Salt comes from dead, dried-up seas or living ones. It can bubble to the surface as brine or crop out in the form of salt licks and shallow caverns. It lies in white veins below the earth's skin, some thousands of feet deep. It can be evaporated from salt "pans," boiled down from brine, or mined, as it often is today, from shafts extending half a mile down.
According to salt, the history of the world is simple: animals wore paths to salt licks; men followed; trails became roads, and settlements grew beside them. When the human menu shifted from salt-rich game to cereals, more salt was needed to supplement the diet. But the underground deposits were beyond reach, and the salt sprinkled over the surface was insufficient. Scarcity kept the mineral precious. As civilization spread, salt became one of the world's principal trading commodities.
Salt routes crisscrossed the globe. One of the most traveled led from Morocco south across the Sahara to Timbuktu. Ships bearing salt from Egypt to Greece traversed the Mediterranean and the Aegean. Herodotus describes a caravan route that united the salt oases of the Libyan desert. Venice's glittering wealth was attributable not so much too exotic spices as to commonplace salt, which Venetians exchanged in Constantinople for the spices of Asia. In 1295, when he first returned from Cathay, Marco Polo delighted the Doge with tales of the prodigious value of salt coins bearing the seal of the great Khan.
As early as the 6th century, Moorish merchants routinely traded salt ounces for gold in the sub-Sahara. In Abyssinia, slabs of rock salt, called 'amôlés, became the coin of the realm. Each one was about ten inches long and two inches thick. Cakes of salt were also used as money in other areas of central Africa.
Not only did salt serve to flavor and preserve food, but it also made a good antiseptic, which is why the Roman word for these salubrious crystals (sal) is a first cousin to Salus, the goddess of health. Of all the roads that led to Rome, one of the busiest was the Via Salaria, the salt route, over which Roman soldiers marched, and merchants drove oxcarts full of the precious crystals up the Tiber from the salt pans at Ostia. A soldier's pay consisting in part of salt came to be known as solarium Argentum, from which we derive the word salary. A soldier's salary was cut if he "was not worth his salt," a phrase that came into being because the Greeks and Romans often bought enslaved people with salt.