Petrography comprises the mineralogical description and analysis of stone and pottery by utilizing the techniques of optical mineralogy and petrology commonly used by earth scientists. This method is carried out to establish the origins of raw materials and, for pottery vessels, how they were fabricated. Knowing the origins of raw material helps the archaeologist establish and understand patterns of trade, manufacturing processes and even political organization. Petrography is done by examining thin-sectioned samples through a microscope. A thin slice from a vessel or a sherd is mounted on a glass slide and ground down to a thickness of 30 microns (0.03 mm). The thin section can then be viewed using a polarizing (petrographic) microscope, allowing the inclusions, (which may comprise minerals, rocks, organic material and man-made materials such as grog 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. The texture and composition of the fabrics in a selected pottery assemblage can be characterized and sherds with similar fabrics can be grouped together. From the identification of the rock and mineral inclusions it may be possible to suggest provenance(s) for the pottery studied, by comparison with the local geology in the area where it was 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 pottery thin sections.