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Direct thermoluminescence chronology for Early Iron Age smelting technology on the Gambaga Escarpment, Ghana
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Archaeological SCIENCE Journal of Archaeological Science 30 (2003) 1037–1050 http://www.elsevier.com/locate/jas
Direct thermoluminescence chronology for Early Iron Age smelting technology on the Gambaga Escarpment, Ghana D.I. Godfrey-Smith a,*, J.L. Casey b a
Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5 b Department of Anthropology, University of South Carolina, Columbia, SC 29208, USA
Received 8 October 2002; received in revised form 25 November 2002; accepted 2 December 2002
Abstract We present luminescence dates which demonstrate a full-blown smelting technology in Northern Ghana, West Africa, during the Early Iron Age. Our chronology is based on thermoluminescence (TL) dating of quartz grains extracted from the walls of three iron smelters located at the Birimi site in the Northern Region of Ghana. Two of the smelters yielded statistically indistinguishable ages of 108070 and 109060 years, while the third yielded a higher age of 1600100 years. All are significantly older than the sole direct radiocarbon date of 550–330 calBP (46090 BP) obtained on a furnace in the Northern Region. The TL ages indicate that iron smelting was well established in the Northern Region before the middle of the first millennium AD, and corroborate the validity of a number of similarly early radiocarbon dates associated with Early Iron Age ceramics but not directly associated with smelting activity, from other sites in northern Ghana. The iron working remains at Birimi are located on a river terrace, only a few tens of meters west of a dense Kintampo occupation. Radiocarbon and TL dates for the Kintampo complex demonstrate that the Iron Age and Kintampo components at Birimi are clearly non-contemporaneous. The occurrence of two distinct layers of goethite within 150 cm of the terrace surface, both of which are clearly exposed in the scarp of the terrace upon which the smelters are situated, indicates that the industrial site was deliberately sited to take advantage of the readily available source material. This premise is confirmed by geochemical analyses of sediment, goethite, and slag from the smelting site. We therefore propose that the frequent proximity of Kintampo and iron production sites is incidental, in the sense that each group exploited different resources offered by a riverbank location. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Early Iron Age; Smelting; Africa; Luminescence; Thermoluminescence; Dating; Geochemistry; Goethite; Fayalite; Kintampo
1. Introduction The data we present here add to the discussion about the Early Iron Age in West Africa in several significant ways. First, although the few claims of a transitional phase between the Late Stone Age and the Iron Age in Ghana have been challenged, the consistent occurrence of Late Stone Age Kintampo Complex sites in the vicinity of iron working sites needs to be addressed. At the Birimi site in northern Ghana, a large Kintampo habitation is flanked by an iron smelting area that is not associated with domestic Iron Age artifacts. Direct * Corresponding author. E-mail addresses: [email protected] (D.I. Godfrey-Smith), [email protected] (J.L. Casey).
dates, using luminescence and radiocarbon dating, obtained on features of both components and coupled with geochemical analyses, allow for a test of the transition hypothesis. Second, at a number of sites (e.g. Daboya, Ntereso) indisputable Kintampo artifacts and Iron Age ceramics plus iron artifacts are found in close stratigraphic association. The direct dating of archaeological remains attributable to one or another cultural phase could make a significant contribution towards clarifying whether the association is contemporaneous or due to post-depositional geological or anthropogenic site disturbance. Third, the use of luminescence to date the materials directly overcomes the problems inherent in the dating of charcoals that may be intrusive, and the issue of ‘old wood’ where fuel woods used in the iron smelting process may predate iron working activity
0305-4403/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0305-4403(02)00292-3
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Fig. 1. Archaeological sites in Ghana with evidence for Early Iron Age iron production.
[25,32]. Finally, there have been relatively few dates for early iron working activity in Ghana. Our data firmly place iron smelting activity in northern Ghana in the early and middle phases of its appearance in West Africa. Contemporary archaeological investigations into the African ‘Iron Age’ have largely moved away from discussions of origins and have concentrated more on the process of smelting, the social, ideological, and technological changes that accompanied the advent of iron production, and the symbolic and ritual dimensions of iron working ([5–7,10,18,20,26,38–41,46–48,58,59,61,62]—to name but a very few). Much of this research is being done through experimentation and ethnographic analysis. This new
approach has given us a multi-dimensional picture of the dynamics of iron working and usage, but there is still a lot of basic archaeology that needs to be done in order to chronicle the presence of iron working in Africa. The Iron Age, which is thought to have been initiated during the second millennium BC by the Hittites in Armenia, spread to Cyprus and Greece by 1200 BC, and, via routes both unknown and still debated, reached Nigeria during the first millennium BC. Regardless of its origins, the techniques used for smelting iron are highly variable throughout pre-colonial Africa, suggesting that the technology has undergone significant regional innovation and adaptation [17,32,49,57,60].
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The majority of the earliest iron working sites in West Africa stretch from Senegal to Chad and from the Sahel to the rainforests. They date to between 2870 and 1830 calBP (2628120 to 196060 BP [21, p. 338]), with the earliest dates occurring in Nigeria and Niger [3,19]. AMS radiocarbon dating of charcoal samples obtained from excavated iron smelting furnaces in Nsukka, Nigeria, yielded calibrated ages that ranged from 765 to 520 BC [36]. Broadly, the earliest phase of the development of iron working is considered to have extended from 600 BC to 600 AD [50]. Early in the mid first millennium AD iron working sites in many parts of West Africa underwent a dramatic change, signaled by intensive construction and increasing social and political complexity [27]. It is this change that has resulted in the massive and most well-known components at JenneJeno [28–30] and Daima [8]. During and after this, iron working and using sites throughout West Africa became much more common. In many parts of Africa the indigenous process of smelting iron continued well into the 20th century. 2. Iron working in Ghana In Ghana, as over all of West Africa, information on the advent of iron working technology comes from a very few, widely spaced sites. Although Davies [11] noted numerous smelting sites throughout Ghana, most of these remain uninvestigated. Information for early iron working activity is limited to about 11 sites (Fig. 1). Although these sites have produced early dates for iron working activity, not all have produced much information about the cultural context in which iron working began in the area. The earliest dates for iron working in Ghana come from Daboya. Shinnie and Kense [53] report evidence for metallurgy at Daboya in the Late Stone Age Kintampo Complex in the early second millennium BC. They concur with Davies [11] that there is continuity between Kintampo and the advent of iron. This view has been criticized on the basis of inconsistencies in dates and provenances at the site [54] and because the frequent association of Kintampo and Early Iron Age ceramics suggests that deposits are likely to have been mixed [31]. Our own analysis of the precise association of iron artifacts at Daboya [53, pp. 206–207] with radiocarbon and thermoluminescence (TL) dates obtained within the same units (units TA10, WA10, and KA12 [53, pp. 249–250]) leads us to conclude that the chronostratigraphic control on the iron occurrences is neither sufficient to support the transition hypothesis nor warrants an Early Iron Age attribution. Elsewhere in Ghana there appears to have been a hiatus of at least 500 years after Kintampo before evidence for iron metallurgy appears. Other early sites for the production of iron include New Buipe in northern Ghana [63] where artifacts and
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a few pieces of slag were found in the course of large scale excavations at a mound site, and radiocarbon dating produced ages of late first millennium AD. The earliest materials, however, were not associated with furnaces [63]. Also in northern Ghana, the Kintampo site Ntereso produced a date of 1950–1705 calBP (1890110 BP, SR-90) on charcoal in a layer that contained mixed Kintampo and Iron Age materials [12, p. 53]. Here, pieces of slag and iron and sherds of Iron Age pottery are thought to be intrusive into the Kintampo layers rather than indicating a transitional phase. Smelting is present at Abam, Atwetwebooso, Begho, Bonoso and Bono Manso [2] along with early dates, but not much is known about these sites. Asantemanso [51] and Adansemanso [52] are both lengthy Iron Age sequences that have produced a few early dates. Evidence for iron smelting on the Gambaga escarpment has been documented by recent archaeological investigations [23,37]. Still, within this region, the sole absolute date directly associated with iron smelting is a radiocarbon date of 550–330 calBP (46090 BP, AECV-710C), obtained for furnace ISM-15 [23] from the escarpment. More general archaeological remains attributed to Middle to Later Iron Age periods span a broad range of 940–300 calBP [23] at this and other northern Ghanaian sites. The vast majority of Iron Age occurrences in Ghana post-date 1000 BP (see Ref. [54]). 3. Birimi site description and cultural context The Birimi site sits upon a 2 m terrace on the northern edge of the Birimi Kuliga, a seasonal stream south of the Gambaga Escarpment, and 3.5 km NW of the village of Nalerigu in the Northern Region, Ghana. A 1996–1997 survey and excavation of this site has revealed the presence of three temporally distinct cultural components. The eastern half of the 20050 m site includes a 3500–3830 calBP Later Stone Age Kintampo component [4] which appears to intrude anthropogenically into the underlying sediments and may extend down to >60 cm b.s. Underlying and at times mixed with it is a Middle Stone Age (MSA) component which at depths of 1 m or deeper appears to be in natural geological sedimentary contexts. Its occurrences in two horizons have been dated to 40,80011,400 years (BRSD3), and 23,6002,900 years (BRSD55) using optical dating [45]. This suggests that the site’s natural setting must be exceptionally favorable to humans, as it was occupied for substantial periods in the distant past. The third and latest component dominates the western half of the site (Fig. 2). It is made up of well-preserved remains of at least five smelters and numerous slag heaps. Unlike the site’s eastern half, which is a settlement area, the western
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Fig. 2. Location map for the Iron Age remains at west Birimi. The grid ties this part of the site to the excavated Kintampo area grid, which spans 20–120 m east and 0–60 m south. Smelters are indicated by circles, slag heaps by # signs. Smelters dated by us are numbered. Smelter 1 lies on the 102.75 m contour line; contour interval is 0.5 m.
