Identifying Clay Sources of Prehistoric Pottery Using Atomic Spectroscopy

MICROCHEMICAL JOURNAL 56, 40–46 (1997)ARTICLE NO. MJ961434

Identifying Clay Sources of Prehistoric Pottery UsingAtomic Spectroscopy

Elizabeth A. Kneisel,* Nancy A. Ciszkowski,* Walter J. Bowyer,*,1

Frank S. Walker,† Thomas G. Huntsberger,† and Richard D. Foust†,1

*Department of Chemistry, Hobart and William Smith Colleges, Geneva, New York 14456; and†Department of Chemistry, Northern Arizona University, Flagstaff, Arizona 86011

Atomic spectroscopy is applied to the compositional analysis of sherds from pottery producedby the Hohokam about 1150 A.D. near modern day Tucson, Arizona. Analysis of NationalInstitute of Standards and Technology standard reference materials is used to develop the neces-sary digestion procedures and to demonstrate that atomic absorbance and inductively coupledplasma emission spectroscopies offer a good substitute for neutron activation analysis in thecompositional analysis of ceramic artifacts. Accuracy and precision of the determination of majorand minor elements are evaluated. Analysis of 16 possible clay sources and 30 sherds suggeststhat the pottery sherds were from locally produced pottery. q 1997 Academic Press

INTRODUCTION

Compositional analysis has long been used by archaeologists to classify prehistoricpottery by elemental composition. Less often, clay sources have been identified forpottery by matching elemental composition of the pottery to that of suspected claysources. Neutron activation analysis (NAA) is the most commonly used analyticaltechnique for compositional analysis for a variety of reasons. NAA has very low limitsof detection for a wide range of elements and requires a relatively small sample size(1). Although pottery sherds are frequently abundant at archaeological sites, using asmall piece of each sherd allows the remainder to be studied by other techniques suchas polarizing light microscopy.

Unfortunately, NAA is an expensive technique and can be performed only in alimited number of laboratories. Therefore, there is increasing interest in the use ofatomic spectroscopy for compositional analysis of pottery (2–5). Atomic spectroscopycombines low limits of detection for many elements with a relatively small samplesize. Inductively coupled plasma (ICP) atomic emission spectroscopy (AES) has theadded advantage of very wide linear dynamic ranges and relatively few chemicalinterferences (6). The principal disadvantage of atomic spectroscopy is the difficultyinherent in the digestion of aluminosilicate clays. One group has used acid ‘‘extrac-tion’’ in place of digestion (7), but this approach is problematic (e.g., 8).

In this paper we describe the use of standard reference materials (SRM) fromthe National Institute of Standards and Technology (NIST) to test various digestionprocedures. We then use digestion and atomic spectroscopy to identify possible claysources for pottery sherds found at the West Branch site near Tucson, Arizona. West

1 Authors to whom correspondence should be addressed.

400026-265X/97 $25.00Copyright q 1997 by Academic PressAll rights of reproduction in any form reserved.

ah0a$$1434 04-15-97 01:40:14 mica AP: MICROCHEM

41SOURCES OF PREHISTORIC POTTERY

FIG. 1. Map of Tucson basin indicating West Branch site and locations of sampled clay sources (squares—possible clay sources; circles—rejected clay sources). Major watercourses are labeled.

Branch was occupied during the Sedentary phase of the Hohokam culture. Pottery withthree types of decoration was analyzed: plainware, red-on-brown, and polychrome.

MATERIALS AND METHODS

Four SRMs were purchased from the NIST: 1646 Estuarine Sediment, 2709 SanJoaquin Soil, 1633a Coal Fly Ash, and 2704 Buffalo River Sediment. Thirty potterysherds of three types (plainware, red-on-brown, and polychrome) and clay samplesfrom 16 possible sources (mixed with temper and fired at 5007C) were generouslyprovided by Statistical Research (Tucson, AZ). Temper in the pot sherds was identifiedas Tucson basin sand by polarizing light microscopy and appropriate samples weremixed (60%) with the clay samples prior to analysis. Figure 1 is a map of the Tucsonbasin and indicates the locations of the West Branch site and the clay sources. Theouter layer of the pottery sherds was removed with a tungsten carbide burr (becauseof possible postdepositional alteration as well as to remove decoration) and the centralpart was ground to a fine powder (particles less than 100 mm by microscopic observa-tion) in an agate mortar and pestle. All samples and SRMs were dried for 2 h at1107C immediately before analysis.

A standard method for digestion by HF was used (4), but is described briefly here.


42 KNEISEL ET AL.

(CAUTION: HF is very corrosive and extremely toxic. Anyone using HF should befully informed of its hazards and be provided with appropriate protective gear and anemergency response kit.) In this method, 0.25 g of sample was shaken with 5 ml HFand 0.5 ml aqua regia for 15 h followed by 1 h of heating at 957C. The solution wasthen neutralized with 75 ml of saturated aqueous boric acid and brought to 100.0 gwith distilled water. These solutions were analyzed for Fe, Mn, Ti, and Zn by atomicabsorption spectroscopy (AA) using a Perkin–Elmer 560 spectrometer. The manufac-turer’s recommended instrument settings were used with one exception. The alternate,less sensitive, iron line was chosen for Fe—analysis was performed at 373.7 nm witha slit of 0.2 and air as oxidant—because iron concentrations were too high for analysisby the high-sensitivity line.

