one’s way.
So even if hand warmers are assur on Shabbos, I could have moved the ones on my chair.
But wait! There may be another type of muktzah to consider.
As mentioned, there is a special category called muktzah machmas issur. This category of muktzah only shares its name, yet is unrelated to classic muktzah and its special injunction. This is when an item is deemed muktzah not because of an assumption of disuse (painting, pen, a rock, etc.), rather because handling this item may lead to a forbidden activity/result. Some common examples are a lit candle – which is muktzah because moving it may cause it to be extinguished; wet clothes – which are muktzah because one may wring them out; and an iPhone – which is muktzah because its backlight may be activated.
If it is indeed forbidden to activate hand warmers on Shabbos and if it is true that moving them may activate them, they would not only be muktzah by dint of being a kli issur, but also muktzah machmas issur!
However, according to many, the fear of violating a rabbinical prohibition would preclude an item from falling into this latter category of muktzah.
II. How Do They Work?
We have been working under the assumption that activating such warmers would certainly be forbidden on Shabbos.
Is this true? How do they work?
There are two types of hand warmers.
- The less common type is a reusable pack filled with a supersaturated solution of sodium acetate in water (gel). When the small metal disc inside is agitated, it releases tiny fragments of metal that act as nucleation sites for sodium acetate, changing the gel from a liquid to a near-solid state, which releases stored heat energy in a process called crystallization.
- The more common type of hand warmer is for one-time use. Inside the pouch are tiny crumbs of iron. When agitated, oxygen is introduced, causing a speedy rust process to begin releasing energy as heat. (Other chemicals such as salt may be added to speed up the process.)
The halachic issues involved are numerous.
In regard to the first type, we must first consider bishul, which on the Torah level is only a concern at high temperatures that likely do not apply here. If indeed such temperatures are reached, it still only reheats the gel, which is a rabbinic concern (bishul achar bishul), and only according to some poskim.
This concern may apply to the second type as well, depending on how we define bishul b’chamah and toldos chamah (cooking with the sun and other odd methods of cooking; famously, this is the problem with microwaving on Shabbos in terms of bishul; see, e.g., shu”t Igros Moshe 3:52).
Furthermore, many people place empty aluminum pans on top of a hot plate on Shabbos. Would bishul even apply to such metals? (See Chazon Ish 37:11, with Chut Shani 22:4.)
Beyond bishul, there are other issues such as molid, creating a new entity on Shabbos or Yom Tov. Molid is the reason we are permitted on Yom Tov to light a candle from an existing fire, but we can’t kindle a new flame.
Finally, there is the biblical concern of makeh b’patish, which involves completing or activating an item by adding the finishing touch.
III. Other Considerations
The chemical reactions in hand warmers and their halachic applications go well beyond warm hands. They are an issue, for instance, when it comes to MRE self-heating meals, which soldiers use (and which I used during the time I traveled for kashrus supervision). One activates the pouch, places the sealed food on it, returns it to the box and waits (which means adding hatmanah to the halachic mix).
May one reheat food this way on Shabbos?
{For those who are interested, Rav Menachem Shimmel published two brilliant essays delving into this technology and its many halachic applications in Yeshurun, volumes 20 and 21}
IV. Lessons Learned
Although people should ask their rav about these many fascinating issues, I am discussing all this for reasons beyond l’hagdil Torah.
I’d like to share what I told the agitated members of my learning group, who were bewildered by the fact that I would even consider investigating the possibility that hand warmers might be permitted on Shabbos.
Of course we should trust our instinct, certainly in regard to the question of when to ask a sh’eilah. Although it is also true that issues such as zilzul Shabbos can be defined by the era or by one’s feeling (specifically, the feelings of the leaders of that generation; see Chazon Ish), one must still be able to discuss the halachah dispassionately and not just because it has real-world applications for cholim and soldiers.
Moshe Rabbeinu understood the danger of making decisions while in an agitated state, even though his anger in that instance was righteous (see Bamidbar 31:22 with Sifri and Rashi).
Knowing the exact status of items such as hand warmers and MRE meals aids us in understanding muktzah, and it may indeed assist a soldier on the field by preventing him from using a much worse mode of bishul. It can also be useful to a choleh or someone who is alone for Shabbos (see Rav Zalman Nechemia Goldberg, quoted by Rav Schimmel at the end of one of his essays).
Food for thought. ●
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