The Maya civilisation flourished in central America from 2500 BC until the arrival of the Spanish in the 17th century. During that time it developed a complex written language as well as impressive architecture and art.
One of the features of this art is a vivid colour called Maya Blue which amazingly survives until today. Maya Blue has long puzzled archaeologists because of its remarkable longevity. How does it survive when other pigments fade away?
Today, Catherine Dejoie at the Néel Institute in Grenoble et amis provide an answer. They use x-ray diffraction and thermogravimetric analysis (which uses changes in weight to determine how the breakdown in materials occur) to determine the structure of the material. This in turn has revealed the secret of its longevity.
Archaeologists know that Maya Blue is made by heating the organic pigment indigo with palygorskite, a type of fibrous clay found in Yucatan. During this heating process, the indigo is somehow absorbed into the fibrous clay and this fixes the colour. But how this processes increases the longevity of the pigment hasn’t been known until now.
Dejoi et amis say that the clay fibres contain channels filled with water molecules. Their analysis shows that the heating process causes the water to boil away, allowing the indigo to enter these channels. When the material cools down, these channels then become sealed by “gatekeeper” molecules that prevent the indigo from getting out again.
That partly explains the longevity but there is another mechanism at work too. Dejoi et amis say that indigo looses its colour and becomes yellow when a carbon-carbon double bond in the pigment is broken. However, this cannot happen in Maya Blue because this bond is protected by the clay channels, a phenomenon known as steric shielding.
“We thus believe that fixing of the indigo molecule to sterically screened sites mainly accounts for the chemical stability of May Blue,” say Dejoi and co.
The discovery of the secret of Maya Blue could have important implications for pigment manufacturers. Dejoi and pals say that now they know why Maya Blue is so long lasting, the same trick can be used for other colours too.
They even reveal the first new pigment designed in this way: a new kind of blue in which indigo is embedded in microporous zeolite which performs the same protective function as the palygorskite clay.
That, they say, is the birth of the new discipline of archaeomimetics in which the molecular structure ancient pigments is inspiring a new generation of long-lasting colour.
Ref: arxiv.org/abs/1007.0818: Revisiting Maya Blue and Designing Hybrid Pigments by Archaeomimetism