Although polished Blackware pottery was known to many of the pueblos, Maria and Julian developed the black-on-black style. The iron-rich clay used as the slip and paint of black-on-black would change color depending upon the conditions under which it was fired.

In an oxygen-rich environment—one exposed to air—the clay would fire red. In an oxygen-free environment—one smothered by soil—the clay would fire black.

Museum of New Mexico