Salt has been a precious commodity throughout most of history,
despite its simple nature. Rome's armies were paid in salt
(the origin of the word 'salary'). Salt is a seasoning, an
essential nutrient, but also a very important preservative.
Table salt is sodium chloride combined with iodine sources (for
nutrition), stabilizers for the iodine, and anti-caking compounds
to make it pour by preventing it from absorbing water from the air.
Potassium iodide is added as a nutrient, to prevent goiter, a
thyroid gland problem caused by lack of iodine, and to prevent
mental retardation associated with iodine deficiency. A project
started by the Michigan State Medical Society in 1924 promoted
the addition of iodine to table salt, and by the mid 1950's,
three quarters of U.S. households used only iodized salt.
Potassium iodide makes up 0.06% to 0.01% of table salt
by weight. Sometimes cuprous iodide (iodide of copper)
is used as the source of iodine.
is a sugar (the main sugar in corn syrup), and is added
in small amounts (0.04%) to salt to prevent the potassium iodide
from breaking down into iodine, which evaporates away (sublimes).
Other potassium iodide stabilizers include sodium bicarbonate
(baking soda), sodium carbonate, and sodium thiosulfate.
Calcium silicate is an anti-caking additive. Salt is hygroscopic,
meaning it absorbs water from the air. This water dissolves the
salt, and the resulting salt water combines with the remaining salt,
cementing the grains together into a solid mass.
Calcium silicate absorbs moisture from the air better than salt,
and does not dissolve in the water it absorbs.
This protects the salt from caking, and ensures that it pours
freely. Less that 0.5% is generally used, so in very humid
weather, the salt may still become lumpy.
Other anti-caking ingredients include
ferric ammonium citrate,
aluminum calcium silicate,
sodium aluminosilicate (also called sodium silicoaluminate),