The study of rocks and similar materials in thin sections under a microscope, using high magnifications, is known as petrography. The method has been applied to pottery since the end of the nineteenth century—for almost as long as it has been in use in geology to examine rocks. To make a thin section, a 5-millimeter-thick slice of the object to be examined is glued to a glass slide and then ground to a thickness of 30 microns (0.03 mm). At this thickness most minerals become transparent to light. The petrographic microscope includes several components that alter the light passing through the thin section and provide each mineral with a unique set of optical properties that can be identified under the microscope. Thin sections are inexpensive to prepare; the training to analyze them is not complicated; and they can be stored for future reference and reexamination as many times as necessary.

Petrography is applied mainly to pottery, but also to plaster, stone, and metal artifacts. The method has been used to identify the sources of raw materials in pottery production, to locate the provenance of vessels, to assess the techniques used by potters, and to estimate firing temperature.

The fabric (the combination of raw materials and texture as seen under the microscope) consists of matrix and temper. The matrix consists of clay (of clay minerals too fine to be identified under the microscope) and silty components. By identifying microfauna and silty minerals, the source of the clay can be determined. The temper is the sand-size fraction that, in most cases, the potter added to the clay. Rock fragments, minerals, fossils, and plant material are natural tempering materials; grog (crushed pottery) and ash are temper assembled by potters. The composition of the temper reveals which rocks were used; its texture (shape, sorting, size, quantity) reveals how it was prepared before being added to the clay. Firing temperature can be estimated using optical changes that occur in different minerals at different temperatures. Grain counting, a useful petrographic technique, is used to obtain quantitative information on the various temper components.

Thus, from a thin section which ingredients the potter mixed and in what proportion can be determined, as well as how the materials were processed and fired and the source(s) of the raw materials. This kind of technological information is the advantage petrography offers over neutron activation analysis (NAA), a method mainly used in provenance studies. [See Neutron Activation Analysis.] Moreover, using petrography to identify the provenance of a sample does not require a large database of vessels from known sources (such as kiln wastes). A thorough knowledge of the regional geology and the raw materials available can tell a petrographer whether a sample is local. Petrography is particularly useful for analyzing coarse, low-fired ware, when abundant information can be obtained; however, it is also applicable to fine ware. It can be used on its own or as a preliminary study preceding other types of analyses to select representative samples for more expensive or less available techniques.

Bibliography

  • Amiran, Ruth, et al. “The Interrelationship between Arad and Sites in Southern Sinai in the Early Bronze Age II.” Israel Exploration Journal 23 (1973): 193–197. Combining typology and petrography to solve an archaeological problem neatly. A classic.
  • Gilead, Isaac, and Yuval Goren. “Petrographic Analyses of Fourth Millennium B.C. Pottery and Stone Vessels from the Northern Negev, Israel.” Bulletin of the American Schools of Oriental Research, no. 275 (1989): 5–14. Petrography used for stone artifacts with unexpected results and an application of the grain-counting technique in pottery analysis.
  • Porat, Naomi. “Petrography of Pottery from Southern Israel and the Negev.” In L'urbanisation de la Palestine à l'âge du Bronze ancien: Bilan et perspectives des recherches actuelles; Actes du Colloque d'Emmaüs, 20–24 octobre 1986, edited by Pierre de Miroschedji, vol. 1, pp. 169–187. British Archaeological Reports, International Series, no. 527. Oxford, 1989. Demonstrates the wealth of information obtained from petrography and gives an example of the variety of fabrics found in pottery within a limited area and period.
  • Porat, Naomi, et al. “Correlation between Petrography, NAA, and ICP Analyses: Application to Early Bronze Egyptian Pottery from Canaan.” Geoarchaeology 6 (1991): 133–149. Petrography used in combination with other analytical techniques, demonstrating its effectiveness.
  • Rothenberg, Benno, and Jonathan Glass. “The Midianite Pottery.” In Midian, Moab, and Edom: The History and Archaeology of Late Bronze and Iron Age Jordan and North-West Arabia, edited by John F. A. Sawyer and David J. A. Clines, pp. 65–124. Sheffield, 1983. Methodical study, using petrography to its utmost capacity.

Naomi Porat