Direct current plasma (DCP) spectrometry provides an effective means of analyzing the chemical composition of ancient pottery. Similar to inductively coupled plasma spectrometry (ICP), a related spectroscopic method, DCP offers a highly accurate means of determining the presence of a wide range of elements in any given sample, a necessary first step in all provenance studies. By obtaining chemical “fingerprints” of reference materials (e.g., kiln wasters) of known origin and finding compositional “matches” to pottery samples, one is often able to pinpoint the source of manufacture.
The first application of DCP on archaeological artifacts was conducted In 1994: select fired and unfired Jerash bowl fragments recovered from deposits associated with the late Byzantine pottery kilns in the hippodrome at the ancient Decapolis city of Gerasa (Jerash, Jordan) were examined (Lapp, 1994). In combination with petrographic thin-section analyses, DCP subsequently proved invaluable for the chemical characterization of clay oil lamp samples collected from sites once comprising Roman Palestine (Lapp, forth-coming). The team of F. A. Hart and S. J. Adams first used ICP to determine the provenance of select Romano-British pottery from Hampshire (Hart and Adams, 1983). The recent examination of early Bronze Egyptian pottery from Canaan by means of petrography, neutron activation, and ICP analyses successfully combines the analytical strengths of these archaeometric methods for determining provenance (Porat et al., 1991).
DCP centers on the analysis of samples prepared as solutions. The sample itself, whether a ceramic fragment of a Roman oil lamp or of a Persian storage jar, is brought into solution using a lithium metaborate fusion technique. Following the method outlined by E. M. Klein (1991), Lapp mixed 0.1 g of powdered Jerash bowl sample with 0.4 g of ultra-pure lithium metaborate flux and fused the mixture in preignited crucibles at 1040°C for 13 minutes (Lapp, 1994). After fusion, the pebble-shaped melt was dissolved in 0.24 ml of nitric acid; the resulting concentrated solution was then ready to be analysed for trace elements. For the major elements, a dilute solution of each sample consisting of 0.25 ml of the concentrated solution with nitric acid plus lithium was prepared.
Once in solution, the sample was nebulized and its aerosol directed at the plasma. The plasma itself is created by initiating an electrical discharge within a stream of argon flowing between two electrodes (Potts, 1987). After nebulization, the aerosol particles of the sample solution experienced desolation, vaporization, and atomization. This occurred in the “excitation zone” where the sample temperature ranges between 5700 and 6000 K (Potts, 1987). At this temperature, the elements' atoms undergo transitions to lower electronic states and emit their excess energy as a quanta of light. Because these wavelengths of light are specific to the emitting element, the identification of a tested element can be discerned by a proper spectrometer. The intensities of emission are measured by the spectrometer, measurements of which indicate the elemental abundances in any given sample. Potts notes that samples atomized in a direct current plasma experience significantly lower atomization temperatures than those atomized in the inductively coupled argon plasma of ICP analysis (Potts, 1987).
DCP spectrometry has proven to be an effective, accessible, and affordable means of major and trace element analysis of terracotta wares and lamps alike. Undoubtedly, it should also prove useful in future characterization studies with respect to nonceramic objects, as has already been demonstrated by the related archaeometric method of ICP applied to metal artifacts from late Bronze Age hoards in Slovenia (Trampuz-Orel et al., 1991).
- Hart, F. A., and S. J. Jones. “The Chemical Analysis of Romano-British Pottery from the Alice Holt Forest, Hampshire, by Means of Inductively-Coupled Plasma Emission Spectrometry.” Archaeometry 25.2 (1983): 179–185. The first landmark analysis of ancient pottery using ICP spectrometry.
- Klein, Emily M., Charles H. Langmuir, and Hubert Staudigel. “Geochemistry of Basalts From the Southeast Indian Ridge, 115°E–138°E.” Journal of Geophysical Research 96. B2 (1991): 2089–2107. The methods of sample preparation and procedure for DCP analysis as presented and outlined by Klein were adopted and followed by Lapp in his analysis of Jerash bowl fragments.
- Lapp, Eric C. “A Comparative Clay Fabric Analysis of Fired and Unfired Jerash Bowl Fragments by Means of Petrography and Direct Current Plasma (DCP) Spectrometry.” In Proceedings of the 1994 Byzantine and Early Islamic Ceramics Colloquium in Syria-Jordan (Vth–VIIIth Centuries), edited by Estelle Villeneuve and Pamela M. Watson. Bibliothèque Archéologique et Historique. Paris, forthcoming. Results of the first application of DCP spectrometry to determine the chemical composition of archaeological artifacts.
- Lapp, Eric C. “The Archaeology of Light: The Cultural Significance of the Oil Lamp from Roman Palestine.” Ph.D. diss., Duke University, forthcoming. Demonstrates the usefulness of DCP spectrometry in combination with archaeology and epigraphic investigations into the cultural function of oil lamps in ancient daily life.
- Plank, Terry A. “Mantle Melting and Crustal Recycling in Subduction Zones.” Ph.D. diss., Columbia University, 1993. Highly technical work, written with the geochemist in mind. A model study for understanding the application of DCP spectrometry as it pertains to clay-sediment analysis.
- Porat, Naomi, et al. “Correlation between Petrography, NAA, and ICP Analyses: Application to Early Bronze Egyptian Pottery from Canaan.” Geoarchaeology 6.2 (1991): 133–149. Provenance findings and the value of multiple archaeometric techniques for determining provenance of manufacture.
- Potts, P. J. A Handbook of Silicate Rock Analysis. Glasgow, 1987. General and succinct introduction to the technique of DCP spectrometry, supplemented with schematic diagrams of the instrumentation involved (see esp. pp. 192–197).
- Trampuž-Orel, N., et al. “Inductively Coupled Plasma-Atomic Emission Spectrometry Analysis of Metals from Late Bronze Age Hoards in Slovenia.” Archaeometry 33.2 (1991): 267–277. Pioneers the application of ICP-AES on metal artifacts, as opposed to pottery.
Eric C. Lapp