Spectroscopic studies of Maya pigments

摘要

The Maya of Central America developed a complex society: among their many achievements they developed a writing system, complex calendar and were prolific builders. The buildings of their large urban centres, such as Copan in Honduras, were decorated with painted stucco, moulded masks, carving and elaborate murals, using a range of coloured pigments. In this study the paints used on the buildings of Copan and some ceramic sherds have been investigated, non-destructively, using micro-Raman spectroscopy, micro-ATR infrared spectroscopy, environmental scanning electron microscopy with energy dispersive X-ray analysis (ESEM-EDX) and FTIR-ATR imaging spectroscopy. The paint samples come from four buildings and one tomb covering three time periods in the four hundred year history of Copan. The main pigment used in the red paint on these samples was identified as haematite, and the stucco as a mixture of calcite particles dispersed throughout a calcite-based lime wash stucco. The composition and physical nature of the stucco changed through time, indicating a refining of production techniques over this period. A range of minor mineral components have been identified in each of the samples including rutile, quartz, clay and carbon. The presence and proportion of these and other minerals differed in each sample, leading to unique mineral signatures for the paint from each time period. ududGreen and grey paints have also been identified on one of the buildings, the Rosalila Temple. The green pigment was identified as a celadonite-based green earth, and the grey pigment as a mixture of carbon and muscovite. The combination of carbon and mica to create a reflective paint is a novel finding in Maya archaeology. The high spatial resolution of the micro-FTIR-ATR spectral imaging system has been used to resolve individual particles in tomb wall paint and to identify their mineralogy from their spectra. This system has been used in combination with micro-Raman spectroscopy and ESEM-EDX mapping to characterize the paint, which was found to be a mixture of haematite and silicate particles, with minor amounts of calcite, carbon and magnetite particles, in a sub-micron haematite and calcite matrix. The blending of a high percentage of silicate particles into the haematite pigment is unique the tomb sample. The stucco in this tomb wall paint has finely ground carbon dispersed throughout the top layer providing a dark base for the paint layer. Changing paint mixtures and stucco composition were found to correlate with changes in paint processing techniques and building construction methods over the four hundred years of site occupation.