Scientists decode Maya blue paint formula

Scientists have discovered the recipe and process for preparing Maya Blue, a highly-resistant pigment used for centuries in palace walls, sculptures, codices and pieces of pottery in Mesoamerica.
The pigment is characterised by its intense blue colour and is highly resistant to chemical and biological deterioration.
The ingredients for its preparation are known to be a plant dye, indigo, and a type of clay known as palygorskite, but scientists did not know how they were 'cooked' and combined together.
Now, a team of chemists from the University of Valencia and the Polythecnic University of Valencia (Spain) have come up with a new hypothesis about how it was prepared.

The dominant theory proposes that there is a single type of Maya Blue that was also prepared in a unique way and that a specific type of bond binds the two components: one organic component, indigo - the dye used for denim that is obtained from the Indigofera suffruticosa plant in Mesoamerica - and another inorganic component, palygorskite, a type of clay characterised by its crystal structure full of internal channels.
But the new study seems to contradict this version.

"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," stated Antonio Domenech, a UV researcher.

"Indigo is blue and dehydroindigo is yellow therefore the presence of both pigments in variable proportions would justify the more or less greenish tone of Maya Blue," Domenech said.
"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," Domenech said in a Spain-based Scientific Information and News Service (SINC) statement.
Another of the unsolved questions is how the dye molecules are distributed in palygorskite's crystal network. According to some scientists, the indigo adheres to the exterior of the clay structure with the 'brick' shape although it could also form a sort of 'cover' on the entrance to the channels.

However, the new study believes that the indigo penetrates into the channels.
The team is also investigating the secret of the unknown chemical bonds that bind the organic to the inorganic component. These bonds are the reason behind Maya Blue's resistance.
The study was published in the journal Microporous and Mesoporous Materials.