The details of the fusion technique were determined by trial and error analysis ofthree different SRMs. Lithium metaborate (LiBO2, 0.75 g) was mixed well with 0.25g of sample and then fused at 9257C for 20 min in a graphite crucible. The resultingbead was dissolved in 10 ml of nitric acid and 20 ml of water with heating andstirring. After dissolution the total volume was brought up to 100.0 ml with distilledwater. The solutions were analyzed by ICP–AES using either a Jobin Yvon 38 or anARL 3410/ ICP spectrometer. Both instruments were fitted with a v-groove nebulizerbecause of the high dissolved solids in the solutions. Even with the v-groove nebulizer,frequent recalibration was necessary (as determined by analysis of a standard solution),presumably due to salt deposition at the capillary tip. Al (at 396.152 nm), Ca (393.367),Ba (455.403) and Sr (421.552) and, for a few samples, Fe (273.955 nm) were deter-mined by ICP.

After the completion of this project, we found that a simple revision of our fusiontechnique allows determination of a much wider range of elements and allows one toavoid the hazardous HF digestion. If the bead produced from the fusion is dissolvedin 8% HCl/8% HNO3 (instead of the 10% nitric acid used in this study), excellentprecision and accuracy are obtained for 10 elements.

Standards were prepared by dilution of ISO 9001 SPEX Plasma Standard solutions;all standard solutions were matrix-matched with the digested samples (e.g., 0.5 gLiBO2 and 10 ml nitric acid for analysis of fusion digestion solutions). All reagentswere purchased either from Fisher Scientific or from Aldrich Chemical Company.

Approximately 80% of the samples were analyzed by duplicate digests. Further-more, digest solutions were frequently analyzed in duplicate. Also, standard solutionswere analyzed regularly as if unknowns to check calibration. Finally, one SRM wasanalyzed in parallel with every batch of sample digests.

Statistics were computed using SYSTAT 5.02 (Evanston, IL).

RESULTS

Table 1 contains the results for replicate digests of SRM 1646 by HF digestion.Fe, Mn, Ti, and Zn are all within {10% of the certified values, indicating that theanalysis is accurate for these elements. However, when analysis of HF digests of SRMwas performed for Al, Ca, Ba, and Sr, values obtained were frequently below thecertified value. Furthermore, duplicate analysis of clays showed that HF digestiongave very poor reproducibility for these elements. We attribute this to the formationof insoluble precipitates (e.g., BaSO4). For this reason, these elements were determined



TABLE 1NIST SRM 1646 Estuarine Sediment Was Analyzed

for Fe, Mn, Ti, and Zn by AA Following HF Digestion(Values Are from Independent Digests)

Element Certified ppm Percent recovery

Fe 33,500 102, 102, 110Mn 375 104, 98, 105Ti 5100 90, 93, 103Zn 138 100, 96

using lithium metaborate fusion. In Table 2 are results for analyses of SRM 1646,2709, and 1633a following lithium metaborate fusion. As can be seen, these valuesare again all within {10%, indicating that lithium metaborate fusion is acceptablyaccurate for these elements. Unfortunately, analysis of the solutions produced byfusion for Fe, Mn, and Ti yielded low values relative to the certified values. Therefore,Fe, Mn, Ti, and Zn were determined by HF digestion and Al, Ba, Ca, and Sr bylithium metaborate digestion.

Samples of the 16 clay sources were analyzed in duplicate for Fe, Mn, Zn, Ti, Al,Ba, Ca, and Sr. It became apparent that Ti was not significantly different betweenclays. Therefore, the 30 sherds were analyzed only for Fe, Mn, Zn, Al, Ba, Ca, andSr. The data for the 30 sherds were tested to see if the elements were nonrandomlydistributed among the three pottery types. F-tests indicated that three elements (Al,Fe, Zn) show significantly greater variance between groups than within groups. F-tests also indicated that Sr concentrations were slightly variable between groups. (SeeTable 3 for a summary of univariate statistics.)

