Petrographic Analysis of Archaeological Ceramics

Abstract

Petrographic analysis is not new to the study of archaeological ceramics, but it is receiving more attention as it is applied to interesting and diverse archaeological problems. Petrography was developed by geologists to identify minerals in rocks. Archaeologists use it to study the minerals that occur in prehistoric clay matrices transformed by heat to a rocklike state.Petrographic analysis requires a polarizing microscope and a piece of ceramic, called a thin section, 30 microns (0.03 mm) thick. As light from this specialized microscope passes through a mineral, it produces optical properties unique to that mineral. Birefringence and pleochroism are two such properties used to identify minerals, while physical properties such as cleavage and relief are also important discriminators. Several references fully explain these properties and relate them to specific minerals and rocks.Using petrographic techniques, as many as five materials can be studied in a ceramic—the clay itself, natural inclusions in the clay, purposefully added inclusions, and glazes or slips on the clay surface. Under certain conditions the clay or inclusions may melt and a fifth material, glass, may form. Depending on the research problem to be addressed and the samples available, the petrdgrapher studies one or more of these materials.