Chemical stability and dehydration behavior of a sepiolite/indigo Maya Blue pigment

Sepiolite is a fibrous clay mineral which, together with palygorskite, is an end-member of the palygorskite–sepiolite polysomatic series. Both palygorskite and sepiolite are renowned in Cultural Heritage studies because when properly complexed to the indigo dye they form Maya Blue, a synthetic blue pigment used in Pre-Columbian America which is famous for its exceptional stability. Freshly synthesized Maya Blue-like composites can be prepared by grinding and heating such clays with indigo. The pigment can be considered a precursor of modern inclusion compounds, with indigo molecules possibly being hosted within nano-tunnels crossing the clay frameworks, usually filled by loosely bound zeolitic H2O and tightly bound structural OH2. While the palygorskite/indigo composite has been widely studied, few papers have been focused on the analogous sepiolite-based adduct. An in-depth research was therefore planned in order to characterize the composite structural features when indigo is hosted on the sepiolite matrix, whose importance may be relevant to both the Cultural Heritage and Materials Science fields. A Maya Blue-alike composite was obtained by properly mixing and heating (190 °C) pure sepiolite with 2 wt.% synthetic indigo. SEM-EDS analysis showed the clay crystal–chemical formula to be coherent with the ideal one. Performed stability tests proved the resistance of the synthesized sepiolite + indigo pigment to be lower than the one typical of palygorskite + indigo adducts. Concentrated HNO3 progressively decolorized and completely destroyed the pigment structure, due to a more pronounced intrinsic fragility of the sepiolite framework compared to palygorskite. Prolonged NaOH attack did not substantially alter the pigment colour but caused the hosting matrix to undergo a progressive phase transformation from sepiolite to loughlinite, a similar clay mineral with intra-framework Na ions. Such a transition is favoured by indigo, whose exact role has yet to be understood. Conventional thermograms recorded for both pure sepiolite and the sepiolite + indigo composite gave indirect evidence about the encapsulation of the dye within the tunnels, as the total amount of zeolitic H2O decreased in the pigment due to the channel volume being partly occupied by indigo. Thermogravimetry coupled with gas chromatography and mass spectrometry proved that organic splinters related to fragmentation of the adsorbed indigo molecules leave the hosting matrix in the 300–500 °C temperature interval. As loss of indigo and OH2 proceeded simultaneously, it is legitimate to suppose that H-bonds may form within the clay channels between Mg-coordinated OH2 and the dye C=O/N–H groups, thus conferring to the pigment its renowned stability.

Research Highlights
► Maya Blue is a precursor of inclusion compounds, indigo being hosted in clay pores.
► Sepiolite-based Maya Blue is less stable than analogous palygorskite + indigo adduct.
► NaOH bleaching causes host phase transformation from sepiolite to loughlinite.
► Indigo loss due to progressive heating occurs in the 300–500 °C temperature range.
► Indigo C=O/N-H groups and sepiolite structural OH2 are mutually linked by H-bonds in the tunnels.