Ancient pigment history is fascinating. From dried beetles (carmine) to sea snails (Tyrian purple) to ground up human and feline mummies (the rather uncreatively-named Mummy brown), colors come from some weird places. I’d heard of Maya Blue before, but didn’t realize that it’s more of a process rather than a specific mineral pigment. The color was made by intercalating indigo (añil) into fine clay over continuous heat. The slow fusing with clay made the paint exceptionally resistant to weather and acidic conditions (and even modern solvents), and the process wasn’t fully understood / rediscovered until a few years ago. Cooking it up may have been ritualistic, as the incense copal was often burned in the same bowls. The color was important in sacrifice rituals as well: when the Sacred Cenote of Chichen Itza was dredged back in 1904, a a layer of blue silt 14-feet-thick was found at the bottom (sensationalism aside, the silt was likely more from all of the blue-painted pots tossed in than the blue-painted people…I hope).
The researchers knew that the Mayans made their blue by heating the pigment with palygorskite (a type of clay); their analysis showed that this heating allowed the pigment to enter tiny channels in the clay which are sealed after the mixture cools, protecting and keeping the pigment true blue for centuries.