Developments in Ceramic Materials Research

8
Leslie G. Cecil
can study “the clay itself, natural inclusions in the clay, purposefully added inclusions, and
glazes or slips on the clay surface” (Childs 1989:24).
Petrographic analysis has been adapted from geological techniques of analysis for the
study of soils and rocks and is useful for archaeological ceramics because, to a large extent,
geological sources differ enough across regions to allow for comparison of different clays
(Blatt 1992). These methods are applicable to pottery analysis because pottery can be
regarded as a metamorphosed sedimentary rock due to the composition of a sherd consisting
of clastic grains imbedded in a clay matrix which has been transformed to “rock” through the
process of firing (Bishop et al. 1982; Rice 1987b:376). Understanding these basic principles
of geology, plus other principles of optical mineralogy, allow the description of pottery pastes
and clays.
Additionally, petrographic analysis can be used to establish technological characteristics
within the Postclassic ceramic complex because petrographic analysis aids in the
classification of sherds into specific groups (Childs 1989:24). This aspect of petrography is
most helpful when a sherd or vessel cannot be assigned to a typology based on surface
decoration, vessel form, or rim diagnostics (Shepard 1956:165). I anticipated that the above
situation would arise because many of the sherds are highly eroded and because the
variability within pastes may be great enough that I could not easily identify the ceramic
group. As a result, paste characteristics (clay matrix and inclusions) were examined to
establish possible differences between types.
Once a sherd or vessel was assigned to a ceramic group, I could develop additional
behavioral information from a classification based on petrography because petrography can
answer process-oriented questions from a diachronic or synchronic perspective (Carr 1990;
Childs 1989). It is possible to create a time series that models shifts in technological
characteristics such as paste and temper characteristics (Braun 1982, 1985; Carr 1990).
All of the samples (described above) were cut with a wet saw for the preparation of thin
section slides. The cut sherds were sent to Spectrum Petrographics where they were
embedded in an epoxy block. The most fragile sherds were vacuum impregnated and then
embedded in an epoxy block. The block was cut in such a manner that a thin section
measuring .03 mm thick resulted. The resulting thin section allowed me to identify minerals
in the clay paste with the use of a polarizing microscope.
Mineral size, shape, roundness, and frequency provide qualitative and quantitative data
by which to infer manufacturing recipes. Mineral size, shape, and roundness (qualitative data)
are established through a comparison of various graphs and tables (Shackley 1975:44-51).
The most common geological method of determining the quantity of minerals in a thin section
(quantitative data) is point counting. Point counting determines the number of different
minerals along a predetermined area (for example, 10 mm) of the length and width of the
section (Chayes 1956). Various studies have employed different methods for counting the
frequency of inclusions (e.g., Middleton et al. 1985; Peacock 1973). However, because I was
not interested in the absolute number of minerals in the thin section, but rather in what
minerals exist and their relative frequency, I used a non-volumetric area frequency counting
method (Cecil 2001; Dickenson and Shutler 1979). Area counting pertains to counting all of
the minerals in the standard image.