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The obsidian of Sierra de las Navajas is well known for its green color and gold hue. In order to relate these features with compositional and microtextural characteristics, we have carried out a microanalytical study by Wave Dispersion System associated to Electron Probe Micro-Analyzer, Scanning Electron Microscope observation, and X-rays micro-tomographic analyses of samples showing different colors (dark to light green, sometimes with bands of different color intensity) and hues (changing, uniform, no hue). In accordance with previous studies, the green color of the obsidian seems to be related to a high iron content, probably in its reduced state. However, no significant difference in composition occurs between dark and light green samples. The SEM observation and microtomographic study revealed the absence of microcrystals and the occurrence of vesicles of different size, shape, and orientation. Lighter green colors are shown by highly vesiculated surfaces, whereas non-vesiculated samples are darker. On the surfaces with a high concentration of coarse vesicles, a uniform golden hue is observed. Decreasing vesiculation gives a hue changing with the angle of incident light. However, when the vesicularity approaches zero, no hue is visible. The iso-orientation of vesicles along preferential directions and their distribution in bands determine the variation in color intensity and hue on differently oriented surfaces. Microvesiculation also influences other characteristics that were important features for the use of obsidian in the past, such as fracture, transparency, and roughness. [ABSTRACT FROM AUTHOR]

Hydraulicity is a well-known characteristic of mortars and plasters from the Roman, Late Roman, and Medieval periods, while the use of hydraulic mortars in pre-Columbian Mesoamerica and Central Mexico in particular, has not received equal attention. To address this issue, in this paper, we discuss the problem of hydraulicity and suggest a method to study it in Mesoamerican plasters. To prove the effectiveness of this method, we selected plaster samples from the archaeological site of Teotihuacan (Mexico), the main city of Central Mexico during the Classic period. The characterisation of plaster samples confirmed that, similar to other Teotihuacan plasters, they are made of two layers: the outermost layer (enlucido), composed of a mixture of lime and volcanic glass shards, and the underlying layer locally called firme , consisting of crushed volcanic scoria (tezontle) mixed with a mud-based binder. The compositional characterisation of the plasters confirmed their hydraulicity and the combination of microchemical (energy dispersive spectrometer (EDS) microchemical analysis), microphysical (Vickers microhardness), and petrographic (optical microscopy) measurements demonstrated that it was not the presence of glass shards in the external plaster layer which produced hydraulicity, as we originally thought. Instead, this was due to the reactivity of the tezontle , present in the lower layer. This suggests that the small thickness of the enlucido layer, in addition to save the amount of lime used, increased its performance, promoting chemical reactions with the tezontle in the contact layer and that the glass shards played another role in the mixture, rather than producing hydraulicity. • The compositional characterization of 11 plasters from Teotihuacan was performed. • The approach used allowed to study the hydraulicity of the plasters. • A correlation between hydraulicity and tezontle (volcanic material) was highlighted. • Through p-XRF analysis were identified the pigments used in some plasters. [ABSTRACT FROM AUTHOR]