Add cal to water and mix to dissolve. Add corn. Bring to a boil, then turn down heat to a low simmer. Stir occasionally.
After 15 minutes test the corn. Check to see if the skins slip off the kernel. Then bite into the corn and see how soft it is. After 30 minutes repeat the test. The skins should be slimy and slip off easily. The corn should be tender with a firm core. If not, cook for another 10 minutes. After 40 minutes the cooking should be done. Take care not to overcook or the corn will become mushy and gelatinized.
After the cooking, cover the pot and let the corn steep 8–10 hours sitting on the stove with the fire turned off.
Drain the corn into a colander and rinse well, rubbing the kernels until the skins are washed off and they are no longer slimy. Repeat until the water runs clear. You now have nixtamal, ready to cook in the posole or to grind for masa, the dough used in tortillas and tamales (but that is another story). Nixtamal can be refrigerated for a few days or frozen for further use.