Characterization of earth building materials from archaeological sites on La Rioja and Catamarca (Argentina)

SPENGLER, GISELA; DO CAMPO, MARGARITA; JUAN JIMÉNEZ-MILLÁN

Madrid-Sevilla

Congreso; Trilateral Meeting on Clays; 2010

Spanish Clay Society (SEA) - Clay Science Society of Japan (CSSJ) - Clay Minerals Society (CMS)

The use of earthen architecture had been widespread across the world in almost all continets since ancient times. In South America it has a long tradition, begining in pre Columbian cultures, but it survives until the present. The use of sediments for the construction of Earthen buildings was a common prehistoric technique, widely employed in arid and semi-arid lands, where other construction materials were scarce.  Earthen building techniques are varied, and they can be classified in three general types: monolithic systems (rammed earth), brickwork (adobe o mudbrick), and mixed systems (wattle and daub). Nevertheless, the recognition of archaeological evidence indicative of these kind of buildings is problematical, because this kind of architecture is vulnerable to destruction by weathering, especially by wind and water erosion. These processes frequently blur the evidence of earthen architecture, leaving fillings or mounds composed by loose and fine-grained  material difficult to attribute to disaggregated earthen architecture. So, the presence of earthen architecture is often assumed rather than demonstrated. Recently, analytical studies of building materials have been enriched by different approaches, like geoarchaeology, archaeometry and conservation, together with advances in archaeology of architecture. Assuming the perspective of archaeometrical approaches, we have carried on several physical and chemical analytical tests in order to characterize archaeological building materials from La Rioja and Catamarca provinces (northwestern Argentina), attempting to define parameters for their determination in archaeological contexts. These materials have been recovered from different sites, belonging both to prehispanic and post-Hispanic times. Materials have been examined with regard to their chemical (organic carbon content by calcinaction and Walkley-Black, phosphate content by Kurtz and Bray1) and mineralogical composition (X-Ray diffraction analysis at bulk sample and in the < 2 µm sub-fraction, optical microscopy, SEM), but also to their phisycal (granulometric analysis, density, porosity ?) and mechanical properties, in order to investigate the nature of the raw materials used, the technology of manufacture and the potential use of specific recipes. In this contribution we present the preliminary results of the analytical tests performed. XRD analyses indicate that the main components of all the samples are quartz and micas, with minor hematite. Building materials from Catamarca sometimes contain calcite, and less frequently heulandite and gypsum. La Rioja materials also contain potassium feldspar and calcite. The XRD study of <2 µm sub-fractions indicates that Catamarca samples are composed of abundant illite-mica, smectite, subordinate chlorite and sometimes minor kaolinite. Samples from La Rioja contain higher proportion of illite-mica and variable smectite contents. The technological properties of earthen building materials should be mainly controlled by the proportion of clay- versus sand+silt-sized material, but also by the kind and relative proportion of expandible clays in raw material.