Optical Mineralogy

Thin-section microscopy is a technique for characterization of the mineralogical and textural composition of materials.  In archaeology, this technique is most commonly applied to ceramics as a means of comparing them to potential raw materials utilized in their manufacture, including both clays and other aplastic materials ("temper") that potters may have deliberately added to modify the working properties of the finished ceramic paste.  Materials analyzed by this technique are attached to glass slides with epoxy, then ground and polished to a uniform thickness of 30 micrometers.  Using a polarizing light microscope, thin-sections are examined by transmitting both plane- and cross-polarized light through them.  Different mineral types reorient light in different ways, and examination of coloration, interference patterns, and other optical properties allow for identification of mineral types and frequencies in ceramic thin-sections.  Additionally, textural properties including grain size, angularity, and orientation are examined.  The information gained provides a strong line of evidence as to the kinds of geological formations from which potters obtained clays and tempering materials.

The EAF's Petrographic Microscopy Lab features a Meiji Techno Polarizing Light Microscope.  In conjunction with chemical analysis by LA-ICP-MS, which provides a chemical signature for the fine fraction of ceramic paste, thin-section microscopy provides a compositional signature for the coarser grain types present in studied ceramics.  For example, thin-sectioning of plainware ceramic samples from Jecosh (Ancash, Peru) revealed the specialized knowledge and making practices of Recuay potters. The particular ceramic shown above, likely used between 100–400AD, was recovered from a domestic trash midden. To make their wares, Recuay artisans combined less commonly used white kaolin clay with terracotta clay, as shown by the white streaks in the ceramic paste (Grávalos et al. 2022). This practice of mixing special clay into plainware bowls used in everyday spaces had not been previously documented, and helps us better understand the cultural, economic, and environmental milieu of the ancient Andeans who made and used this pottery.”

 Grávalos, M. Elizabeth, Rebecca E. Bria, and George F. Lau. 2022. “An Examination of Recuay Kaolin Pottery Production and Exchange through Petrography and LA-ICP-MS (100–700 CE; Ancash, Peru).” Archaeometry 64 (6): 1340–58.