Today salt is inexpensive and universally available, but that wasn't always the case. Because of its importance in food preservation and the fact that the human body requires it (for the regulation of fluid balance), salt has been an extremely valuable commodity throughout the ages. It was even once used as a method of exchange — Roman soldiers received a salt allowance as part of their pay. Salt was valued by the ancient Hebrews and Greeks, throughout the Middle Ages and well into the 19th century when it began to become more plentiful and therefore reasonable in price. Salt (sodium chloride) comes either from salt mines or from the sea. Most of today's salt is mined and comes from large deposits left by dried salt lakes throughout the world. Table salt, a fine-grained refined salt with additives that make it free-flowing, is mainly used in cooking and as a table condiment. Iodized salt is table salt with added iodine (sodium iodide) — particularly important in areas that lack natural iodine, an important preventative for hypothyroidism. Kosher salt is an additive-free coarse-grained salt. It's used by some Jews in the preparation of meat, as well as by gourmet cooks who prefer its texture and flavor. Sea salt is the type used down through the ages and is the result of the evaporation of sea water — the more costly of the two processes. It comes in fine-grained or larger crystals. Rock salt has a grayish cast because it's not as refined as other salts, which means it retains more minerals and harmless impurities. It comes in chunky crystals and is used predominantly as a bed on which to serve baked oysters and clams and to combine with ice to make ice cream in crank-style ice-cream makers. Pickling salt is a fine-grained salt used to make brines for pickles, sauerkraut, etc. It contains no additives, which would cloud the brine. Sour salt (see citric acid), also called citric salt, is extracted from acidic fruits, such as lemons and limes. It's used to add tartness to traditional dishes like borscht. Seasoned salt is regular salt combined with other flavoring ingredients, examples being onion salt, garlic salt and celery salt. Salt substitutes, frequently used by those on low-salt diets, are products containing little or no sodium.