half is manifested solely by industrial remains, and shows no evidence of living areas. 4. Smelting activity and thermoluminescence dating Traditional smelting in Africa depended on the bloomery process, in which the ore must reach temperatures high enough (1100–1300 (C) to separate iron from slag [20]. Ore smelted at such temperatures does not reach the melting point of iron, but produces a solid, spongy iron mass called the bloom. The slag, composed largely of non-ferrous residues of the ore, does liquefy in the smelting process. The two components were often separated by directing the slag to run off, leaving only the bloom in place (though other approaches, including mechanical separation of the cooled bloom–slag mixture, were also used). TL dating is particularly well suited to the dating of anthropogenically heated materials such as ceramics, cooking hearths, ovens, kilns and other types of furnaces, fire-cracked rock used in cooking, and heattreated flints used in toolmaking [1]. It measures the time since the material forming the matrix of the items mentioned was last exposed to a ‘clock-resetting’ event. Such an event takes place when the material is heated to a temperature in excess of 500 (C. For an artifact which has been heated, the TL signal is proportional to the yearly rate at which new TL is created (which is propor-
tional to the radiation dose absorbed by the sample and depends on the concentration of radioisotopes in its immediate surroundings; this is measured by independent means), times the number of years since the sample was last heated. Thus: Age (years) 5
Total TL (TL per unit dose)#(dose per year)
Because smelting activity is perforce associated with very high temperatures, remains of the smelters themselves, or indeed even far simpler fire-associated installations [14] are uniquely well suited to direct geochronometric analysis using TL dating. In addition, large well-heated features such as these provide plentiful material for TL dating. We therefore chose to apply TL dating to three smelters at the site. These appeared to be sufficiently well preserved to still retain their circular shape and their wall remnants protruded 5–10 cm above the ground surface. In one case, a bellows inlet was also clearly evident (Fig. 3). In addition to the chronological studies, we investigated the sediments forming the matrix of the Birimi site. To accomplish this, we employed scanning electron microscopy, major and trace element determination by X-ray fluorescence, and mineralogical analysis with X-ray diffraction.
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Fig. 3. Smelter 1, looking south. Dashed white curves follow the outside of the visible wall remnants and tuyere opening. The TL dating sample was collected above the left end of the lower curve. Hammer and African hoe are included for scale.
5. Sample preparation Large fragments (300–500 g) of smelter wall were removed and immediately sealed in plastic to preserve their natural water content. They were weighed in the field, and re-weighed upon receipt in the laboratory to determine their water loss during transit (about 6 months delay). Their natural and saturation water contents were determined as described by GodfreySmith et al. [15]. The smelter walls are coarse in texture, composed primarily of silty clay, with a high void volume which includes many well-delineated moulds of incinerated grass or twig temper. A decision was made to use
sand-sized quartz grains for the following reasons: quartz grains were readily apparent on visual inspection; plenty of smelter wall material remained available for quartz extraction; the walls were not vitrified, and their high void volume made them relatively easy to disaggregate with gentle crushing; quartz has favorable TL properties in the age and dose range expected for these structures; and the use of etched sand-size quartz grains and the quartz-inclusion technique obviates the need for additional tedious alpha efficiency measurements. In addition, should these smelters have proved to be younger than 1000 years, accurate and precise ages could have been obtained for them using the pre-dose technique [1].
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Fig. 4. (A) TL glow curves for quartz extracted from smelter 1. Glow curves with naturally accumulated TL (N) and with additional artificial radiation (N+1.2, 2.6, and 5.2 Gy, respectively) are shown. For clarity at low doses, glow curves with higher added doses are omitted. (B) Growth curve and linear least squares fit to the data of the first glow TL at 275 (C, for smelter 3. Inverse weighting with respect to the TL intensity was used. The past dose estimate De is indicated by the two vertical lines marking its 1, at x⫽⫺2.4 Gy, y⫽0.
Quartz extraction was performed on an 80 g fragment of each sample. The outer 5 mm layer was removed using a slow-speed Dremel drill. This ensured that the outer surface which had been exposed to light during excavation and later handling and therefore had its natural TL partially erased, was not included in the analysis. It also ensured that quartz grains which had accumulated beta radiation from both the wall itself and the surrounding sediments, i.e. those that resided in the outermost 2 mm of the wall, were not included in the analysis. The removed material was used in the determinations of K, U and Th activities of the smelter walls.
The remaining sample was gently crushed using a benchtop hydraulic press. This process ensured that the piece was disaggregated without crushing any singlemineral grains. The resulting aggregate was dry sieved to obtain typically 6 g of 125–150 µm grains, which were ultrasonically wet sieved in a methanol bath through a 125 µm sieve. The cleaned fraction was treated overnight with concentrated HCl to remove carbonates and disaggregate silts and clays. These two steps ensured a very clean quartz extraction when treated with 48% HF, with no precipitates forming on the quartz. Heavy liquid and magnetic separations completed the quartz purification process.
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6. Thermoluminescence principles and measurements When minerals such as quartz and feldspar are exposed to radiation energy from their surroundings, they absorb and store it in their crystal lattice. When heated in the laboratory, the minerals release the absorbed energy in the form of light which is known as TL. When the emitted TL is plotted as a function of temperature, a reproducible ‘glow curve’ results. The amount of TL emitted is proportional to the amount of radiation energy absorbed, and varies in a consistent manner with temperature (Fig. 4A). Simplifying the above relationship to the age equation standard to all dosimetric dating methods, age is deduced from the ratio of the radiation dose De (accrued since the last clock resetting event) divided by the dose rate R: Age⫽De/R. The total dose De accrued in the minerals is determined through TL glow curve analysis, here of quartz grains extracted from a smelter wall. The dose rate R is determined by measuring the concentrations of the radioisotopes U, Th, K, of each wall fragment, and the sediments immediately surrounding it. Activities of the U and Th radioisotopes and their decay chains were measured using thick-source alpha counting, calibrated with known-concentration standard powders from New Brunswick Laboratories, Maine, USA. K concentrations were measured at a commercial laboratory (Bondar-Clegg of Vancouver, BC). The dose rate calculations include other variables, including the water contents of wall and sediment, the wall thickness, the depth of the sampled wall into or above the ground surface, and the size of quartz grains used for TL analysis. The measured dosimetric values are given in Table 1. 6.1. First glow thermoluminescence
Fig. 5. Plateau plots for smelters 1, 2, and 3. The temperature range of each plateau is indicated by the horizontal line.
