Place the corn in a colander and rinse under cold running water to wash off the dusty chaff. Combine the water and cal in a large nonreactive saucepan. Stir well to dissolve and add the corn, discarding any kernels that float.
Bring to a boil over high heat, then reduce the heat to low. The corn will be noticeably yellow (even blue corn will turn somewhat yellowish). Cook for 15 minutes, remove from heat, and let cool to room temperature. Let the corn soak, covered, for 4 hours at room temperature or overnight in the refrigerator. The kernels will now be visibly softened and swelled.
The last step is to remove the outer skins, now soft and gelatinous. Pour the contents of the saucepan into a large colander and rinse thoroughly under cold water. With the cold tap running, rub the kernels between the palms of your hands until the last gelatinous coating rinses off to reveal the kernel and corn germ (the inset pointed tip, which Mexicans often remove but I usually leave intact). When the coating is completely gone, thoroughly drain the nixtamal. It is now ready to use.