edited bv:
chem I ~upplement
HELEN J. JAMES Weber State Cdlege Ogden. Utah 84408
Cracked Pot Chemistry The Role of Analytical Chemistry in Archaeology Ralph 0. Allen University of Virginia, Charlottesville, VA 22901 In an age of great technological advances it is not difficult t srecognize the role of the chemist in solving today's problems and in shaping the future. Less well known is the role of the chemist in helpine us understand the past. As memben of an archaeological expedition to Egypt inthe spring of 1980, my colleagurs and L uscd chemical techniqurs tu study an imnortant archaeolwical site. Our studies illustrated the value of chemical analysis in the study of ancient people and their technology and provided a good example of the scientific method a t work in a "cultural lahoratory."Most of us are familiar with some of the scientific measurements ~erformed in archaeological studies (archaeometry), s u c h a s carbon datine and maenetometer suwevs. This article shows the way we used chem&.try to describehow prehistoric people used chemical principles to change the properties of matter in an ancient technology.
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Chemists on an Archaeological Expedition Our expedition took us to the prehistoric Egyptian town of Hierakonpolis (or Nekhen) on the upper Nile River. According to ancient Egyptian legends Hierakonpolis was the home of Narmer, the pharaoh who united upper and lower Egypt in humanity's first nation-state over 5100 years ago (around 3100 B.C.). Over the last 80 years members of a series of expeditions have attempted to discover what happened in Predynastic Egypt that gave rise to this new political structure (1,2). The 1980 expedition to this largest Predynastic settlement comdex in all of Ewwt --. included a varietv of scientists: a geochemist (3),a geologist, a botanist, a zoologist, an ecologist, an anthropologist, architects, and archaeologists.' The interdisciplinary research a t Hierakonpolis is an example of reeionallv based. chronoloaicallv and ~ r o b l e m . mounded. o~ented&chaeology(2). For centuries the River Nile has carried fertile clays and sediments from the south and deposited them along its banks by flooding. The 6800-mile-long Nile carries the suspended sediments from the Equatorial system (the White Nile between Lake Victoria and Khartum) and the Ethiopian system (the Blue Nile between Lake Tana and Khartoum). Over thousands of years the amounts and conditions of such deposits have changed, making it possible for the geologist to distinguish among various periods when silt was laid down,
' Leading the expediion was Michael Honman, an archaeologist from me Un versily of SoufhCarolina's Earth Science and Resource lnstitne Marianne Rogers, a chemistry graduate student from the University of Virqinia. and~~any Hamroush, a geologist from Cairo University and gr&uate studentBt the university of Virginia, and I performed the chemical analyses.
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and to use chemical and sedimentoloeical nros~ectine . " techniqtte iu find new archaeological sites from Predvnastic times. After an archneoloeiral site is discovered. chemical methodologies can contribke to the understanding of the artifacts found on the site and their relationships with similar artifacts found elsewhere. The large Predynastic sites around Hierakonpolis contained numerous artifacts made of stone and clay, an ideal setting for using chemical analysis to obtain cultural information ( 4 ) . Archaeologists helieve that. due t t ~the uniqu(mess of the natural drainaee svstem and the presence of Pulneolithic sites. the protectedembayment of the Nile flood plain (called the Great Wadi or Abu Suffian) may have been an important settlement area for the last quarter million years (Fig. I). Occupation of the site seems to have been most extensive during the Amratian period (5000-2180 B.C.). The site covered 100 acres and was home to several thousand people (5). An enormous amount of broken pottery, estimated to be over 50 million pieces, and a number of cemeteries have been found along the dried stream bed of the Great Wadi. The cemeteries have heen studied in detail because of the large tombs which seem to be confined to one area, suggesting that they were resewed for the elite of the community. One grave is the oldest example of a stone-cut tomb found in Egypt, while another grave is lined with mud bricks (2). Although these tombs were looted in ancient times, they still contained valuable artifacts, including fine black-topped clay jars of what is called Plum Red Ware. This type of pottery seems to have been an important part of the ancient funerary practices. TWOKinds of Predynastic Ponery
Archaeoloaical investieations of the site revealed the remains of at least 15 kilns and led to our conclusion that during Amratian times this was the center of an i m ~ o r t a nDotterv t industry. Although the industry was localized, the piesence of different tvpes of potterv indicated that it became mecialized in stilk and fkctio" a t a very early time. ~ m r a c a n pottery was made in two distinctive styles: Straw-Tempered Ware and Plum Red Ware. The coarser Straw-Tempered Ware was thick, rough, porous, and yellowish or reddish brown. Straw was added to the clay to make it less fluid during shaping and, when fired, the carbonaceous material burned. What remained was not the straw but silica skeletons (6).
