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