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The mishna in Shabbos 4(1) tells us which materials may not be used to insulate foods for Shabbos because they "add heat". One of these materials is salt. Can anyone explain how salt can add heat other than by saying that the salt in the time of the Mishna was not the same as our salt?

Isaac Moses
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Avrohom Yitzchok
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  • in Baba Basra 17:1 (http://www.mechon-mamre.org/b/l/l4302.htm) it is also mentioned that salt makes same type of destruction to wall as olive waste, so there we see also that it adds heat. – jutky Aug 18 '11 at 08:49
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    I don't understand why the question is limited to only salt. The Mishna bans peat, foliage, salt, lime or sand, straw grape-skis, soft flocking or moist-herbage. The question should be how any of these materials add heat. – zaq Aug 18 '11 at 13:24
  • Organic material can get quite hot from fermentation, as Shalom noted. But salt would not have any such effect. (Presumably, though, the question would apply to lime and sand.) – Dave Aug 18 '11 at 18:17
  • The heat was not the same as our heat. – Double AA Feb 01 '12 at 21:22
  • http://www.seriouseats.com/2010/04/how-to-bake-in-a-salt-crust-fish-chicken-potatoes-duck-snapper.html – rosends Jul 08 '15 at 18:30
  • @Danno Thanks for being interested in the question. Trouble is that it is not the salt that adds heat here because it says "When the dish comes out of the oven .." – Avrohom Yitzchok Jul 08 '15 at 19:55
  • I figured that the encasing in the salt keeps moisture in and therefore encourages higher internal temperature and continued cooking after the food is taken out. – rosends Jul 08 '15 at 22:13
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    Hevel means steam, not heat. I have no idea why discussion of insulating in something that adds hevel is usually phrased in terms of things that add heat. More natural to the word itself: it's things that trap the pot in a cloud of its own steam, vs things that let the steam escape, or even wool wicking it away. – Micha Berger Feb 06 '18 at 16:04

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I vaguely recall seeing some discussion about the whole "heat adding" thing years ago (maybe an old AOJS article?), but unfortunately don't have the details. Honestly, I'm still not certain how to understand the mishna. But off the top of my head ...

Leo Levi's The Science in Torah suggests that something like manure worked by fermenting, and fermentation is exothermic. He suggests that sand contained some organic matter that fermented, causing the same effect.

So perhaps the salt contained fermenting organic matter?

Perhaps the salts contained other minerals, causing a reaction similar to that found in self-heating meals?

A third option may involve the salt dissolving in (or coming out of?) water, involving heat of enthalpy? Maybe?

Shalom
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Can't say for sure, but salt lowers the temperature at which water freezes (which is why in cold areas, people clear the snow and ice from their walks using salt). This can be casually observed as a type of heat even if it does not have a burning sensation.

YDK
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  • This is what I was going to answer. But I'll expand on it, unless you want to. – avi Aug 18 '11 at 08:47
  • Be my guest. Anything else I had to say was just distracting from the above very simple answer. – YDK Aug 18 '11 at 13:11
  • Eretz Yisrael was not exactly a cold area, and even in Bavel, I doubt they used salt to clear their driveways. In any event, you're suggesting that salt was mistakenly perceived as adding heat; if so, you could have explained much more simply that salt was viewed as "hot" because of its strong flavor. (Ironically, by the way, adding salt to ice actually lowers its temperature.) – Dave Aug 18 '11 at 18:10
  • @Dave, although I don't know if my answer has validity, I'm not sure if we're on the same page. Regarding temperature Mid-Eastern temperatures, all you need is someone to drop a rock of salt on snow, which isn't rare even for the location. Re: strong flavor- I don't see how one would relate that to heat for bishul. Re: ice temperature- it is certainly lowering the surrounding temperature as the ice draws in excessive heat which causes it to melt. – YDK Aug 18 '11 at 19:42
  • All I'm saying is that "salt melting ice" was not a common occurrence then, certainly nothing that would be obvious to the masses. The strong flavor logic is related to מליח כרותח with regard to issur v'heter. It's not a stretch to say that this "resichus" was interpreted as an energy that could heat a pot. Re: ice lowing temperature, it's a reaction similar to an a/c evaporator, the state change causes it to absorb energy (or something like that). See e.g. here: http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-cools-icewater.shtml – Dave Aug 18 '11 at 21:37
  • The last comment was for you, @YDK. – Dave Aug 18 '11 at 22:48
  • @Dave, I'm not sure how common a circumstance had to be to gain halachic attention, though I now see the direction of your comment. I think the article is saying that you can get the ice cream colder because the heat is pulled into the ice/salt mix. – YDK Aug 19 '11 at 03:12
  • Btw, @avi, looking forward to your expansion! – YDK Aug 19 '11 at 03:15
  • I put it as a new answer, which got severely down voted... don't quite understand why though. – avi Aug 19 '11 at 06:25
  • By the way, salt melting ice is an endothermic reaction which means that it actually makes things colder, not hotter. – Eliyahu Mar 14 '18 at 14:46
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I have several suggestions.