J This featuepresents applications of chemistry relevant to everyday life. The information presented might be used directly In class, pasted an
bulletin boards, or otherwise used to stimulate student involvement in aCtiviti~5related to chemistry. Canbibutionsshwld be sent tome featve
editors.
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Number 1 January 1985
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If the difference between the two types of pottery resulted from the use of clays from different sources, then trace element nnalvsis (nrovenance studies) could confirm this ex~ ~ ~ ~ planation. The kilns used for firing Plum Red Ware were located ahove Nile~ sediments. hut there were deoosits of . . ~ ~ the - ~ crumbly shale or marl from a much older geologic& period available. During sample collection, we noted that this soft shale was layered by deposits of a white salt. In the areas around the kilns there was evidence that lavers of the soft shale had either been dug out or weathered but from under layers of harder rock. We also noted that the kilns appeared to have been located to take advantage of the wind blowing down the cliffs between rocks producing a wind tunnel effect. These observations led to the hypothesis that the production of Plum Red Ware required deposits of the crumbly shale and a "wind tunnel." We tested this theory by searching the Great Wadi for another area where shale was present and the wind passed between large rock masses. We found just such a place and there we discovered a previously unknown site of a kiln surrounded by Plum Red ware w a s & + ! Thus. we were convinced that Plum Red Ware was made with theshale while Straw Tempered Ware was made with the Nile silts upon which the habitation sites were located. Samples of the shale and silts were analyzed and compared to the pottery. Unfired pottery and overfired wasters were also analyzed. Neutron activation analysis was used to measure some of the trace elements present in these materials. When such samples are placed in the core of a nuclear reactor some of the stable isotopes capture neutrons and become radioactive. By measurine the enerw and intensitv of the ?-raw . " emitted. the chemist can determine the elements present and their concentrations. The technique is especially sensitive for the determination of the lanthanides or rare earth elements (REE), a familv of elements oarticularlv useful in the studv - of eeochemich processes (f).
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F i v e 1. CMItwr msp of Hiwakonpolis ( U p p Egypt) showing Qeal Wadi which extends to the len side. Straw-Tempered Ware was produced at several kilns including mose at locality 11, located on Masmas famatim silts. Plum Red Ware was produced at kilns such as those indicated at localities39 and 59 near me cemetery at locality 6. 1 and 2 are the stone-cut and brick-lined tombs discussed in the tea.