Once a quartz extract was obtained, a single layer of grains was deposited by sprinkling them onto oiled, 9.8 mm aluminum disks, resulting in aliquots of w5 mg. Typically 44 aliquots were prepared for each sample. TL measurements were carried out at the Thermally and Optically Stimulated Luminescence (TOSL) Research Laboratory at Dalhousie University.
For first glows, 24 aliquots were irradiated with 0.04 Gy and promptly glowed to 150 (C to yield the 100 (C TL peak. This provided one of the normalizing values for the raw TL glow curves; the other normalizing value used was aliquot mass. Twenty aliquots were irradiated in batches of four. Laboratory doses of 1.5– 24 Gy were administered using on-plate irradiation from a 90Sr/90Y source with a calibrated strength of 0.99 Gy/min. Afterwards, each aliquot was glowed out under a flow of high purity dry nitrogen in a Risø multi-sample TL/OSL instrument. Samples were preheated to 150 (C, then glowed at 2 ( s1 to 500 (C. At this heating rate the nominal 325 (C TL peak of relevance to dating of heated archaeological materials appears at 295 (C. Luminescence was detected through a red-absorbing, violet-transmitting glass filter pack (Kopp 7-59+Schott BG39). To remove any contribution to the glow curves from thermal blackbody radiation which is naturally emitted by all matter at high
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Table 1 Dosimetry values for Birimi smelters 1, 2, 3 and their associated sediments
Smelter Smelter Smelter Smelter
walls: 1 (BRSM1) 2 (BRSM4) 3 (BRSM5)
Grain size (µm)
Depth b.s. (m)
Water content
K2 O%
Alpha counts Total (ks1 cm2)
Th (ks1 cm2)
U (ppm)
138 138 138
0.04 0.00 0.00
0.12 0.12 0.12
0.33 0.36 0.58
1.2120.014 0.9160.014 1.1770.015
0.5570.050 0.3480.045 0.4530.050
5.11.1 15.01.3 4.41.1 9.41.2 5.71.2 12.21.3
0.10 0.05 0.04
0.21 0.23 0.66
0.7680.010 0.9640.011 1.3600.013
0.3210.030 0.4110.038 0.3750.033
3.50.8 8.60.8 4.30.9 11.01.0 7.71.0 10.10.9
Smelter-associated sediments: Smelter 1 (BRSS1&2) Smelter 2 (BRSS3) Smelter 3 (BRSS4)
Equivalent Th (ppm)
An error of 5% of the measured K2O concentration, a 10% error in cosmic ray dose, and an error of 0.05 in water contents, are included in the dose rate calculation. Equivalent U and Th concentrations were calculated using the conversion from alpha count rates: U=[(total countsTh counts)/ 0.128] ppm ks cm2; Th=[Th counts/0.0372] ppm ks cm2.
temperatures, reheats wereb subtracted from all glows prior to data analysis. Raw glow curves of all three smelters were normalized to aliquot mass (in mg) and, for smelters 2 and 3, to the 100 (C TL peak. For each 5 ( interval, a growth curve of normalized TL versus added laboratory dose was created, and the past dose at that temperature deduced by extrapolation to y⫽0 of a linear least squares fit to the data (Fig. 4B). A plateau, consisting of a graph of past dose De as a function of glow curve temperature was constructed. A good plateau should show a De near zero at low temperatures, gradually increasing between 200 and 300 (C and thence remaining nearly constant over a span of 100–150 (C, and indicates that the material being tested was exposed to a sufficiently high temperature in the past to have erased completely any natural TL present in its constituent minerals prior to the heating event being dated. Linear growths, and excellent TL plateaus spanning 100 (C or more were observed for all three smelters (Fig. 5). 6.2. Second glow thermoluminescence Because the dose-dependent TL increase in quartz sometimes shows an initial period of very slow TL growth known as supralinearity, it is necessary to perform second glow analysis to determine the dose equiva-
lent value, also known as the ‘non-linearity intercept’, of this period. The second glow, or non-linearity intercept is deduced through TL analysis of aliquots which had first been heated once to 500 (C to remove their natural TL, re-irradiated with calibrated doses, and glowed to 500 (C a second time (hence, second glow TL). For each smelter 20 aliquots of 5 mg were heated to 500 (C, given fresh radiation doses of 0.5–12 Gy in batches of four aliquots, and glowed to 500 (C at 3 ( s1. The natural high-temperature TL was used to normalize the second glow TL. Non-linearity intercepts were deduced analytically from the second glow data for a 20 (C integral centered on the 325 (C peak. 7. Results The smelters and their associated sediments are remarkably uniform in their relevant radioisotope compositions. K concentrations are low, ranging from 0.3 to 0.6 weight percent K2O, while both U and Th activities are moderately high and equivalent to U 3–5 ppm and Th 10–15 ppm. Although the samples were collected during the rainy season, the water contents of the sediments were low, ranging from 0.04 to 0.09 (water weight/dry sediment weight). Slightly higher water contents of the smelter walls are consistent with their higher clay contents. Elsewhere at Birimi sediments
Table 2 Plateau, first and second glow (I) doses, past dose De, dose rate R, and age results for Birimi smelters Smelter Smelter Smelter Smelter Smelter Smelter
1 2 2 3 3
Normalization
Plateau ((C)
First glow dose (Gy)
I (Gy)
Past dose De (Gy)
Dose rate R (Gy ka1)
Age (years)
Mass 100 (C TL Mass 100 (C TL Mass
240–340 253–313
1.520.06 2.510.09 2.400.09 2.410.11 2.150.10
0.30 0.14 0.14 0.03 0.03
1.820.06 2.650.09 2.540.09 2.440.11 2.180.10
1.730.08 1.620.08 1.620.08 2.130.10 2.130.10
105662 163199 156794 114575 102369
250–340
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Fig. 6. (A) Chemical analysis of the Birimi site sediments to 180 cm b.s. Nodule horizons at 57 and 136 cm b.s. show high concentrations of Fe, V, and Cr; (B) X-ray diffractogram of a 1 cm nodule from the 57 cm b.s. layer. Goethite (G) and quartz (Q) peaks are labeled.
