Archeometric analysis of clay building materials from the iron age of the nw of Portugal: a comparative study

Abstract

Archaeometric analyses have been widely applied in studies of Iron Age building materials. Clayed plasters and pavements are rarely found in Iron Age settlements from Northwest Portugal, why few archeometric studies were done, to the date, on this type of building materials. This research aims to present the results from microanalysis of clayed plasters and pavements from three Iron Age archaeological sites located in NW Portugal, namely São João de Rei, Castro Máximo and São Paio settlements and the ceremonial site (?) of Frijão. The materials collected for analysis comprise a chronology from the fourth to the second centuries BCE and from the second BCE to the first AD centuries, which corresponds to the Early and Later Iron Age of the region. The study was done by means of mineralogical petrographic analysis, X-ray powder diffraction (XRD) and Scanning Electron Microscopy/Energy Dispersive X-Ray Spectroscopy (SEM-EDS). The main components of the studied materials are micas, particularly muscovite, feldspar and quartz, although small amounts of phyllosilicates, such as kaolinite and illite were also identified. Some of the pavements show signs of having being fired. The mineralogical data indicates that firing was done at temperatures lower than 950º C. The minerals from these materials came from the surroundings of the archaeological sites as they are commonly found in the granitic soils and weathered granitic overburden that characterize the region. This investigation offers fresh insight into proto-historic building materials from the Iron Age context of NW Portugal and provides useful knowledge of the constructive characteristics and skills of this chronology.