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.

Two Belgian early Neolithic ceramic thin-sections illuminated with cross-polarized light, showing contrasting suites of minerals.

The Museum'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 instance, thin-sectioning of early Neolithic ceramic sherds from Belgian Linienbandkeramik culture villages reveals different suites of mineral types in sherds that were previously studied by LA-ICP-MS (figure), providing a second line of evidence as to how these ceramics were produced, and what locally or non-locally available materials Neolithic potters utilized to produce their wares.