sampled at 0–1.8 m b.s. yielded water contents of 0.06– 0.15 [45]; the higher values were obtained for sediments at depths >50 cm b.s. The coarse, highly porous, and sharply drained nature of the sediment makes it unlikely that average yearly moisture contents could have fluctuated outside the moisture content’s assumed error of 0.05. Since all assumed and measured errors are propagated in quadrature, a 0.05 assumed error in water content contributed 20 years to the standard deviation in the age for smelters 1 and 3, and 40 years for smelter 2, i.e. 1.9% and 2.5% of their respective ages. Measurements of each wall yielded its beta dose rate, while the values of adjacent sediment were used to calculate its gamma dose rate. Because the wall fragments were collected at 0–4 cm above surface, the gamma dose rate to the wall at these elevations ranged from 0.45 to 0.5 of an infinite-matrix rate. The cosmic dose rate was calculated according to Prescott and Hutton [43]. No correction was made for any possible variations in surrounding or infilling sediment depth in antiquity, which would have affected the gamma and cosmic dose rates. However, the presence, close to the modern surface, of a tuyere inlet in one of the smelters, the fact that slag heaps nearby appear to sit on the modern surface, and the absence of any sediment infilling the spaces between the slag fragments in the slag heaps, all suggest that the present surface has not varied significantly from its prehistoric level (Table 2). Excellent TL properties, including a high dose response luminescence sensitivity and wide plateaus spanning w100 (C, were observed. The high luminescence efficiency (TL per unit radiation dose), suggests that good precision ages may be obtained on significantly younger Iron Age installations in the study area in future. For smelters 2 and 3, the TL data were analyzed using both mass and 110 (C TL peak normalization. The ages normalized to the TL peak were older than the
corresponding mass-normalized ages by 32 and 61 years, respectively, but these differences remained well within the standard deviation in each age. Smelter 1 was analyzed using mass normalization alone. Extrapolating from the results of the other two, it is reasonable to expect that its TL-normalized age would have been w46 years older, or 110262 years. Averaging the mass- and TL-normalized ages, the mean ages for the three features are: Smelter 1 Smelter 2 Smelter 3
1090 60 years 1600100 years 1080 70 years
These ages are consistent with the external appearance of the smelter remains. Smelter 1, by far the best preserved of the three, falls within the younger subgroup. Smelter 2, the oldest of the group by w500 years, was rather worse for wear. Birimi site sediments are derived locally, from a well sorted and heavily oxidized medium to coarse sandstone which forms the bedrock of the Gambaga Escarpment and is sporadically exposed in outcrops. The sedimentary column of the terrace is dominated throughout by fine quartz sand and silt. Sedimentological analysis with scanning electron microscopy indicates the terrace sediments accreted fluvially [44,45]. Thin horizons of nodular iron hydroxide are found at 57 and 136 cm below surface. These high concentrations of iron oxide, of up to 14% and 21% by weight, are also evident in major element analyses of the sedimentary column (Fig. 6). Using X-ray diffraction (Fig. 6), we determined that the nodules are composed of precipitated mineral goethite [FeO(OH)], which binds together the sedimentary quartz grains. It is possible that each nodule horizon represents the lower limit of a downward leaching zone or the upper limit of a reduced-state saturated zone; they would have formed during two much wetter
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Table 3 Element compositions of the sediment layers (S 57 cm, S 136 cm) containing goethite nodules, a cleaned goethite nodule, and a fragment of slag from smelter 1 Element
S 57 cm
S 136 cm
Goethite
Slag
SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Cl L.O.I. Total incl. LOI
(%) 62.52 0.83 12.97 13.10 0.02 0.16 0.08 0.00 0.35 0.05 0.00 8.90 98.99
(%) 51.93 0.81 16.00 20.56 0.09 0.19 0.23 0.03 0.30 0.06 0.03 9.40 99.63
(%) 14.55 0.17 3.38 68.47 0.08 0.08 0.04 0.10 0.05 0.09 0.01 12.40 99.43
(%) 26.05 0.32 5.54 68.22 0.11 0.15 0.16 0.09 0.03 0.13 0.03 3.20 104.02
Sc V Cr Co Zr Ba La Ce Nd Ni Cu Zn Ga Rb Sr Y Nb Sn Pb Th U
(ppm) 16 105 75 35 535 339 81 116 49
(ppm) 20 405 346 46 569 318 68 136 58
(ppm) 14 222 73 21 60 370 48 90 41 8 29 0 18 41 5 26 9 17 18 5 3
(ppm) 16 198 38 9 210 244 6 59 27 0 21 0 27 43 8 29 12 16 6 10 3
The SiO2 to Nd determinations were made on lithium borate fused glass disks, while Ni to U were measured on pressed powders. The analyses were carried out using a Philips PW2400 wavelength-dispersive X-ray spectrometer at the Regional Geochemical Centre, St. Mary’s University, Halifax.
periods in the region’s past history, possibly during the Pleistocene. Chemical analysis (Table 3) shows the clean goethite to be a very rich source of iron in comparison to the bulk sediment of the horizon containing the goethite nodules. The trace element composition of slag from smelter 1 confirms its genetic relationship to the goethite. Its major element composition is very similar to the fayalitic slags from Nsukka (for example, see samples Lej/2/90 and Lej/4/90 of Okafor [35, p. 443]). Based on these analyses, there is no doubt that the location of the smelting activities is directly related to the presence and easy access to this easily extractable
natural source of iron. In contrast to the eastern half of the site, where the terrace slope is laterally and vertically smooth (excluding obvious erosional gullies), the terrace scarp in the western half is severely disturbed, forming an irregular, hummocky and disorganized surface that slopes down to the streambed. Clearly, the iron oxides were being mined out of the terrace scarp, and smelted on top of the terrace. Because the sediments are uniformly fine textured whereas the goethite nodules are typically 0.5–1.5 cm in diameter, the ore would have been easily concentrated by sieving and washing. Thus, the riverbank location appears to have provided the raw ore, the direct means of pre-processing it, a level site for the iron smelters and, likely, clay for their construction. 8. Discussion Our investigations of Birimi suggest that the presence of Iron Age and Kintampo sites in close proximity to each other [23,24] may be spurious, in the sense that although both groups exploited some of the resources offered by the river-bank environment, they probably did not utilize the same resources. It is our belief that the smelters were located so as to take advantage of the goethite deposit readily available in the terrace bank. The Kintampo people, on the other hand, surely had no interest in the goethite, but were likely interested in the ready availability of water and natural resources associated with it. The fact that smelting sites are not associated with their makers’ habitation sites, which typically are located well away from streams, offers support for the premise that different resources were of interest to each of these cultures. A luminescence and radiocarbon chronology has been developed for the Kintampo complex in east Birimi [16,44,45]. Presenting and discussing fully the entire set of these dates is beyond the scope of this paper, however, we note here that we have 30 independent absolute dates from east Birimi, which we can compare to the three smelter ages from west Birimi. The youngest TL age on a Kintampo ceramic sherd is 2270130 years [44], while the youngest TL age on a fragment of burnt house daub is 2640160 years ([16]; manuscript in preparation). Four radiocarbon ages on charcoal fragments collected by one of us (DIGS) from two Kintampo features have a two standard deviation range of 3260–3960 calBP [45]. Two additional radiocarbon ages on single millet seeds from a Kintampo pit feature span the range 2180–4290 calBP, also at two standard deviations of their calibrated ranges [9]. The ceramic and daub TL chronologies overlap fully with the radiocarbon chronology, yet a comparison of the age ranges at the 95% confidence interval leaves a gap of 210 years between the Kintampo complex and the earliest iron smelting activity at the site (Fig. 7). Thus, despite the fact that TL and radiocarbon ages at east Birimi are
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Fig. 7. Comparison of Birimi Kintampo complex and iron smelting ages. Hatched bars show 14C dates, shaded bars TL dates. The uncalibrated 14C ages are shown numerically. The burnt house daub chronology extends beyond 4700 years. The short vertical lines through the burnt house daub and Kintampo ceramics TL ranges show the position of the youngest TL date in each set. Ranges of 2 are shown throughout. The radiocarbon 2 calBP ranges were obtained using the calibration curve of Stuiver et al. [55]. The vertical gray bar marks the position of a 210 year gap (at 95% confidence) between the Kintampo occupation and the Iron Age at Birimi.
