I have a discussion on Roman salt in my upcoming book on the Tiber river, due out November. If I may quote:

Salt was indispensable, the reason for Rome’s first colony at Ostia at the mouth of the Tiber, where sandbars created a barrier between the wider Mediterranean and the Italian mainland, lagoons of brackish water, a place where flats could be laid out and shallow water allowed to be steamed off by the sun, leaving salt crystals behind. Livy claims that Ancus Marcius established the first saltworks (salinae Ostienses) there.[i] Some suggest that the Pons Sublicius was built at least in part to expedite the transport of salt from downriver across to the city proper.

Salt is crucial; it allowed fishermen to preserve the daily catch. Mountain people used it to preserve excess game. It was useful in making cheese. It provides iodine, a vital nutrient, and brings out flavor in otherwise insipid food.

For all these reasons, salt can be extremely lucrative. So much so that in the first year of the Roman republic, the senate, eager to get support from the lower classes who still had to be convinced that throwing off the king was in their interest, appropriated the privately owned salt pans and established a government monopoly, providing the stuff to the people at cost.[ii]

Rutilius (fl. fifth century AD) describes the arrangement:

“It is on this score that value is set upon the salt marsh,

where the sea-water, running down through channels in the land, makes entry,

and a little trench floods the many-parted ponds.

But after the Dog-star has advanced his blazing fires,

when grass turns pale, when all the land is athirst,

then the sea is shut out by the barrier-sluices,

so that the parched ground may solidify the imprisoned waters.[iii]

The dog-star being Sirius, prominent in the hot months, the time best suited for evaporation. Natural heat is best; also cheap.[iv]

The process is straightforward enough. Salt in water breaks down into sodium and chloride ions, which are then surrounded by water molecules. When water evaporates, it cannot take the ions along for the ride, which leaves them free to recombine into salt and precipitate to the bottom of the pans. Salt gatherers can at that point rake the crystals into moist piles, scoop them out, and work on packaging and sales. The less water there is to break down the salt, the more salt solids precipitate to the bottom of the pans.

From the earliest days, the main road heading inland from Rome has been called the Via Salaria, the Salt Road—note that it starts from Rome, where salt ships would have brought the product upstream from the river’s mouth. An entire district was called the Salinae, translated as either “saltworks” or “salt flats,” where presumably the stuff was packaged and sold. The area retained that name well into the fifteenth century, long after its salt connection was gone.[v] Saltworks were a significant source of revenue for anyone who controlled them (the church in Middle Ages, which leased them out).

Taking from nature is taking from the gods, and a suitable acknowledgment is always in order. In Rome, a duty of the Vestal Virgins was to prepare mola salsa, sacred meals, the leftover grains and salt of which were solemnly tossed from the Pons Sublicius. Salt has, in its way, as strong a magic as water; tossing salt over the left shoulder is an attack on whatever devils might be perching there.

There is a widely held myth that Roman soldiers were paid in salt or that salt was used as currency or that the word “salary” comes from sal, salt. This bit of folk etymology goes back at least as far as Pliny, but nothing supports it.[vi] Soldiers like salt as much as anyone, but they prefer precious metals. At the height of Rome’s financial crisis in the third century AD, they were the only government employees who could demand, and get, their pay in gold. You can buy a lot of salt with gold.

[i] Livy, Ab urbe condita 1.33.

[ii] Livy, Ab urbe condita 2:11.

[iii] Rutilis Namatianus, De reditu suo 476–482, trans. J. Wright Duff and Arnold M. Duff (Cambridge, MA: Harvard University Press, 1934), 148.

[iv] The process is described nicely in Georgius Agricola, De Re Metallica, Translated from the First Latin Edition of 1556 (New York: Dover, 1950), 546.

[v] Biondo Flavio, Roma instaurata (Turin, 1527), 40r-v, Salinas in locum quo sal populo vendendum conservaretur fuisse ei in Tyberis ripa / quae a Ponte nunc Sanctae Mariae in Aventinum pertinet.

[vi] Pliny, Natural History 31.7.41.