Electron microprobe analysis of ancient ceramics: A case study from Romania

Electron microprobe analysis (EMPA) is a widely applied technique in the geological sciences and can also be used successfully for archaeometric purposes, e.g. for the study of ceramics. The main advantage is that it allows the identification of components with a wide range of sizes, independent of their nature as primary minerals or firing products. In addition, the minerals can be chemically mapped using Backscattered Electron Image (BSEI) coupled with precisely positioned electron microprobe analysis. Limitations of the method, however, include difficulties with the identification of extremely fine-grained or poorly crystallized phases. Low analytical totals and the lack of any structural information are additional problems.As an example, we focus here on Late Bronze Age ceramic pottery from Ilişua, Transylvania (Romania) and particularly consider the compositions of certain mineral phases such as non-plastic inclusions, matrix and newly-formed compounds with a view to obtaining information on the source materials and their provenance, as well as the technological conditions of ceramic production. The microprobe data indicate the presence of plagioclase (ranging from oligoclase to labradorite), K-feldspar, partly altered muscovite, quartz and an illite-type matrix. In addition, several phases were formed during the firing process, such as a Ca-“illite”, An-rich plagioclase, K-feldspar and a SiO2–Al2O3 compound close to pyrophyllite. The matrix minerals are commonly associated with high concentrations of elements such as Fe, Al, or P, which could either be held within a new lattice structure or may simply adhere to the mineral surface. The main finding of our study is that illite-type silty clays and alluvial sands from the neighbourhoods of the site were used to make pottery that was fired at ~ 950 °C temperature. Our results also indicate considerable potential for related follow-up studies.

Research highlights
► Microprobe highlights small-scale firing transformations in ancient ceramics.
► Ca-“illite”, An-rich plagioclase and K-feldspar form upon firing.
► The newly-formed phases are nonstoichiometric and metastabile.
► Mineralogical constrains indicate a firing temperature around 950 °C.
► Neogene illitic silty clays are the inferred raw materials for pottery.