wholly consistent with each other, our chronology offers no support for suggestions made by Davies [11,12], or Shinnie and Kense [53], among others, that the Early Iron Age in Ghana is a continuous development of the Kintampo complex. On the contrary, the Kintampo and Iron Age components at Birimi fail to overlap even at the 99% confidence interval (three standard deviations) of the available chronological data. The smelter TL ages presented here offer important and independent support for the presence of Early Iron Age in northern Ghana, as suggested by absolute dates at other sites ([3,13,42,53,56,63] and others). Still, we believe that anthropogenic site disturbance may be responsible for a number of spurious associations of iron artifacts and very early radiocarbon ages. Extensive and deep pits uncovered in the excavations in east Birimi contained mixed deposits that included artifacts diagnostic of both Kintampo and Middle Stone Age toolkits. As there is reason to believe that Iron Age people were at least as adept at disturbing the soil, we urge extreme caution in interpreting the on-site presence of iron and Kintampo (or Kintampo-like) ceramics and other artifacts as contemporaneous. For this reason, investigations into the chronology of iron smelting technology should focus on materials directly associated
with the activity, such as the remains of smelters or the slags themselves. 9. Conclusions The TL dates presented here demonstrate a fullblown smelting technology in Northern Ghana during the Early Iron Age. Iron smelting at Birimi was established 1600 years ago, and continued until w1080 years ago. This time period predates any previous direct ages for iron smelting on the Gambaga Escarpment by 500–1000 years, and demonstrates that this technology flourished in the Northern Region of Ghana centuries earlier than previously suspected. The appearance of smelting technology at Birimi post-dates by 670 years the latest evidence for the Kintampo complex at the site. Further chronological work is needed to determine if Early to Late Iron Age iron production on the Gambaga Escarpment was episodic or continuous. Based on these results, we feel that the methods chosen for chronological investigations of the Iron Age in future should focus more on materials directly associated with this industrial process. One of such methods is luminescence dating. Optical dating (OSL), although originally developed for the dating of
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unheated sediments [22] can equally be applied to date heated materials, since the OSL signal is completely erased by heating to a high temperature. The recently developed SAR technique [34] offers an optical dating alternative to the classical TL method. Luminescence dating (TL and OSL) may be applied not only to smelters themselves, such as we have done here, but also to other smelting paraphenelia, such as fragments of tuyeres, slag-coated pottery sherds, and many other remains clearly associated with smelting activity. Because the products of smelting (slag and bloom) retain temperatures in excess of 1000 (C during processing and for some time afterwards, we suggest that luminescence may even be applied to the surfaces of hollowed out depressions in the soil into which slag had been directed while still molten. At these temperatures and volumes of slag [35,47] sufficient heat will have been transferred to the underlying earth to have baked a layer at least a few centimeter thick to a temperature in excess of 500 (C. This temperature is sufficient to completely erase any pre-existing luminescence signal in the minerals of interest, and allow the heating event to be dated. We postulate therefore that luminescence dating may be applied to chambers for slag collection, provided that the in situ association of slag with the underlying sediment can clearly be demonstrated. Such proof may take the form of reddened and hardened crust of earth directly underlying the slag, matching cast-and-mould shapes of slag and earth depression, or adhesion of the chamber’s upper surface to the slag. The additional benefit of dating smelting activity by luminescence is that the old wood effect, a potentially confounding and difficult to assess accurately issue inherent in the radiocarbon dating of smelter residues and slags, is avoided [32]. Indeed, with careful selection of material unquestionably associated with smelting activity, luminescence dating may be useful in confirming the very early dates for iron metallurgy in Nigeria, and finally validating the existing radiocarbon chronology. Since smelter remains and associated paraphenelia represent a closed physical system with respect to the last firing event even if the site suffers some disturbance, questions based on suspicion of recent carbon contamination or raised due to doubtful contextual grounds (see, for example, Ref. [33, p. 466]), may also be resolved by this method.
Acknowledgements The TOSL Laboratory is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through equipment and research grants to D.I.G.S. The excavations at Birimi were supported by a Social Sciences and Humanities Research Council of Canada (SSHRC) research grant to J.L.C., while the
archaeometric studies were supported through a SSHRC research grant to D.I.G.S. Technical assistance of Mrs P. Scallion and Mr K.B. Vaughan in the TOSL Laboratory is warmly acknowledged.
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Archaeological SCIENCE Journal of Archaeological Science 30 (2003) 1037–1050 http://www.elsevier.com/locate/jas
Direct thermoluminescence chronology for Early Iron Age smelting technology on the Gambaga Escarpment, Ghana D.I. Godfrey-Smith a,*, J.L. Casey b a
Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5 b Department of Anthropology, University of South Carolina, Columbia, SC 29208, USA
Received 8 October 2002; received in revised form 25 November 2002; accepted 2 December 2002
Abstract We present luminescence dates which demonstrate a full-blown smelting technology in Northern Ghana, West Africa, during the Early Iron Age. Our chronology is based on thermoluminescence (TL) dating of quartz grains extracted from the walls of three iron smelters located at the Birimi site in the Northern Region of Ghana. Two of the smelters yielded statistically indistinguishable ages of 108070 and 109060 years, while the third yielded a higher age of 1600100 years. All are significantly older than the sole direct radiocarbon date of 550–330 calBP (46090 BP) obtained on a furnace in the Northern Region. The TL ages indicate that iron smelting was well established in the Northern Region before the middle of the first millennium AD, and corroborate the validity of a number of similarly early radiocarbon dates associated with Early Iron Age ceramics but not directly associated with smelting activity, from other sites in northern Ghana. The iron working remains at Birimi are located on a river terrace, only a few tens of meters west of a dense Kintampo occupation. Radiocarbon and TL dates for the Kintampo complex demonstrate that the Iron Age and Kintampo components at Birimi are clearly non-contemporaneous. The occurrence of two distinct layers of goethite within 150 cm of the terrace surface, both of which are clearly exposed in the scarp of the terrace upon which the smelters are situated, indicates that the industrial site was deliberately sited to take advantage of the readily available source material. This premise is confirmed by geochemical analyses of sediment, goethite, and slag from the smelting site. We therefore propose that the frequent proximity of Kintampo and iron production sites is incidental, in the sense that each group exploited different resources offered by a riverbank location. 2003 Elsevier Science Ltd. All rights reserved. Keywords: Early Iron Age; Smelting; Africa; Luminescence; Thermoluminescence; Dating; Geochemistry; Goethite; Fayalite; Kintampo
1. Introduction The data we present here add to the discussion about the Early Iron Age in West Africa in several significant ways. First, although the few claims of a transitional phase between the Late Stone Age and the Iron Age in Ghana have been challenged, the consistent occurrence of Late Stone Age Kintampo Complex sites in the vicinity of iron working sites needs to be addressed. At the Birimi site in northern Ghana, a large Kintampo habitation is flanked by an iron smelting area that is not associated with domestic Iron Age artifacts. Direct * Corresponding author. E-mail addresses: [email protected] (D.I. Godfrey-Smith), [email protected] (J.L. Casey).
dates, using luminescence and radiocarbon dating, obtained on features of both components and coupled with geochemical analyses, allow for a test of the transition hypothesis. Second, at a number of sites (e.g. Daboya, Ntereso) indisputable Kintampo artifacts and Iron Age ceramics plus iron artifacts are found in close stratigraphic association. The direct dating of archaeological remains attributable to one or another cultural phase could make a significant contribution towards clarifying whether the association is contemporaneous or due to post-depositional geological or anthropogenic site disturbance. Third, the use of luminescence to date the materials directly overcomes the problems inherent in the dating of charcoals that may be intrusive, and the issue of ‘old wood’ where fuel woods used in the iron smelting process may predate iron working activity
0305-4403/03/$ - see front matter 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0305-4403(02)00292-3
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Fig. 1. Archaeological sites in Ghana with evidence for Early Iron Age iron production.
[25,32]. Finally, there have been relatively few dates for early iron working activity in Ghana. Our data firmly place iron smelting activity in northern Ghana in the early and middle phases of its appearance in West Africa. Contemporary archaeological investigations into the African ‘Iron Age’ have largely moved away from discussions of origins and have concentrated more on the process of smelting, the social, ideological, and technological changes that accompanied the advent of iron production, and the symbolic and ritual dimensions of iron working ([5–7,10,18,20,26,38–41,46–48,58,59,61,62]—to name but a very few). Much of this research is being done through experimentation and ethnographic analysis. This new
approach has given us a multi-dimensional picture of the dynamics of iron working and usage, but there is still a lot of basic archaeology that needs to be done in order to chronicle the presence of iron working in Africa. The Iron Age, which is thought to have been initiated during the second millennium BC by the Hittites in Armenia, spread to Cyprus and Greece by 1200 BC, and, via routes both unknown and still debated, reached Nigeria during the first millennium BC. Regardless of its origins, the techniques used for smelting iron are highly variable throughout pre-colonial Africa, suggesting that the technology has undergone significant regional innovation and adaptation [17,32,49,57,60].
