I am copying below an answer that the Kollel wrote about this about 14 years ago. Let me know if you have anything to add.

Best regards,

Mordecai Kornfeld

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brought to you by Kollel Iyun Hadaf of Har Nof

Rosh Kollel: Rabbi Mordecai Kornfeld

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Shabbos 47b: Salt and sand adding heat

Robert A. Miller asked:

Dear Rabbi Kornfeld:

The Mishna on hatmana includes salt and sand as materials that could

ADD heat to the wrapped object.

Ordinary salt (sodium chloride, NaCl) and ordinary sand (silica or

silicon dioxide SiO2) don't do this, when moist or dry, as far as I

know. Was there a different type of salt or sand that could add

heat? Or was there perhaps a physical resemblance between salt or

sand on the one hand and known heat-adding substances on the

other hand, that could make one want to ban salt or sand as a

precaution?

I've been puzzled by this for a long time. Thanks for your help!

Sincerely,

Robert A. Miller

Manager, Materials Development

TAFA Incorporated

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The Kollel replies:

Your question is a good one. Your suggestion that perhaps the Mishnah really means that a ban was made on salt and sand because they resemble some material that could add heat is insightful, and we would have liked to be able to suggest such an answer, but the straightforward meaning of the Mishnah and Gemara is that salt itself and sand itself add heat.

I brought your question to Dr. I. Asher, a fellow Dafyomi learner (who presently serves as special consultant to the Israeli Ministry of Science and Technology), who said that he was also bothered by the question, and the only thing that he and his Chavrusa could suggest was that either the nature of the physical world in the times of the Mishnah was different (which we do find in a number of instances; it could be that the salt the Mishnah mentions was Melach Sedomis ("Sodomite" salt), which may have had heat-inducing properties), or that we have to re-evaluate the definition of "Mosif Hevel" and say that perhaps it does not mean that the insulatory material actually adds heat, but that it is just extra-effective at conserving heat. This does not fully answer the question, though, as he pointed out, because (a) salt and sand are not known to be particularly effective at even doing that, and (b) this answer goes against the simple way of understanding "Mosif Hevel."

For now, then, it seems we will have to leave it as a question. If we find something, we will certainly let you know. And if you find something, please let us know.

All the best,

Yisrael Shaw

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The Kollel adds:

While I do not claim to be as knowledgeable in science as any of the people

mentioned above, I am not surprised that salt is called "Mosif Hevel." We

find that salt in Halachah is said to have similar properties to fire. This

is clearly implied when we say in Pesachim (76a) "Meliach k'Rosei'ach" --

"salting is like boiling." This means it causes a similar action to that of

roasting (see Meiri Pesachim 76a). Just as fire dries out a piece of meat

and causes it to become harder (i.e. think burned), so too salt draws out

the blood of a piece of meat and causes it to become hard (devoid of blood).

In other words, since the way salt interacts with other items is similar to

the way fire interacts with items, it is understandable that it is

considered Mosif Hevel. Rav Ben Tzion Uziel (Mishpatei Uziel vol. 2, Y.D #7)

explicitly states that this interaction of salt and other items "is from the

basic foundation of fire," concluding that this is why salt is called Mosif

Hevel. (He discusses the fact that salt is like fire at length in this

Teshuvah.)

All the best,

Yaakov Montrose