Microscopic examination revealed that the mineral g r i n s were not wrlded tneether vers well. Because Straw-'l'emr~rred Ware was usually lfound whkre people apparently live& we concluded that it was the everyday household ware. Plum Red Ware had the beautiful color from which its name was derived and was often decorated with an uneven black color around the top. The pots were smooth, often shiny, and very strong since the fine mineral grains were welded together hy the partial melting of minerals during the firing process. Sherds of this type were found primarily in the cemetery areas. The remains of a number of pottery kilns were located in what seemed to be a massive industrial area covering nearly a thousand square meters. Around each of the 15 Amratian pottery kilns we found scraps of ruined pottery fired too long or a t too high a temperature, called wasters. At some of the larger kilns we found clay bricks used in kiln construction and other objeds used in firing clay pots. We noticed that around those kilns in and near the habitation sites, the sherds and wasters were primarily from the production of Straw-Tempered Ware. Other kilns, situated high in the desert cliffs on the northwestern side of the Great Wadi near the cemeteries (Fig. I), were surrounded by sherds and wasters of Plum Red Ware. We surmised that the smaller amounts of StrawTempered Ware present a t these sites were used in the production and firing processes rather than having been produced in these kilns. The location of these kilns near a cemetery suggested that they were used to manufacture funerary objects. Were special conditions needed to produce this fine Plum Red Ware used as grave offerings? Were the cemeteries and kilns located together for that reason? Trace Element Analysis Determines Clay Source T o answer these auestions. we examined the kilns and nearby artifacts and collected iamples for chemical analysis. The contrasts in the two t .w.e s of Dotterv indicated either diffrrtmt starting material or different production techniques. The i~n,duc.:ionof ootterv rwuirrd 1 .1 clav. .. which was wndilv avaiiable in the Gile deposks or earlier sediments in thk lavered cliffs alone the Nile and Wadi: 2) water. presumablv more readily availabl~:in ancient time than it is today-nearly 5 km out onto the d~sert!and 3) fuel to firp the ootterv in kilns. presumably more readily available during ~ r l d y n a i t i ctimes and may have been wood, grass, or animal dung.
38
Journal of Chemical Education
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Distrlbutlon of REE in Geological Materials The rare earth elements. as well as manv other elements. occur at low concentrations.& the silicate minerals and oxides found in " eeoloeical materials. When the crvstal lattice of a mineral forms, some of the major cations (c;, Fe, h a , K, etc.) can he replaced bv cations which are present at much lower concentrations, as trace elements. substitution of trace elementsinto the crystal lattice is favored when the charge and size of the trace element ion is similar to that of the majorelement ion. Trace elements are partitioned or separated from each other during geological processes because of differences in size and char~e.Geochemists can use the relative concentrations of tracedements to describe the chemical conditions under which a rock was formed (7). The rare earth elements usually occur as 3+ ions but they are partitioned from each other in minerals because of the differences in ionic size. (The smooth decrease in size with atomic number is known as the lanthanide contraction). The relative enrichments of these elements are difficult to observe by simply measuring the concentrations because the REE abundances in nature are not alwavs the same. The REE with even atomic numbers are moreabundant than those with odd atomic numbers. This is a result of the greater nuclear stability associated with even numbers of protons. This "even-odd" effect obscures the effects of ionic size on the chemical enrichments of these elements. If the REE concentrations in geological materials are plotted as a function of REE ionic radii or the atomicnumber (which is inversely related to the radii), the "even-odd" effect results in a sawtoothed pattern such as shown in Figure 2: The REE concentrations shown in Figure 2 are for chondritic (stone)meteorites. which are assumed to represent the relative REE concentrations in the primordial matter from which our solar svstem was formed. As the earth evolved these elements were partitioned by a variety 01' grorhrmiral processes, including nrck weuthc&g iind the *epnration uf mineral grains.
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Pr NII Pin
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Er ~m ~b
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When the absolute concentrations of rare earth elements measured in one of rhr Nile silts are plotted against atomic numt~ers(Fic. 2 ) , the enrichment of the KEF:over the presumed original concentrations (chondrites) becomes apparent. Unfortunately, relative enrichments of the elements are not as clear. When the concentration of each REE in the Nile siit is divided by the concentration of the same element in the chondritic meteorite, the normalized concentrations show the enrichments of different elements relative to each other. In Figure 3 (curve B) the normalized REE concentration for the Nile silt shown in Figure 2 (cuwe B)appears. For comparison, the normalized REE concentrations in the chondritic meteorites would have a value of one for each element and would lie along the lower axis of Figure 3. REE in Nile Silts
The Nile silt shown in Figure 3 was deposited in what the aeoloaists - call the Arkin formation some fi.OOO to 10,000 sears ago and is exposed on the surface along the Nile ne& the present cultivation zone. Nearby are the remains of a large ~ m r a t i a npottery kiln (locality 24 in Fig. 1). Another, muih older (40,000 years old) Nile silt formation, called the Masmas formation, was exposed further from the Nile on the floor of the Great Wadi (locality 11). Normalized REE concentrations formation are also shown in for the silt from the Figure 3. The difference in the two sediment layers was the result of differences in the contributions of suspended particles from the two sources of the Nile or differences in the sortine of mineral erains in the denositional nrocesses. We used tKe differencesYinthe REE as "ch;omatograms" (or "fineernrints") to distineuish between these two sediment formations.
