Thin-Section Petrography or Petrographic Analysis:

How is this done?

Minerals are identified by passing polarized light through a thin section (30 microns thick) of the potsherd or stone. The polarizer on the microscope polarizes the light so that it vibrates in only a single plane. The light passes through the sample that is placed in a stage that is capable of being rotated 360 degrees. The light then passes through a second polarizing filter, which permits only passage of light vibrating in a plane that is perpendicular to that of the first polarizing filter. The atomic structure and orientation of the types of mineral crystals in the rock or pot sherd placed on the rotating stage changes the direction of the light path, thus determining how the polarized light will be transmitted to the microscopes optics.

Why should archaeologists be interested in thin-section petrography?

Thin-section petrography will allow the for the identification of minerals in lithic tools and ceramics that can be compared with regional geological information and with previous petrographic studies to identify potential sources of raw materials and artifacts. The resulting information can be used to address research questions regarding regional mobility, exchange, and specialization in production. Petrographic analysis is often conducted in conjunction with bulk compositional techniques, such as Neutron Activation Analysis, to aid in identifying the sources of variation between composition groups.

What kind of samples are required for petrographic analysis?

Pot sherds and lithic samples need be no larger that 2x3 centimeters in size and can be smaller. A larger sample provides for a better estimate of the variability in the inclusions present in a pottery sherd or rock fragment. The sample preparation technique requires that the sample be ground to a thickness of thirty microns after its attachment to a microscope slide. Ceramic samples frequently require stabilization as part of the preparation process. Petrographic analysis is destructive to the sample, but the thin section will stand as a permanent record of the artifact.

What else is important to know regarding petrographic analysis?

The origin of mineral grains and rock fragments observed in ceramics should be treated as a research question. When studying the sources of a particular type of pottery, it is frequently useful to collect clay samples near the site area, fire them, and analyze the fired clay tiles in conjunction with the sample of ceramics. You may collect these clay samples and submit them to us to be fired and analyzed. Also geological formations or clay sources with very low or very high compositional variability are difficult to characterize and compare with archaeological materials. When this is the case it is best to combine petrography with another technique such as Neutron Activation or X-ray Fluorescence.

Updated 01-13-2008