Thin-section microscopy has proven most useful in the analysis of archaeological ceramics. Yet, not all pottery is suitable for thin-section analysis. There is a common misconception that petrography is used to identify clay. On the contrary, standard thin-section, polarized light microscopy is used to identify sand-sized inclusions (.063 mm – 2 mm) in the clay body. Coarse silt (.03 mm – .063 mm), though visible is frequently too small to identify with confidence and everything smaller than .03 mm is thinner than the thin-section and therefore not visible. Since the analysis is incumbent on the identification of sand-sized inclusions it is essential that the sample submitted contains enough inclusions to provide a statistically viable sample particularly when resource provenance is the goal of the analysis. If the ceramic assemblage is characterized by sparse inclusions then the size of samples to be thin-sectioned must be as large as possible, around 22 mm x 42 mm. This requires that the sample be cut parallel to the vessel surfaces rather than perpendicular as is typically done. We will cut each sample to the optimal size for a nominal fee (50 ¢). Please contact us if you would like to have a ceramic sample analyzed but are unsure whether it is appropriate for a resource provenance study.
Some ceramic traditions are characterized by crushed shell, grit (crushed rock) or grog (crushed pottery) temper. Typically there are sands naturally occurring in the paste, but not necessarily. In such circumstances it may be prudent to first have a ceramic thin-section analyzed prior to submitting sediment samples for a provenance study. For instance, a ceramic sample can be thin-sectioned and a Basic Paste Characterization analysis conducted. The analysis will identify the type and volume of temper/inclusions present as well as other useful technological variables of the sample and provide data as to the feasibility of a resource provenance study.
Thin-section microscopy is a destructive analytical technique. Many curation facilities as well as federal and state agencies have regulations or guidelines pertaining to destructive analysis. It is the client’s responsibility to ensure that all regulations and guidelines are followed when submitting archaeological material for thin-sectioning.
Resource provenance studies are not suitable in all regions due to homogenous mineral compositions of sediments. Please review the Information on Resource Provenance Studies to ascertain whether a provenance study is appropriate for the assemblage in question. In cases where sand petrofacies have already been identified in a region, such as southern Arizona there is no need to submit sediment samples as the results of published studies can be used for provenance determinations. Likewise, our office has samples from many drainages in Wyoming, eastern Colorado and western Nebraska. Please call ahead to see if we already have a sample.
At least one, and preferably two or more sediment samples are required if you would like us to evaluate whether a ceramic sample was manufactured using locally available sediments. If the goal of the analysis is to develop a petrofacies model of a region or locale then 10 or more sand samples from multiple adjacent drainages is typically required. As with all statistical analyses, the larger the sample the higher the confidence one can place in the results. Please review the Resource Provenance Studies section for more information on petrofacies modeling.
Sediment samples should consist of sand from a fluvial source, such as a sandbar or wash, within the same drainage and as close as possible to the site from which the pottery was recovered. In order to obtain a representative sample, dig a trench two to four meters wide and 20-30 cm deep or several holes of the same depth and place the excavated sand on a tarp and mix thoroughly by hand. Once the pile of sand is well mixed screen a couple liters through a 2 mm sieve (No. 10) to remove gravel and granule making sure to collect all the sediment that passes through. Next screen the sample through a 63 µm (No.230) to remove silt and clay. Submit approximately ½ liter of the sand along with UTM coordinates and a map of the sample location. A sample of clay may also be useful if naturally occurring clay deposits are present in the area. A 50-60 ml sample will suffice. Again please submit the UTM coordinates and a map of the sample location. If you do not have a set of standard geological sieves we will process the sample for a $5 fee. Please call ahead if you have any questions or concerns.
Ceramic samples should be individually bagged, labeled, wrapped in shock-absorbent packing material and placed in a box. Please ensure that the package is labeled “Fragile.” Sediment samples should be clearly labeled and placed in a sealed container, or in two (double bagged) ziplock-type bags. Please include the Petrographic Services Order Form, a map showing the location of the site and/or sediment samples as well as a table or list of the UTM coordinates.