m as mas
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RARE EARTH ELEMENT
RARE EARTH ELEMENTS Figure 2. Absolute concernrationsof REE for an average chondritic metmite (A) given on weight basis (ppm = gmlgm) as a l~nctionof RE€ atomk Rmber. For -parim me ~ ~ k in s Nile n sin shm (he Alkin formaton at locality 24 are shown (8).
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La Ce PI Nd
Figure 3. Chondrlte normalized REE disb'ibution in two Nile silts from Hierakonpolis. Masmas formationsiit (A) is hom iacalii 11 while Ihe Arkin famation sin (6) is from locality 24.
We tested our theory that Plum Red Ware was made from the crumbly shale exposed along the edge of the Great Wadi (e.g., locality 39) by analyzing Plum Red Ware sherds from several kilns and comoarine them to the crumblv salt-rich sediment deposits near the kilns. Examplrs of the resulting REE distrihution curves are shown in Firure 4. The i'lum Rrd Ware sherds (A) did not match the nea& crumbly shale (B), hut they did match the relative REE concentrations for the Masmas formation sediment deposited by the Nile ( C ) .The Masmas sediment was collected from the area around a kiln in the Wadi (locality 11 in Fig. 1) that was used to produce Straw Tempered Ware (based uuon the t m e of wasters ohserved near'the kiln). When the Straw-Tempered Wnrr from around this kiln war malvzed, the KF:Econrentrationspn~ved that the local Masmas silt (clay) was used for its production (D, Fig. 4). These results indicated that the same Nile sediments were used for both the Plum Red Ware and the Straw-Tempered Ware. While clay or formed pottery may have been taken to special kilns for firing Plum Red Ware, the results from the analysis of other artifacts from around the kilns suggest that the manufacture of pottery was very localized. Although the analysis has not been extensive, the Plum Red Ware and Straw-Tempered Ware found out on the Wadi (southwest of localitv 11) . annear .. to have been nroduced usine clav from the Masmas formation which is expbsed in that area. samples of both tvoes of notterv and clav from localities closer to the Nile (e.g., fdcalitie's 29 ahd 24) had different REE patterns. This is consistent with the nostulate that the clavs from the immediate vicinity of the kilns were used in the production of pottery. For example, the Masrnas silt (whosr REE pattern isshown in Fig. 3) was used ro produce 110ththe Straw-Temoeri-d Ware and the Plum Red Warr whosr REE patterns are shown along with the Masmas silt in Figure 4.
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Volume 62
Number 1 January 1985
39
geothite (HFe02)which formed a brownish coat over the other mineral grains. In the Straw-Tempered Ware the earthy red color resulted from the Fe being oxidized to form hematite (Fe203).The mineralogy of Fe in the plum-red-colored portion of the finer wares was not determined, hut the black color of the tops was due to the more reduced magnetite (Fe(I1). Fe(lII)204).Further reduction to fine-grained metallic iron was apparent in an overfired waster from one of the kilns where Plum Red Ware was fired. These and other observed phase changes were compared to those observed when clays were heated in the laboratory (6).The firing temperature estimated for the Plum Red Ware (>lOOO°C) was higher than that required for the StrawTemoered Ware (500-800%). I t mav have been possible to have'produrrd the Plum Red Ware at lower temperatures if the anrient Eev~tirtnrraftsmen had discovered chemical methods to effect phase transformation. This possibility was suggested by the discovery of unusual overfired wasters near several of the kilns used to produce Plum Red Ware. The overfired Plum Red Ware wasters from the kilns along the edge of the Great Wadi were hlack and contained finegrained metallic iron (evidence of heating in a highly reducing environment). More significantly, these wasters had a very different appearance from those around the kilns where Straw Tempered Ware was produced. The Plum Red Ware wasters resembled volcanic pumice, a glassy mass which solidified when it was frothy and full of gas bubbles. The minerals present in the clays used to produce this pottery (kaolinite clav. and eeothite) normallv would not .. auartz.. ulaeioclase. . .. produce gases upon hcating hut would producc wasters such as those uh.;ened around the Straw-Temuered Ware kilns. The proximity of the salt deposits in the shhes suggested that salt could not have been added to the Plum Red Ware ceramics. Analysis of the salt showed that it was anhydrite (CaSOa)with some calcite (CaC03), both of which decompose at high temperatures to form CaO and a gas (SO2 or COz). The decomposition of anhydrite proceeds rapidly a t temperatures above 1200°C, especially in the presence of clay and silica (8).