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The majority of the earliest iron working sites in West Africa stretch from Senegal to Chad and from the Sahel to the rainforests. They date to between 2870 and 1830 calBP (2628120 to 196060 BP [21, p. 338]), with the earliest dates occurring in Nigeria and Niger [3,19]. AMS radiocarbon dating of charcoal samples obtained from excavated iron smelting furnaces in Nsukka, Nigeria, yielded calibrated ages that ranged from 765 to 520 BC [36]. Broadly, the earliest phase of the development of iron working is considered to have extended from 600 BC to 600 AD [50]. Early in the mid first millennium AD iron working sites in many parts of West Africa underwent a dramatic change, signaled by intensive construction and increasing social and political complexity [27]. It is this change that has resulted in the massive and most well-known components at JenneJeno [28–30] and Daima [8]. During and after this, iron working and using sites throughout West Africa became much more common. In many parts of Africa the indigenous process of smelting iron continued well into the 20th century. 2. Iron working in Ghana In Ghana, as over all of West Africa, information on the advent of iron working technology comes from a very few, widely spaced sites. Although Davies [11] noted numerous smelting sites throughout Ghana, most of these remain uninvestigated. Information for early iron working activity is limited to about 11 sites (Fig. 1). Although these sites have produced early dates for iron working activity, not all have produced much information about the cultural context in which iron working began in the area. The earliest dates for iron working in Ghana come from Daboya. Shinnie and Kense [53] report evidence for metallurgy at Daboya in the Late Stone Age Kintampo Complex in the early second millennium BC. They concur with Davies [11] that there is continuity between Kintampo and the advent of iron. This view has been criticized on the basis of inconsistencies in dates and provenances at the site [54] and because the frequent association of Kintampo and Early Iron Age ceramics suggests that deposits are likely to have been mixed [31]. Our own analysis of the precise association of iron artifacts at Daboya [53, pp. 206–207] with radiocarbon and thermoluminescence (TL) dates obtained within the same units (units TA10, WA10, and KA12 [53, pp. 249–250]) leads us to conclude that the chronostratigraphic control on the iron occurrences is neither sufficient to support the transition hypothesis nor warrants an Early Iron Age attribution. Elsewhere in Ghana there appears to have been a hiatus of at least 500 years after Kintampo before evidence for iron metallurgy appears. Other early sites for the production of iron include New Buipe in northern Ghana [63] where artifacts and
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a few pieces of slag were found in the course of large scale excavations at a mound site, and radiocarbon dating produced ages of late first millennium AD. The earliest materials, however, were not associated with furnaces [63]. Also in northern Ghana, the Kintampo site Ntereso produced a date of 1950–1705 calBP (1890110 BP, SR-90) on charcoal in a layer that contained mixed Kintampo and Iron Age materials [12, p. 53]. Here, pieces of slag and iron and sherds of Iron Age pottery are thought to be intrusive into the Kintampo layers rather than indicating a transitional phase. Smelting is present at Abam, Atwetwebooso, Begho, Bonoso and Bono Manso [2] along with early dates, but not much is known about these sites. Asantemanso [51] and Adansemanso [52] are both lengthy Iron Age sequences that have produced a few early dates. Evidence for iron smelting on the Gambaga escarpment has been documented by recent archaeological investigations [23,37]. Still, within this region, the sole absolute date directly associated with iron smelting is a radiocarbon date of 550–330 calBP (46090 BP, AECV-710C), obtained for furnace ISM-15 [23] from the escarpment. More general archaeological remains attributed to Middle to Later Iron Age periods span a broad range of 940–300 calBP [23] at this and other northern Ghanaian sites. The vast majority of Iron Age occurrences in Ghana post-date 1000 BP (see Ref. [54]). 3. Birimi site description and cultural context The Birimi site sits upon a 2 m terrace on the northern edge of the Birimi Kuliga, a seasonal stream south of the Gambaga Escarpment, and 3.5 km NW of the village of Nalerigu in the Northern Region, Ghana. A 1996–1997 survey and excavation of this site has revealed the presence of three temporally distinct cultural components. The eastern half of the 20050 m site includes a 3500–3830 calBP Later Stone Age Kintampo component [4] which appears to intrude anthropogenically into the underlying sediments and may extend down to >60 cm b.s. Underlying and at times mixed with it is a Middle Stone Age (MSA) component which at depths of 1 m or deeper appears to be in natural geological sedimentary contexts. Its occurrences in two horizons have been dated to 40,80011,400 years (BRSD3), and 23,6002,900 years (BRSD55) using optical dating [45]. This suggests that the site’s natural setting must be exceptionally favorable to humans, as it was occupied for substantial periods in the distant past. The third and latest component dominates the western half of the site (Fig. 2). It is made up of well-preserved remains of at least five smelters and numerous slag heaps. Unlike the site’s eastern half, which is a settlement area, the western
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Fig. 2. Location map for the Iron Age remains at west Birimi. The grid ties this part of the site to the excavated Kintampo area grid, which spans 20–120 m east and 0–60 m south. Smelters are indicated by circles, slag heaps by # signs. Smelters dated by us are numbered. Smelter 1 lies on the 102.75 m contour line; contour interval is 0.5 m.
half is manifested solely by industrial remains, and shows no evidence of living areas. 4. Smelting activity and thermoluminescence dating Traditional smelting in Africa depended on the bloomery process, in which the ore must reach temperatures high enough (1100–1300 (C) to separate iron from slag [20]. Ore smelted at such temperatures does not reach the melting point of iron, but produces a solid, spongy iron mass called the bloom. The slag, composed largely of non-ferrous residues of the ore, does liquefy in the smelting process. The two components were often separated by directing the slag to run off, leaving only the bloom in place (though other approaches, including mechanical separation of the cooled bloom–slag mixture, were also used). TL dating is particularly well suited to the dating of anthropogenically heated materials such as ceramics, cooking hearths, ovens, kilns and other types of furnaces, fire-cracked rock used in cooking, and heattreated flints used in toolmaking [1]. It measures the time since the material forming the matrix of the items mentioned was last exposed to a ‘clock-resetting’ event. Such an event takes place when the material is heated to a temperature in excess of 500 (C. For an artifact which has been heated, the TL signal is proportional to the yearly rate at which new TL is created (which is propor-
tional to the radiation dose absorbed by the sample and depends on the concentration of radioisotopes in its immediate surroundings; this is measured by independent means), times the number of years since the sample was last heated. Thus: Age (years) 5
Total TL (TL per unit dose)#(dose per year)
Because smelting activity is perforce associated with very high temperatures, remains of the smelters themselves, or indeed even far simpler fire-associated installations [14] are uniquely well suited to direct geochronometric analysis using TL dating. In addition, large well-heated features such as these provide plentiful material for TL dating. We therefore chose to apply TL dating to three smelters at the site. These appeared to be sufficiently well preserved to still retain their circular shape and their wall remnants protruded 5–10 cm above the ground surface. In one case, a bellows inlet was also clearly evident (Fig. 3). In addition to the chronological studies, we investigated the sediments forming the matrix of the Birimi site. To accomplish this, we employed scanning electron microscopy, major and trace element determination by X-ray fluorescence, and mineralogical analysis with X-ray diffraction.
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Fig. 3. Smelter 1, looking south. Dashed white curves follow the outside of the visible wall remnants and tuyere opening. The TL dating sample was collected above the left end of the lower curve. Hammer and African hoe are included for scale.
5. Sample preparation Large fragments (300–500 g) of smelter wall were removed and immediately sealed in plastic to preserve their natural water content. They were weighed in the field, and re-weighed upon receipt in the laboratory to determine their water loss during transit (about 6 months delay). Their natural and saturation water contents were determined as described by GodfreySmith et al. [15]. The smelter walls are coarse in texture, composed primarily of silty clay, with a high void volume which includes many well-delineated moulds of incinerated grass or twig temper. A decision was made to use
sand-sized quartz grains for the following reasons: quartz grains were readily apparent on visual inspection; plenty of smelter wall material remained available for quartz extraction; the walls were not vitrified, and their high void volume made them relatively easy to disaggregate with gentle crushing; quartz has favorable TL properties in the age and dose range expected for these structures; and the use of etched sand-size quartz grains and the quartz-inclusion technique obviates the need for additional tedious alpha efficiency measurements. In addition, should these smelters have proved to be younger than 1000 years, accurate and precise ages could have been obtained for them using the pre-dose technique [1].
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Fig. 4. (A) TL glow curves for quartz extracted from smelter 1. Glow curves with naturally accumulated TL (N) and with additional artificial radiation (N+1.2, 2.6, and 5.2 Gy, respectively) are shown. For clarity at low doses, glow curves with higher added doses are omitted. (B) Growth curve and linear least squares fit to the data of the first glow TL at 275 (C, for smelter 3. Inverse weighting with respect to the TL intensity was used. The past dose estimate De is indicated by the two vertical lines marking its 1, at x⫽⫺2.4 Gy, y⫽0.
