Inside the matrix

Abstract

The differentiation between various fine-grained and clay-rich materials, also described as argillaceous inclusions, specifically mudstones and grog, presents considerable challenges in the analysis of archaeological ceramics. These difficulties arise from overlapping characteristics and the mineral uniformity of raw materials in certain environments. This study addresses these challenges by developing an interpretative framework to identify robust characteristic features to distinguish between mudstone and grog as well as recurring pitfalls. To this end, we constructed an experimental reference set comprising 26 testbars, which are analyzed using thin section petrography and SEM-EDS to examine the features of mudstone, grog and clays. The materials derive from the Kur River Basin in Southwest Iran, as this is an area with evidenced presence and use of extensive argillaceous inclusions through time. Additionally, we studied thin sections from a variable selection of archaeological ceramics (n = 11) with argillaceous inclusions from the same region that date from the Banesh to Late Bronze Age periods (ca. 3500 to 900 BCE) for comparison. The aim of this research was threefold: (1) assess optical identifiable characteristics of the argillaceous materials, (2) determine the compositional variability in the mudstones and grog fragments in this region and (3) document and test the potential of mudstone analysis for characterization and provenance purposes. Our findings indicate that distinguishing features are influenced by the granularity of the grog and sintering process. Notably, characteristics associated with mudstone inclusions comprise a compact, fine-grained texture, homogeneity with few to no constituents, solid or polygonal cracking, higher roundness, variability in particle sizes, and a pronounced likelihood of birefringence at lower firing temperatures where sintering has not yet occurred. The differentiation challenges intensify once sintering develops at higher firing temperatures, particularly when fine grog is present. In such cases, the angularity of the inclusions emerges as the primary criterion. SEM-EDS analysis further corroborated the established criteria, demonstrating a consistent chemical difference between grog and mudstone, aside from one overlapping type. Moreover, it supports a typological approach for determining these characteristics in thin section petrography and highlights a potential for provenance determinations of mudstone within a regional framework. Finally, analysis of the argillaceous inclusions in the archaeological ceramics suggests that no grog was used in these ceramics and that an intra-basin variability of mudstone was present that allows for higher resolution provenance studies.