Limestone, seashells, coral, chalk, marble, eggshells, pearls, stalactites, and stalagmites all consist mainly of a remarkably versatile and plentiful chemical compound called calcium carbonate (CaCO3 ).
It constitutes about 7 percent of our planet’s crust, the 20-mile-or-so-thick top layer. When heated to 1520 to 1650°F (825 to 900°C), calcium carbonate decomposes into carbon dioxide gas (CO2) and calcium oxide or lime (CaO). Lime has been used for centuries to make mortar, glass, and many other useful materials.
When lime is added to water, it forms calcium hydroxide, Ca(OH)2 , also known as limewater or slaked lime. It is quite alkaline, but not as much so as lye.
The Aztecs used an even more easily obtained alkaline material to treat their corn: wood ashes. All plant materials, including wood, contain potassium (it’s the “potash” in fertilizers), and when they are burned, their ashes are rich in the alkaline chemical potassium carbonate.
The Aztecs didn’t know all that, because Chemistry 101 wasn’t scheduled to be taught for another five hundred years. We can only guess at why they started boiling their corn in water containing wood ashes.