Raman and infrared studies of synthetic Maya pigments as a function of heating time and dye concentration

Abstract

Maya Blue is a famous indigo-based pigment produced by the ancient Mayas. The organic/inorganic complexes inspired by Maya Blue have led to a new class of surface compounds that have novel applications to pigment industries. Materials analyzed in the present work are made by a synthetic route, and demonstrate chemical stability similar to that of the ancient Maya Blue samples. However, we have learned that stable complexes can be synthesized at much higher dye concentrations than used by the Mayas. Analysis by FT-Raman and FT-IR spectroscopy demonstrates the partial elimination of the selection rules for the centrosymmetric indigo, indicating distortion of the molecule. This distortion accounts for the observed color changes, as the molecular orbital structure is modified, allowing the complex to stabilize. The spectroscopic data also shows the disappearance of the indigo N-H bonding, as the organic molecules incorporate into palygorskite material. A structural change of indigo to dehydroindigo during heating is suggested by this result. Infrared data confirm the loss of zeolitic water and a partial removal of structural water after the heating process. Evidence of bonding between cationic aluminum and dehydroindigo through oxygen and nitrogen is revealed by FT-Raman measurements at higher dye concentrations.

A combined synchrotron powder diffraction and vibrational study of the thermal treatment of palygorskite–indigo to produce Maya blue

“…• The surface silanols are not modified in the presence of indigo and rehydrate fully after MB formation, a result that is incompatible with the description of MB as a "surface compOlll1 d," as suggested in some works [18,21].…”

“…It has been proposed that indigo could mostly bond to AIH rather than Mg 2 + [26]. This interaction is supposed to play a capital role in the indigo -clay association [20,21]. However, the associ ation clay -indigo via the octahedral Ae+ seems improbable (if not completely inexistent), because there is no evidence that this cation can occupy the external octahedral positions at the tunnel edges [12].…”

“…A second possible structural approach proposed by Kleber et al [17] would allow indigo to penetrate (partially or deeper) into the clay tunnels. It can be found literature supporting Van Olphen [18][19][20][21][22] and Kleber [13,[23][24][25][26] models. More recently, Hubbard et al [6] suggested the covering of the opening of nano-tmmels by the indigo molecules.…”

The Incorporation of Indigo Molecules in Sepiolite Tunnels

“…This weak band could be assigned to indigo decomposition during heating. [39,40] The thermal decomposition in the presence of oxygen is known to generate dehydroindigo, a yellow-brown product that is soluble in ethanol. [32,41,42] The sample then turns blue-green.…”

Diffusion Of Indigo Molecules Inside The Palygorskite Clay Channels

“…Signet non défini., 31 . Transformation of indigo into an intermediate oxidized form, the dehydroindigo, has also been introduced 32,42 with possible bonding between nitrogen of the organic dye and aluminum site of palygorskite 33 . This study focalizes onto the dehydration/rehydration process and the structural evolution of the palygorskite clay in the absence and the presence of the indigo molecule.…”