Ancient residents of Mexico decorating their pottery and homes used a very stable blue pigment now known as ‘Maya Blue’. A mixture of indigo dye and palygorskite clay, the vivid pigment provides a dramatic background for some of the most impressive murals throughout Mesoamerica.

The pigment is very resistant to attack by acids, alkalis and chemical solvents although it can vary in colour due to mixing of indigo and dehydro-indigo during preparation. The strong chemical bonding of the organic indigo molecules to the surface of the clay gives the possibility of creating entirely new organic/inorganic materials for future applications.

Central to understanding ‘Maya Blue’ and related complexes, is a reliable appreciation of the vibrational spectra from pure indigo, particularly the low energy spectral region that is most sensitive to changes in the local environment.

Tomkinson et al. have released a definitive description of the vibrational spectra of indigo measured with the Tosca instrument at the ISIS neutron source [Tomkinson et al., Vibrational Spectroscopy, (2009) 50 268].

As well as giving a clear interpretation of the indigo spectrum, they also identity two ‘fingerprint’ modes that can be used to get a better understanding of ancient artefacts at a molecular level as well as understanding the processes at work when creating organic/inorganic materials.

Molecular vibrational spectroscopy with neutrons is a well established technique with the advantage that observed intensities can be simply compared to calculated spectral intensities. Intensities are easily obtained from the mean square vibrational atomic displacements that appear as output from standard first principles calculations. The neutron technique has a clear advantage over optical spectroscopies where matching calculated and observed intensities can be difficult.