The nature of the colour centres in 'Maya blue' - the incorporation of organic pigment molecules into the palygorskite lattice

It is shown by optical spectroscopy that indigo molecules, most likely incorporated into the channels of the palygorskite host structure, are the solely colour centres in the ancient Maya Blue pigment - in use during the Maya cultural period in Yucatan and Guatemala. This study also shows that the blue colour of solid indigo shifts toward turquoise and greenish hues when fastened to the host,- it is suggested that the corresponding spectral modifications originate from the energy when single indigo molecules undergo hydrogen bonding with the hydroxy-groups in the channels (or in the open channels on the surface). Indeed artefacts from the Maya culture exhibit rather greenish than pure blue colours. Similarly a tetrachloro-derivative of thioindigo forms hydrogen bonds with palygorskite, as evidenced by an again distinct colour change from red-violet to blueish-violet. Bulkier pigment molecules bearing hydroxy-groups, such as anthrachinon derivatives, seem to directly react with the host surface, but without producing very significant colour shifts.