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RARE EARTH ELEMENl Fioure 4. Chandritenormalired REE distributions showina that both Plum R e d Ware (A) ana Ssaw-TempereoWare ID) were prcducm framthsMasmss larma1.on 311 fmnd 81loca ily 1I (CJ me CrUrnO y sham (8)hmnd m a Phm Red Ware k m at local ly 39 co~lonot have been uoeaaslns c ay for prcducmglhls Plum ~ e Ware. d
Ancient Ceramic Technology: Two Types of Ware from One Source of Clay? I t is very important to note that in addition to the differences in REE patterns, the Fe concentrations in the clay also differed. The Masmas sediments had less Fe (an average of 8.3% Fen03) than the sediments nearer the Nile (e.g., Arkin which averaged 10.5%FezO3). When averages were calculated for Fe content in the pottery, the variability was greater than that observed for the sediments themselves. However, the ceramics from localities nearer the Nile had significantly higher Fe concentrations than those from further out on the desert in the Great Wadi. The Fe concentrations are important because Fe plays a soecific role in uroducine the color of the ceramic. However, the fact that thh range or~ccunrentrationsfound for ~ t r a w : 'I'emuered Ware sherds in each localitv o\erlarwed the ranue of w k r r a t i o n s measured in Plum ked W& sherdi from the same Iocalitv showed that the color dtff~*rencc.s were no1 due to differences in the Fe concentrations. Instead, different temoeratures used in the firina urocess were resuonsible for not bnly the texture of the ceramic but also, by changing the com~oundsof Fe oresent, the color of the ceramic. Comuarina the minerals in the unfired clay, pottery, and overfirej wasters is one way to deduce the firing temperatures since difierent mineral ihmes are pn~ducedand arestable a1 higher temperatures ( 6 ) .In the sdiments and a piere nf unfired clay the Fe bearing mineral phase was hard to identify by X-ray diffraction, even though Fe was oresent in high c&entIations. (Note: the concentrations In the range o f 7-12% were reported as Fez03 even though the actual oxidation state and chemical formula were not known). The most likely form of Fe in these sediments was poorly crystalline 40
Journal of Chemical Education
A Technological Advance Why would early Egyptian potters h a w added a salt to their clay when they were producing the special Plum Red Ware pottery? They could fire the pottery a t lower temperatures in the same wav that elass manufacturers add comnonenu to silica 1Si02)to lower the mrlting point of the glass (8). C.pon hedtinga mixrureofnnhgdrite lor CaO) and kaolinite. the kaolinire hrgins to react and melt at temperatures below thosr required for pure kaolinitr. In the range of ROlJ-500°(: the mixture reacts to form plagioclase (a solid solurion which varies ill cotnrro;irion hetween Na.41SiaO~and Ca.4lSi~Oa1. - - The formation of this new mineral phase welds the grains together giving the ceramic its strength. Another process which can occur during firing is the formation of partial melt a t the melting point of the least refactory component. When the melt cools the glass can also weld grains together. Most clays begin to melt in the 750-800°C range, hut the rate is often low until temperatures reach 1100-1200°C (8).Thus the addition of anhydrite would allow the Egyptian potters to produce the greater melting or reaction necessary to weld the grains together to form a stronger ceramic at temperatures just above 800°C. When we examined the Plum Red Ware pottery with a scanning electron microscope (SEM),we observed that the mineral grains were welded together hy a material which had flowed around the grains. This flow-like material was then analyzed in the SEM by focusing the electrons on the area of interest and measuring the X-rays emitted. The results showed that the material contained Ca, Al, and Si (the 0 cannot he measured by this X-ray technique), which could he glass or fine-grained plagioclase (CaAlzSinOs). When we measured the elemental concentra-