  1. This is a Gzeira. Which means it does not actually add heat. But if these are allowed, someone may end up using live coals.

  2. If the sun was baking any of these items like sand or salt all day long, the sand and salt are indeed hotter than the pot of food and can cook the food by adding heat.

Thanks Josh

user14245
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If you put the pot on hot sand or salt it will add heat. Such as a hot beach or salt mine area where the sun heated the salt all day

Josh
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    Welcome to Mi Yodeya Josh! As noted in the comments on this page, it isn't clear why sand would be different from other materials, which as warning labels everywhere alert us, are 'hot when heated'. Do you meant to imply that sand is more likely than other materials to become heated by the sun? If so, consider editing that information into your answer. – mevaqesh Feb 21 '17 at 15:42
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New answer: Insulation materials that "add heat" are heated up and then either put around a pot, or a pot is put on top of them, resulting in the pot getting hotter than it was. This is in contrast to a towel that only keeps the heat of the pot in. (Shabbos, 34a)

They are prohibited to use even if the pot is set in them before Shabbat, because you can put coals into these materials and then on Shabbat you may stoke them. (Shabbos, 34b)

Before Shabbat, a pot can be removed from the oven and put in something to keep it warm, but not if the insulation is one of these materials which 'adds heat': olive-peat, foliage, salt, lime or sand (wet or dry), straw, grape-skins, soft-flocking or moist-herbage.

These materials either allow coal to be added, [or they're a certain level of 'hot' to begin with, (hot as olive-peat is after making oil) and then used to heat an approved insulation].

Two situations Rabbi Dovid Grossman gives in his Daf yomi shiur where these materials "add heat":

  1. If you cover a pot before shabbat with something, like sand or salt, you can put a coal inside the pile of salt before shabbat, and then you may come to rake the coals to make them hotter after shabbat starts. (The salt in this situation is a material which makes it possible to "add heat" in the form of a hot-coals.)
    • He says this because of Shabbos 34b : one must not insulate in a substance which adds heat, for fear lest he put it away in hot ashes (or sand, or salt...) containing a burning coal. Ashes "add heat", because you can put a coal in it.

So any type of material which "adds heat" was banned because you may put hot coals in it and then stoke it on Shabbat.

2. A pot wrapped in a towel (an allowed form of insulation) cannot be put on hot-olive-peat (or salt...) because the peat "adds heat" to the towel, which is already insulating the pot. (Hot olive peat is used as the example because there is a question of it is hot enough.) < Striken because the first reason applies more to salt.

Talmud Shabbos, 47b-48a

zaq
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    Could you please clarify? I didn't understand any of your points. – Dave Aug 17 '11 at 21:52
  • Salt is a form of insulation that adds heat for three reasons. First, it makes it possible to add a coal (a pot is buried in salt before shabbat, then Shabbat starts and you decide to bury a coal in the salt to make the pot hotter - it's assur to add the coal). Second it can add heat to an approved form of insulation (a pot wrapped in a towel and buried in hot-salt - it is assur to heat the towel). Third, the salt traps too much heat, which is called 'added' because it would have otherwise escaped. – zaq Aug 17 '11 at 22:03
  • Your first point would apply to anything that insulates well, e.g. fiberglass. So would you say that fiberglass is "mosif hevel" because it allows you to later add a coal? I think not. Your second point seems to be that the salt is pre-heated (as I suggested in my answer); if so, it has nothing to do with "heating the towel." Your third point is that the salt "traps too much heat." So where do you draw the line - styrofoam etc. are now assur? That's besides the fact that this is not "mosif hevel" by any stretch. (BTW, you may seal aluminum foil closed, as long as it's before Shabbos.) – Dave Aug 17 '11 at 22:24
  • The silver foil becomes a davar she'mostif hevel when you wrap meat with it and put it ontop of a pot to get warm. Because the pot first heats the foil and then the food. This is what rabbi David Grossman says at the top of the daf – zaq Aug 18 '11 at 00:08
  • This discussion is not about insulating food on a heat source. In that case it would be forbidden to use even things that are normally not considered mosif hevel. Here we are dealing with hot food that is being kept warm by enclosing it with salt. – Dave Aug 18 '11 at 01:20
  • It's not only about salt, we need to identify what it means to "add heat" to understand how salt "add heat". Also listed in the same Mishna are peat, foliage, lime or sand, straw grape-skis, soft flocking or moist-herbage. They can't be used because they "add heat", but that doesn't mean that just piling these material on makes it hotter. None of these materials actually get hot on their own. It means they either create a situation where you could add a coal, or it itself can be heated and an approved insulator is put on top of it too, heating the insulator before the food. – zaq Aug 18 '11 at 02:50
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    Do you have any source whatsoever for this novel interpretation? – Dave Aug 18 '11 at 03:08
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    Novel? It's the reason the gemara gives for not using these materials which add heat, including salt. – zaq Aug 18 '11 at 11:40
  • I updated my response to clarify how the Gemara says this. – zaq Aug 18 '11 at 14:29
  • With all due respect to Rav Grossman, his explanation does not reflect what the Gemara actually says. "Remetz" is not coals, it is ashes that might contain a coal (as stated in Rashi on 34b). [I checked to see what RG said on that daf, and surprisingly, he just skipped over the words שיש בה גחלת!] The meaning of the Gemara is that if people were allowed to insulate with heat-adding items (like salt etc.), they might think it's also permissible to insulate with hot ash that contains some coals. This, in turn, could lead to forbidden stoking of the coals. – Dave Aug 18 '11 at 22:24
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    (cont.) The Gemara does not mean that one might put a coal in the salt. Besides not fitting into the wording of the Gemara, that explanation would not be specific to "mosif hevel"; if people are liable to add coals to insulation, we should be concerned even when the insulating material is not adding heat. – Dave Aug 18 '11 at 22:28
  • Rav Grossman has it correct. I was misunderstanding why the material is called "adding heat", and I revised the answer. – zaq Aug 19 '11 at 01:02
  • I can't go on forever about this. Please see Rashi on 39a, where the concept is explained clearly as I am saying:......... שמא יטמין ברמץ. דמאן דחזי סבר מכדי האי לבשולי והאי לבשולי, ותרוייהו דרך הטמנה, מה לי רמץ מה לי חול – Dave Aug 19 '11 at 02:29
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As YDK said, salt lowers the freezing point of Water. It also raises the boiling point. This means that you need 'less heat' to boil water with salt than you do without salt. In the times of the Mishna, this can be understood to be that the salt itself 'adds it's own heat'

