How salt loses its savour

Dead Sea salt

Ye are the salt of the earth; but if the salt have lost his savour, wherewith shall it be salted? It is thenceforth good for nothing, but to be cast out, and to be trodden under the foot of men.

Even at school I remember being puzzled by this saying from the Sermon on the Mount, as I am sure have many chemists. Neither you nor I have had to throw salt out because it lost its taste; yet Jesus refers to this as if it were a common experience to his audience.

Much theological ink will have been spilt on this passage but here's a natural explanation which I came across in the British Library this week, in a copy of New Scientist from 1965.

The suggestion is that salt in biblical times was a very impure product obtained by evaporation of Dead Sea water, and included insoluble solids like sand and calcium sulphate. 

And, importantly, a high concentration of magnesium chloride, which attracts and absorbs water. (Deliquescent, a word I’ve not used since school chemistry lessons.) As a result it might, on keeping, easily absorb enough water to gradually leach out all the soluble sodium chloride (common salt) leaving only an insoluble and tasteless residue.

A 17th century traveller Henry Maundrell is quoted: “I broke a piece of it, of which that part that was exposed to the sun, rain and air, though it had the sparks and particles of salt, yet it had perfectly lost its savour: the inner part, which was connected to the rock, retained its savour, as I found by proof.”

References: Gospel of Matthew, ch 5 v13 (King James version); New Scientist, 15 July 1965, p 155