tions, we found that the flow-like material welding the ceramic together had higher Ca/Al and Ca/Si ratios than either the center of the unreacted mineral grains or the grains of the untreated Nile silt (6). In the Nile silt Ca is already present in plagioclase grains (although in these sediments the compositions are more Na rich) and absorbed on the clay. The addition of CaSOa would increase the CaIAI and C&i ratios. It reacts with the Ala(OH)sSiaOlo (kaolinite) to form CaAlzSizOs (plagiocase) or glass, which welds the particles together upon cooling to give the fine ceramic texture of the Plum Red Ware. The presence of sulfate and/or carbonate also affected the redox conditions during the firing process and helped produced the characteristic colors of the Plum Red Ware. Was the inrenrionol addition o l a new component (anhgd r i t r ~to the wmmon local Ndesilr rhe trvhnolo~icaladuance that gave these ancient Egyptian craftsmen a& advantage over other Erouus (5)?As is often the case, a scientific investigation leads to more questions than answers. We do not know how important this technology was to the rise of the fmt national city-state, hut it does appear that nearly 6,000 years ago the ancient Egyptians discovered and used the knowledge of phase transformations. The addition of a salt-like anhydrite to a clay mixture may have been accidental. However, when they observed that this allowed them to produce a much more suhstantial form of pottery that was also pleasing to the eye, they continued to make it. Its durability made it a g o d choice
to be included in the graves, helping prepare for the all-imoortant afterlife. In a sense the Plum Red Ware nottew found at Hierakonpolis is a pn:cursur to the fabulous ;reasu>es discovered in the tomb of Kinr Tutankhamen. While there remains much to learn, this st;dy not only shows how chemistry can aid us in understanding our past, hut also shows how important the knowledge of chemical transformations may have been to prehistoric craftsmen. Acknowledgment The field work discussed in this paper was supported by the Smithsonian Institution Office of Fellowship and Grants. The assistance of M.A. Hoffman in the preparation of the figures and the editorial assistance of A. Heath are gratefully acknowledged. Literature Cited 11) H o f h . M . A.''FmptBeforethe Pharsoha: ThePr&storicFouodatititititif E ~ p t i s n Civilieslian."Alfred A. Knopf, New York, 1979. (2) Hoffman, M. A,, "The Predynastie of Hierakonpalis--An lntprim Studies Assoc. Puhl. No. 1. Cairo Univ. Herbarium, 1982.
Rewrt."Egyptian
13) Allen. R. O.,snd Pennell,S.E.,"Adusncesin Chemistry(ArehseologiealChemistn.11)
171." American Chemical Society, 1978, pp. 230-257. (4) Allen, R. 0..and Hsmroush, H., "Aduancs in Chemistry 1ArchaoologiealChemistry 111) 205."American Chemical Society, 1984. pp. 5146. ( 5 ) Hoffman, M. A., Science 83.4 42 11883). (6) Allen, R. O., Ropers. M. S., Mitchell. R. %and Hoffman, M. A.,Archoeomatry,Z4.199 ,,oo,> \.m"*,.
171 Haskin, L. A., Holmke. P.A..Pmter,TP.,aodAllen,R.0..in''ActivatIm Andyakin Goochamistry and Cosmochemistry: (Editors: Brunfelt, A. 0.. and Steinnes, E.), Oslo, 1971,pp. 201.218,
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