Palace walls, sculptures and pieces of pottery produced by the ancient Maya incorporate the enigmatic Maya Blue, and although scientists have known the ingredients are a plant dye, indigo, and a type of clay known as palygorskite, the recipe of how they were "cooked" and combined has been a mystery.
This pigment, also used by other Mesoamerican cultures, has proven highly resistant to chemical and biological deterioration and centuries-old samples appear virtually unchanged.
Researchers from the University of Valencia and the Polytechnic University of Valencia report their analysis of Maya Blue has turned up an unexpected third ingredient.
"We detected a second pigment in the samples, dehydroindigo, which must have formed through oxidation of the indigo when it underwent exposure to the heat that is required to prepare Maya Blue," researcher Antonio Domenech said.
"Indigo is blue and dehydroindigo is yellow," he said, "therefore the presence of both pigments in variable proportions would justify the more or less greenish tone of Maya Blue.
"It is possible that the Maya knew how to obtain the desired hue by varying the preparation temperature, for example heating the mixture for more or less time or adding more of less wood to the fire."
The researchers said they were investigating the secret of the unknown chemical bonds that bind the organic to the inorganic component and are the reason behind Maya Blue's centuries-long permanence.