Quartz extraction was performed on an 80 g fragment of each sample. The outer 5 mm layer was removed using a slow-speed Dremel drill. This ensured that the outer surface which had been exposed to light during excavation and later handling and therefore had its natural TL partially erased, was not included in the analysis. It also ensured that quartz grains which had accumulated beta radiation from both the wall itself and the surrounding sediments, i.e. those that resided in the outermost 2 mm of the wall, were not included in the analysis. The removed material was used in the determinations of K, U and Th activities of the smelter walls.
The remaining sample was gently crushed using a benchtop hydraulic press. This process ensured that the piece was disaggregated without crushing any singlemineral grains. The resulting aggregate was dry sieved to obtain typically 6 g of 125–150 µm grains, which were ultrasonically wet sieved in a methanol bath through a 125 µm sieve. The cleaned fraction was treated overnight with concentrated HCl to remove carbonates and disaggregate silts and clays. These two steps ensured a very clean quartz extraction when treated with 48% HF, with no precipitates forming on the quartz. Heavy liquid and magnetic separations completed the quartz purification process.
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6. Thermoluminescence principles and measurements When minerals such as quartz and feldspar are exposed to radiation energy from their surroundings, they absorb and store it in their crystal lattice. When heated in the laboratory, the minerals release the absorbed energy in the form of light which is known as TL. When the emitted TL is plotted as a function of temperature, a reproducible ‘glow curve’ results. The amount of TL emitted is proportional to the amount of radiation energy absorbed, and varies in a consistent manner with temperature (Fig. 4A). Simplifying the above relationship to the age equation standard to all dosimetric dating methods, age is deduced from the ratio of the radiation dose De (accrued since the last clock resetting event) divided by the dose rate R: Age⫽De/R. The total dose De accrued in the minerals is determined through TL glow curve analysis, here of quartz grains extracted from a smelter wall. The dose rate R is determined by measuring the concentrations of the radioisotopes U, Th, K, of each wall fragment, and the sediments immediately surrounding it. Activities of the U and Th radioisotopes and their decay chains were measured using thick-source alpha counting, calibrated with known-concentration standard powders from New Brunswick Laboratories, Maine, USA. K concentrations were measured at a commercial laboratory (Bondar-Clegg of Vancouver, BC). The dose rate calculations include other variables, including the water contents of wall and sediment, the wall thickness, the depth of the sampled wall into or above the ground surface, and the size of quartz grains used for TL analysis. The measured dosimetric values are given in Table 1. 6.1. First glow thermoluminescence
Fig. 5. Plateau plots for smelters 1, 2, and 3. The temperature range of each plateau is indicated by the horizontal line.
Once a quartz extract was obtained, a single layer of grains was deposited by sprinkling them onto oiled, 9.8 mm aluminum disks, resulting in aliquots of w5 mg. Typically 44 aliquots were prepared for each sample. TL measurements were carried out at the Thermally and Optically Stimulated Luminescence (TOSL) Research Laboratory at Dalhousie University.
For first glows, 24 aliquots were irradiated with 0.04 Gy and promptly glowed to 150 (C to yield the 100 (C TL peak. This provided one of the normalizing values for the raw TL glow curves; the other normalizing value used was aliquot mass. Twenty aliquots were irradiated in batches of four. Laboratory doses of 1.5– 24 Gy were administered using on-plate irradiation from a 90Sr/90Y source with a calibrated strength of 0.99 Gy/min. Afterwards, each aliquot was glowed out under a flow of high purity dry nitrogen in a Risø multi-sample TL/OSL instrument. Samples were preheated to 150 (C, then glowed at 2 ( s1 to 500 (C. At this heating rate the nominal 325 (C TL peak of relevance to dating of heated archaeological materials appears at 295 (C. Luminescence was detected through a red-absorbing, violet-transmitting glass filter pack (Kopp 7-59+Schott BG39). To remove any contribution to the glow curves from thermal blackbody radiation which is naturally emitted by all matter at high
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Table 1 Dosimetry values for Birimi smelters 1, 2, 3 and their associated sediments
Smelter Smelter Smelter Smelter
walls: 1 (BRSM1) 2 (BRSM4) 3 (BRSM5)
Grain size (µm)
Depth b.s. (m)
Water content
K2 O%
Alpha counts Total (ks1 cm2)
Th (ks1 cm2)
U (ppm)
138 138 138
0.04 0.00 0.00
0.12 0.12 0.12
0.33 0.36 0.58
1.2120.014 0.9160.014 1.1770.015
0.5570.050 0.3480.045 0.4530.050
5.11.1 15.01.3 4.41.1 9.41.2 5.71.2 12.21.3
0.10 0.05 0.04
0.21 0.23 0.66
0.7680.010 0.9640.011 1.3600.013
0.3210.030 0.4110.038 0.3750.033
3.50.8 8.60.8 4.30.9 11.01.0 7.71.0 10.10.9
Smelter-associated sediments: Smelter 1 (BRSS1&2) Smelter 2 (BRSS3) Smelter 3 (BRSS4)
Equivalent Th (ppm)
An error of 5% of the measured K2O concentration, a 10% error in cosmic ray dose, and an error of 0.05 in water contents, are included in the dose rate calculation. Equivalent U and Th concentrations were calculated using the conversion from alpha count rates: U=[(total countsTh counts)/ 0.128] ppm ks cm2; Th=[Th counts/0.0372] ppm ks cm2.
temperatures, reheats wereb subtracted from all glows prior to data analysis. Raw glow curves of all three smelters were normalized to aliquot mass (in mg) and, for smelters 2 and 3, to the 100 (C TL peak. For each 5 ( interval, a growth curve of normalized TL versus added laboratory dose was created, and the past dose at that temperature deduced by extrapolation to y⫽0 of a linear least squares fit to the data (Fig. 4B). A plateau, consisting of a graph of past dose De as a function of glow curve temperature was constructed. A good plateau should show a De near zero at low temperatures, gradually increasing between 200 and 300 (C and thence remaining nearly constant over a span of 100–150 (C, and indicates that the material being tested was exposed to a sufficiently high temperature in the past to have erased completely any natural TL present in its constituent minerals prior to the heating event being dated. Linear growths, and excellent TL plateaus spanning 100 (C or more were observed for all three smelters (Fig. 5). 6.2. Second glow thermoluminescence Because the dose-dependent TL increase in quartz sometimes shows an initial period of very slow TL growth known as supralinearity, it is necessary to perform second glow analysis to determine the dose equiva-
lent value, also known as the ‘non-linearity intercept’, of this period. The second glow, or non-linearity intercept is deduced through TL analysis of aliquots which had first been heated once to 500 (C to remove their natural TL, re-irradiated with calibrated doses, and glowed to 500 (C a second time (hence, second glow TL). For each smelter 20 aliquots of 5 mg were heated to 500 (C, given fresh radiation doses of 0.5–12 Gy in batches of four aliquots, and glowed to 500 (C at 3 ( s1. The natural high-temperature TL was used to normalize the second glow TL. Non-linearity intercepts were deduced analytically from the second glow data for a 20 (C integral centered on the 325 (C peak. 7. Results The smelters and their associated sediments are remarkably uniform in their relevant radioisotope compositions. K concentrations are low, ranging from 0.3 to 0.6 weight percent K2O, while both U and Th activities are moderately high and equivalent to U 3–5 ppm and Th 10–15 ppm. Although the samples were collected during the rainy season, the water contents of the sediments were low, ranging from 0.04 to 0.09 (water weight/dry sediment weight). Slightly higher water contents of the smelter walls are consistent with their higher clay contents. Elsewhere at Birimi sediments
Table 2 Plateau, first and second glow (I) doses, past dose De, dose rate R, and age results for Birimi smelters Smelter Smelter Smelter Smelter Smelter Smelter
1 2 2 3 3
Normalization
Plateau ((C)
First glow dose (Gy)
I (Gy)
Past dose De (Gy)
Dose rate R (Gy ka1)
Age (years)
Mass 100 (C TL Mass 100 (C TL Mass
240–340 253–313
1.520.06 2.510.09 2.400.09 2.410.11 2.150.10
0.30 0.14 0.14 0.03 0.03
1.820.06 2.650.09 2.540.09 2.440.11 2.180.10
1.730.08 1.620.08 1.620.08 2.130.10 2.130.10
105662 163199 156794 114575 102369
250–340
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Fig. 6. (A) Chemical analysis of the Birimi site sediments to 180 cm b.s. Nodule horizons at 57 and 136 cm b.s. show high concentrations of Fe, V, and Cr; (B) X-ray diffractogram of a 1 cm nodule from the 57 cm b.s. layer. Goethite (G) and quartz (Q) peaks are labeled.
