Micromorphology of earth building materials: Toward the reconstruction of former technological processes (Protohistoric and Historic Periods)

Abstract

Earth building materials are often identified in the field, but very few microscopic studies focus on the identification of the technological processes, which produced them, especially in temperate contexts. The identification of such processes is crucial as it is the basis of the archaeological investigations concerning both the origin and the diffusion of building technologies. The aim of this paper is to describe and expand the definitions of the microscopic features typically associated with the steps associated with the “chaînes opératoires” of earth building on the basis of micromorphological analyses and development of a reference collection of earthen materials. The results have identified a range of characteristics associated with past earthen construction processes (including microscopic features linked to earth moisture when mixing, degree of mixing, the moisture of the earth when applied to the walls or on the ground, as well as those related to compaction and shear stress) that are all important factors that explain former processes. Strong compaction of earth in a dry state is typical of rammed earth, medium compaction together with a higher degree of moisture is typical of cob wall, and perpendicular organization of the earth on the face of the wall indicates the use of a formwork or shuttering. Thus, the use of microscopic features identified from analysis of vernacular and ancient materials helps to characterize and identify processes such as Roman rammed earth at the site of Rirha (Morocco). It also highlights cultural specificities, such as the diversity of cob wall and earth flooring processes during the Iron Age, as on the site of Lattara (southern France), and the degree of compaction of rammed earth wall for the Roman Period. Analyses carried out in a robust archaeological framework reveal that each wall present a specific vertical and horizontal organization linked to the building processes at macroscopic and microscopic scales, which should be taken into account when studying and sampling earthen walls in order to correctly identify former techniques.