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?
Answer
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?
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