Application of Petrofabric and Phase Equilibria Analysis to the Study of a Potsherd

This paper describes how two standard materials analysis techniques using a petrographic microscope and X-ray diffractometer can be used to determine the fabrication method and firing temperature of a ceramic vessel even when only a small sherd is available. The amount clay is deformed during construction of a vessel depends on the fabrication method. Use of the coiling technique, for example, would involve more deformation than the slab method. The amount of stretching can be calculated from the degree of preferred orientation of elongate pieces of temper in the ceramic, as measured in a thin section cut through the sherd. The method is applied to a Late Woodland vessel from South Windsor, Connecticut, which was tempered with shell, quartz and grass. The shell has completely dissolved away and the grass burned during firing, to leave elongate cavities in the ceramic. The quartz grains, which are also elongate, are interpreted to be knapping debitage. The maximum degree of stretching calculated from the preferred orientation of this temper is only 2·3, which indicates that the vessel was fabricated from slabs rather than from coils. An X-ray image of a large part of the reconstructed vessel confirms the presence of these slabs, which are about 10 cm long and range in height from 2 to 5 cm. This image also indicates that the slabs were added in a counterclockwise direction, which suggests manufacture by a right-handed potter. Although the surface of the vessel is tan-coloured, most of the ceramic is black due to the reduced state of iron in the interior of the vessel. Burning of the grass temper during firing produced reducing conditions in the interior of the vessel wall that allowed wüstite (Fe xO) to form along with magnetite (Fe 3O 4). The wüstite has a unit cell dimension of 4·25 Å (as measured by X-ray diffraction), which indicates a composition that can coexist with magnetite only at 880°C. This is believed to be the maximum temperature reached during firing of the vessel. The black, reduced ceramic is not wet by water, whereas the red, oxidized surface material is. The grass temper, which produced the reduced ceramic, may therefore have been added to the clay to make a more impervious vessel.