A multivariate discriminant analysis model was created using the data for Al, Fe,Zn, and Sr for the 30 sherds. (Inclusion of the other elements or exclusion of Sr didnot significantly change the results.) Both the Wilks’ lambda and Pillai Trace tests

TABLE 2Three NIST SRMs Analyzed for Al, Ba, Ca, and Sr by ICP–AES Following Fusion

NIST SRM Element Certified ppm Percent recovery

1646 Al 62,500 100, 110Ba Not certified —Ca 8300 99, 99Sr Not certified —

2709 Al 75,000 100, 94Ba 968 91, 90Ca 18,900 97, 94Sr 231 92, 96

1633a Al 143,000 94Ba Not certified —Ca 11,100 95Sr 830 97


44 KNEISEL ET AL.

TABLE 3Univariate F-Test Comparing Variance of Elemental Concentrations

between Groups to Variance within Groups(For All Other Elements p ú 0.23)

Element F p

Al 13.2 0.000Fe 3.69 0.038Zn 3.48 0.045Sr 2.18 0.132

indicated that the compositions of the three groups of pottery were significantly differ-ent (p õ 0.001). When the model is applied to the three groups, 25 of the 30 sherdsare accurately assigned. The 5 incorrectly assigned sherds could have reasonablybelonged to any of the three groups. These data are illustrated in Fig. 2, which is aplot of the two factors determined by discriminant analysis. (Factors are linear combi-nations of the elemental concentrations which maximize differences between the threetypes of pottery (9)). As can be seen, the three types form three clusters on the plot,but the clusters are not completely separated.

When the model generated by the pottery sheds is applied to the 16 clay samples,it is possible to reject 12 of the clays as possible sources. Four of the clay sources(indicated by an inverted triangle in Fig. 2) have an elemental concentration whichcannot be confidently discriminated from the composition of the pottery. The geo-

FIG. 2. Plot of factor 1 vs factor 2 for discriminant analysis of three groups of pottery sherds. Circles:plainware. Diamonds: polychrome. Open triangles: red-on-brown. Closed triangles: possible clay sources.



TABLE 4Accuracy and Relative Standard Deviation (rsd)

for ICP–AES Analysis of SRM 2704 River SedimentUsing the Revised Fusion Digestion

(10 Replicate Digests)

Element Recovery (%) rsd (%)

Mg 100.8 1.5Ca 100.1 2Al 102.5 2Sr 100.1 2Ba 98.3 2.5Zn 99.0 3Mn 105.8 3.5Fe 100.4 2.5Na 106.4 6.5K 99.8 2

graphical location of these four sources is indicated by the solid squares in Fig. 1.The four sources are within 10 km of the site where the sherds were found, whichstrongly suggests that the pottery was locally made.

After the completion of this project, we found that a simple revision of our fusiontechnique, in which the bead is dissolved in 8% HCl/8% HNO3, provides excellentprecision and accuracy for 10 elements, as indicated by the data in Table 4. Data inTable 4 were obtained entirely by ICP–AES, but we would expect the digestion wouldbe applicable to AA studies as well. In our future studies, we plan to use this digestionmethod.

DISCUSSION AND CONCLUSIONS

These results indicate that ICP–AES and flame AAS provide an acceptable, eco-nomical substitute for NAA in the compositional analysis of prehistoric pottery.

Because trade among southwestern Native Americans was known to be extensive(10), it is very possible that the locally made pottery (especially polychrome ware)was being exported to neighboring villages. We hope to expand this study to othervillages in the Tucson basin occupied during the Sedentary phase to determine if WestBranch pottery was being exported.

ACKNOWLEDGMENTSWe thank Karen Harry at Statistical Research for the sherds and clay samples, and we thank Kevin

Carney for helpful suggestions on the fusion technique. We thank John Sherman for removing the outerlayer of the sherds. Hobart and William Smith Colleges and Northern Arizona University provided fundingfor the project.

REFERENCES1. Glascock, M. D. Characterization of archaeological ceramics at MURR by neutron activation analysis

and multivariate statistics. In Chemical Characterization of Ceramic Pastes in Archaeology (H. Neff,Ed.), pp. 11–26. Prehistory Press, Madison, WI, 1992.

2. Hart, F. A.; Adams, S. J. Archaeometry, 1983, 25, 179–185.


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3. Storey, J. M. V. J. Arch. Sci., 1988, 15, 35–50.4. Foust, R. D.; Ambler, J. R.; Turner, L. D. Trace element analysis of Pueblo II Kayenta Anasazi sherds.

In Archaeological Chemistry IV (R. O. Allen, Ed.), ACS Advances in Chemistry, No. 220 pp. 125–143. Am. Chem. Soc. Washington, DC, 1989.

5. Shingleton, K. L.; Odell, G. H.; Harris, T. M. J. Arch. Sci. 1994, 21, 343–358.6. Skoog, D. A.; West, D. M.; Holler, F. J. Fundamentals of Analytical Chemistry. Saunders, Fort Worth,

TX, 1992.7. Burton, J. H.; Simon, A. W. Am. Antiquity 1993, 58, 45–59.8. Bishop, R. L. Comments on Section II: Variation. In Chemical Characterization of Ceramic Pastes in

Archaeology (H. Neff, Ed.), pp. 167–170. Prehistory Press, Madison, WI, 1992.9. Harris, R. J. A Primer of Multivariate Statistics. Academic Press, New York, 1985.

10. Cordell, L. S. Prehistory of the Southwest. Academic Press, New York, 1984.


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Identifying Clay Sources of Prehistoric Pottery Using Atomic Spectroscopy