No abstract

Maya blue is a marvelous pigment with extraordinary properties. It was invented by the Maya around VII-VIII century and used by many Mesoamerican peoples in prehispanic times. It is made by encapsulating natural indigo into an inorganic clay matrix of palygorskite. The palygorskite-indigo mixture becomes acid-resistant when a moderate thermal treatment is applied. The chemical reasons of the unusual stability of the pigment and the exact mechanism of interaction between the indigo and the clay are not well understood. We present a Raman study of different preparations of Maya blue and other mixtures of indigo with other inorganic materials. We found that the unheated mixture of indigo with palygorskite presents the same Raman spectrum as Maya blue, indicating that the differences with respect to the indigo spectrum are not due to the interaction produced during the thermal treatment, which makes the mixture acid-resistant. Moreover, indigo mixed with other clays, like sepiolite or montmorillonite, presents a Raman spectrum very similar to that of Maya blue. Some chemical mechanisms that could explain these spectra, and the suitability of Raman spectroscopy for identifying Maya blue are discussed.

A new version of the popular X-ray tracing code SHADOW is presented. An important step has been made in restructuring the code following new computer engineering standards, ending with a modular Fortran 2003 structure and an application programming interface (API). The new code has been designed to be compatible with the original file-oriented SHADOW philosophy, but simplifying the compilation, installation and use. In addition, users can now become programmers using the newly designed SHADOW3 API for creating scripts, macros and programs; being able to deal with optical system optimization, image simulation, and also low transmission calculations requiring a large number of rays (>10 6 ). Plans for future development and questions on how to accomplish them are also discussed.

Maya blue is an organo‐clay artificial pigment composed of indigo and palygorskite. It was invented and frequently used in Mesoamerica in ancient times (eighth to 16th centuries). We analyse in this paper one of the characteristics of Maya blue that has attracted the attention of scientists since its rediscovery in 1931: its high stability against chemical aggression (acids, alkalis, solvents, etc.) and biodegradation, which has permitted the survival of many works of art for centuries in hostile environments, such as the tropical forest. We have reproduced the different methods proposed to produce a synthetic pigment with the characteristics of the ancient Maya blue. The stability of the pigments produced using either palygorskite or sepiolite has been analysed by performing acid attacks of different intensities. The results are analysed in terms of pigment decolouration and destruction of the clay lattice, revealed by X‐ray diffraction. Palygorskite pigments are much more resistant than sepiolite pigments. It is shown that indigo does not protect the clay lattice against acid aggression. We show that Maya blue is an extremely resistant pigment, but it can be destroyed using very intense acid treatment under reflux.

Association of indigo to palygorskite and sepiolite phyllosilicates, forming Maya Blue-like systems, is studied by means of electron microscopy, solid-state multinuclear magnetic resonance ( 1 H-13 C CP MAS, 27 Al, 29 Si NMR), visible and infrared spectroscopies (ATR-FTIR, VIS), and solid state electrochemistry. Combination of such techniques suggest that Maya Blue must be viewed as a complex polyfunctional organic-inorganic hybrid material in which different topological isomers of indigo and dehydroindigo molecules, distributed in the surface of the palygorskite framework and in the clay channels, as suggested by size-excluding electrochemical experiments, are involved. Such isomers can tentatively be assigned to different dye-clay interactions involving hydrogen bonding with interaction with Mg 2+ and Al 3+ ions mediated by structural water and interaction of dye molecules with silanol units of the clay.