That is, if you look at this from a simple observation point of view, instead of a modern science language...

If I have a pot of water it takes 10 burning coals to boil the water. If I add salt to the pot, then I only need 9 burning coals to boil the water. Therefore, halachicly, the salt, "adds heat"

Another way to say this.. if my fire is no hot enough, I can add salt to make the food cook better / faster.

This means that in general, salt 'insulates' a mixture that is wet. From the perspective of the halacha, salt reduces the amount of heat needed to cook things, and people might therefore use the salt to help a wet mixture heated by coals or other fires to cook better. This will then lead people to adjust the coals themselves, as all forms of insulation are likely to do.

avi
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    Your science seems fine, but the mishna is talking about adding heat in the context of insulating hot food before and on Shabbos. – Avrohom Yitzchok Aug 18 '11 at 16:34
  • Whats your point? Once the halacha says that salt adds heat, its going to 'add heat' in all situations. – avi Aug 18 '11 at 19:08
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    I apologise, I gave a poor explanation. What I meant was a phrase in the Bartenuro on "Rabbi Yehuda oseir bedako". Bartenuro says (my translation) "the reason that Chazal forbade insulating before shabbos with something that adds heat is because it may lead him to stir up the coals under the pot eventually on Shabbos". The connection to the coals led me to think that the mishna is speaking about really adding heat and not requiring less heat to boil a salt solution. – Avrohom Yitzchok Aug 19 '11 at 09:11
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    @Avrohom I see, that's an important piece of information to add to your question. (And not directly part of the mishna) And I can see now why my answer was not satisfactory, I will update it. – avi Aug 19 '11 at 10:10
  • Actually, avi, adding salt increases the boiling point of water. Of course, it also thereby causes the liquid to be a bit hotter, but that differential is basically negligible (see here:http://en.wikipedia.org/wiki/Boiling_point_elevation#Uses). – Dave Aug 19 '11 at 14:40
  • @avi, if you are insulating with any substance while the coals are going, that is called mosif hevel. The issue at hand is insulating without coals with a heat-increaser could lead to placement of coal ash within, a violation of shema yechate. – YDK Aug 19 '11 at 16:40
  • @YDk, Avrohom is asking the question based on the Bartenuro, who says "it may lead him to stir up the coals under the pot" That is the understanding of the halcha that he is asking it on. Regardless, the way that salt 'adds heat' is the same. The later rational of when the ashes/coals get stoked is really almost irelevant to the question. – avi Aug 20 '11 at 18:35
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    @avi, the Bartenura does not say "under the pot". He is using the phrase shema yechate bagechalim in reference to the remetz which was commonly used as a heat increasing insulation. Because of remetz, the chashash of shema yechate was applied across the category of heat increasing insulation. – YDK Aug 21 '11 at 03:45
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    @avi - Now that you've modified the beginning of the answer, the rest of it is kind of contradictory. – Dave Aug 21 '11 at 04:07