Intentional clay-mixing in the production of traditional and ancient ceramics and its identification in thin section

Abstract

Ethnographic and historical accounts have indicated that traditional potters intentionally mix two or more sources of clay in order to prepare composite pastes for pottery production. It has long been assumed that a such practice also took place in the past, but detecting evidence for clay mixing in ancient ceramic sherds has proved challenging due to thorough blending, as well as the existence of natural heterogeneity within clay sources. Very few detailed studies on clay mixing exist within the fields of ceramic petrography and ancient pottery technology, and both diagnostic criteria and descriptive terminology are therefore poorly defined. The present paper attempts to fill this gap in our knowledge by reviewing the methods of clay mixing from ethnographic sources as well as archaeologists attempts to identify it in thin section. By studying naturally heterogeneous clay under the polarising light microscope and experimentally mixing pastes according to the various methods reported ethnographically, the study attempts to identify possible lines of evidence with which to detect intentional clay mixing in ancient ceramics. These are then applied to archaeological ceramic sherds from a range of periods and geographic regions. The study reaffirms the difficulties in identifying intentional mixing, which can result in a diverse set of compositional and microstructural phenomena that may also occur naturally, and proposes additional criteria with which to distinguish these two processes. It also highlights the overlap between clay mixing and the technological practice of adding non-plastic temper to the clay body.