Ceramic petrography utilises the techniques of optical mineralogy and petrology commonly used in the geological sciences. A thin slice from a pot sherd is mounted on a glass slide and ground down to a thickness of approximately 30 µm (0.03mm). This thin section can be viewed using a polarising (petrographic) microscope, allowing the inclusions (which may include minerals, rocks, organic material, and man-made materials, such as grog (chamotte) and slag) to be identified. The inclusions may be naturally occurring in the clay, or they may have been added intentionally by the potter as a temper to improve the properties of the clay during working, firing and subsequent use of the vessel. A ceramic is typically composed of 70-80% clay, 20-30% non-plastic inclusions and about 5% voids. The texture and composition of the fabrics in a selected pottery assemblage can be characterised and sherds with similar fabrics grouped together. These fabric groupings reflect similar raw material usage and technological traditions, which can then be related to the typology of the pottery under analysis. This may reveal correlations between fabric group, and vessel or ware type. From the identification of the rock and mineral inclusions it may be possible to suggest possible provenance/s for the pottery studied, by comparison with the local geology in the area where the pot sherds were found, and over larger areas for exotic non-local fabrics. Information on the manipulation of the raw materials by the potter, forming techniques used in the construction of the vessels, and the conditions under which the pottery was fired may also be gained from the study of thin sections of pottery.
When the upper polarising filter on a polarising microscope is inserted, the polarising filters are said to be crossed. Crossed polars produce darkness when nothing is on the stage and when viewing isotropic materials. Anisotropic minerals produce a range of interference colours in most positions between crossed polars. (See also PPL.)
These are the rock, mineral, man-made materials (e.g. grog, slag) and organic particles in a ceramic body. They may be a natural part of the clay used, or they may have been intentionally added as a temper (see below).
A descriptive term used to define the degree to which the non-plastic inclusions are graded according to grain size. If most of the grains have a limited size range it is said to be well-sorted. The classification often used is that devised for sedimentary rocks by Pettijohn, F.J., Potter, P.E. and Siever, R. (1973) Sand and Sandstone. Springer, Berlin. See also Udden-Wentworth Grain Size Scale.