Washington: 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 Maya Blue, a highly-resistant pigment used for centuries in Mesoamerica, was prepared.
Though it is known that the ingredients are a plant dye, indigo, and a type of clay known as palygorskite, scientists do not know how they were `cooked` and combined together.
Palace walls, sculptures, codices and pieces of pottery produced by the ancient Maya incorporate the enigmatic Maya Blue. This pigment, which was also used by other Mesoamerican cultures, is characterised by its intense blue colour but, above all, by the fact that it is highly resistant to chemical and biological deterioration.
There is no document that verifies how this paint was prepared and so it remains a mystery. Archaeologists and scientists have sought to uncover the mystery in recent years but it seems that researchers cannot come to an agreement.
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 work of a team from the University of Valencia (UV) and the Polytechnic University of Valencia (UPV) seem to contradict this `monoist` 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," the expert explained, "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."
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, other researchers believe that the indigo penetrates into the channels. This is the theory supported by the team from Valencia.
They suggested that two stages occur when both components are heated to temperatures between 120 and 180 degree C. In the first and fastest of the two stages water evaporates from the palygorskite and the indigo bonds to the clay, although a part oxidises and forms dehydro indigo.
In the second stage it would appear that the dye disperses through the channels in the clay.
"The process is similar to what happens when we pour a drop of ink into a glass of water," Domenech said, drawing a comparison, although he acknowledges that "this is a hypothesis" at present.
The researcher`s team, like other groups in other parts of the world, 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 appeared in the "Microporous and Mesoporous Materials" journal.