sampled at 0–1.8 m b.s. yielded water contents of 0.06– 0.15 [45]; the higher values were obtained for sediments at depths >50 cm b.s. The coarse, highly porous, and sharply drained nature of the sediment makes it unlikely that average yearly moisture contents could have fluctuated outside the moisture content’s assumed error of 0.05. Since all assumed and measured errors are propagated in quadrature, a 0.05 assumed error in water content contributed 20 years to the standard deviation in the age for smelters 1 and 3, and 40 years for smelter 2, i.e. 1.9% and 2.5% of their respective ages. Measurements of each wall yielded its beta dose rate, while the values of adjacent sediment were used to calculate its gamma dose rate. Because the wall fragments were collected at 0–4 cm above surface, the gamma dose rate to the wall at these elevations ranged from 0.45 to 0.5 of an infinite-matrix rate. The cosmic dose rate was calculated according to Prescott and Hutton [43]. No correction was made for any possible variations in surrounding or infilling sediment depth in antiquity, which would have affected the gamma and cosmic dose rates. However, the presence, close to the modern surface, of a tuyere inlet in one of the smelters, the fact that slag heaps nearby appear to sit on the modern surface, and the absence of any sediment infilling the spaces between the slag fragments in the slag heaps, all suggest that the present surface has not varied significantly from its prehistoric level (Table 2). Excellent TL properties, including a high dose response luminescence sensitivity and wide plateaus spanning w100 (C, were observed. The high luminescence efficiency (TL per unit radiation dose), suggests that good precision ages may be obtained on significantly younger Iron Age installations in the study area in future. For smelters 2 and 3, the TL data were analyzed using both mass and 110 (C TL peak normalization. The ages normalized to the TL peak were older than the
corresponding mass-normalized ages by 32 and 61 years, respectively, but these differences remained well within the standard deviation in each age. Smelter 1 was analyzed using mass normalization alone. Extrapolating from the results of the other two, it is reasonable to expect that its TL-normalized age would have been w46 years older, or 110262 years. Averaging the mass- and TL-normalized ages, the mean ages for the three features are: Smelter 1 Smelter 2 Smelter 3
1090 60 years 1600100 years 1080 70 years
These ages are consistent with the external appearance of the smelter remains. Smelter 1, by far the best preserved of the three, falls within the younger subgroup. Smelter 2, the oldest of the group by w500 years, was rather worse for wear. Birimi site sediments are derived locally, from a well sorted and heavily oxidized medium to coarse sandstone which forms the bedrock of the Gambaga Escarpment and is sporadically exposed in outcrops. The sedimentary column of the terrace is dominated throughout by fine quartz sand and silt. Sedimentological analysis with scanning electron microscopy indicates the terrace sediments accreted fluvially [44,45]. Thin horizons of nodular iron hydroxide are found at 57 and 136 cm below surface. These high concentrations of iron oxide, of up to 14% and 21% by weight, are also evident in major element analyses of the sedimentary column (Fig. 6). Using X-ray diffraction (Fig. 6), we determined that the nodules are composed of precipitated mineral goethite [FeO(OH)], which binds together the sedimentary quartz grains. It is possible that each nodule horizon represents the lower limit of a downward leaching zone or the upper limit of a reduced-state saturated zone; they would have formed during two much wetter
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Table 3 Element compositions of the sediment layers (S 57 cm, S 136 cm) containing goethite nodules, a cleaned goethite nodule, and a fragment of slag from smelter 1 Element
S 57 cm
S 136 cm
Goethite
Slag
SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Cl L.O.I. Total incl. LOI
(%) 62.52 0.83 12.97 13.10 0.02 0.16 0.08 0.00 0.35 0.05 0.00 8.90 98.99
(%) 51.93 0.81 16.00 20.56 0.09 0.19 0.23 0.03 0.30 0.06 0.03 9.40 99.63
(%) 14.55 0.17 3.38 68.47 0.08 0.08 0.04 0.10 0.05 0.09 0.01 12.40 99.43
(%) 26.05 0.32 5.54 68.22 0.11 0.15 0.16 0.09 0.03 0.13 0.03 3.20 104.02
Sc V Cr Co Zr Ba La Ce Nd Ni Cu Zn Ga Rb Sr Y Nb Sn Pb Th U
(ppm) 16 105 75 35 535 339 81 116 49
(ppm) 20 405 346 46 569 318 68 136 58
(ppm) 14 222 73 21 60 370 48 90 41 8 29 0 18 41 5 26 9 17 18 5 3
(ppm) 16 198 38 9 210 244 6 59 27 0 21 0 27 43 8 29 12 16 6 10 3
The SiO2 to Nd determinations were made on lithium borate fused glass disks, while Ni to U were measured on pressed powders. The analyses were carried out using a Philips PW2400 wavelength-dispersive X-ray spectrometer at the Regional Geochemical Centre, St. Mary’s University, Halifax.
periods in the region’s past history, possibly during the Pleistocene. Chemical analysis (Table 3) shows the clean goethite to be a very rich source of iron in comparison to the bulk sediment of the horizon containing the goethite nodules. The trace element composition of slag from smelter 1 confirms its genetic relationship to the goethite. Its major element composition is very similar to the fayalitic slags from Nsukka (for example, see samples Lej/2/90 and Lej/4/90 of Okafor [35, p. 443]). Based on these analyses, there is no doubt that the location of the smelting activities is directly related to the presence and easy access to this easily extractable
natural source of iron. In contrast to the eastern half of the site, where the terrace slope is laterally and vertically smooth (excluding obvious erosional gullies), the terrace scarp in the western half is severely disturbed, forming an irregular, hummocky and disorganized surface that slopes down to the streambed. Clearly, the iron oxides were being mined out of the terrace scarp, and smelted on top of the terrace. Because the sediments are uniformly fine textured whereas the goethite nodules are typically 0.5–1.5 cm in diameter, the ore would have been easily concentrated by sieving and washing. Thus, the riverbank location appears to have provided the raw ore, the direct means of pre-processing it, a level site for the iron smelters and, likely, clay for their construction. 8. Discussion Our investigations of Birimi suggest that the presence of Iron Age and Kintampo sites in close proximity to each other [23,24] may be spurious, in the sense that although both groups exploited some of the resources offered by the river-bank environment, they probably did not utilize the same resources. It is our belief that the smelters were located so as to take advantage of the goethite deposit readily available in the terrace bank. The Kintampo people, on the other hand, surely had no interest in the goethite, but were likely interested in the ready availability of water and natural resources associated with it. The fact that smelting sites are not associated with their makers’ habitation sites, which typically are located well away from streams, offers support for the premise that different resources were of interest to each of these cultures. A luminescence and radiocarbon chronology has been developed for the Kintampo complex in east Birimi [16,44,45]. Presenting and discussing fully the entire set of these dates is beyond the scope of this paper, however, we note here that we have 30 independent absolute dates from east Birimi, which we can compare to the three smelter ages from west Birimi. The youngest TL age on a Kintampo ceramic sherd is 2270130 years [44], while the youngest TL age on a fragment of burnt house daub is 2640160 years ([16]; manuscript in preparation). Four radiocarbon ages on charcoal fragments collected by one of us (DIGS) from two Kintampo features have a two standard deviation range of 3260–3960 calBP [45]. Two additional radiocarbon ages on single millet seeds from a Kintampo pit feature span the range 2180–4290 calBP, also at two standard deviations of their calibrated ranges [9]. The ceramic and daub TL chronologies overlap fully with the radiocarbon chronology, yet a comparison of the age ranges at the 95% confidence interval leaves a gap of 210 years between the Kintampo complex and the earliest iron smelting activity at the site (Fig. 7). Thus, despite the fact that TL and radiocarbon ages at east Birimi are
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Fig. 7. Comparison of Birimi Kintampo complex and iron smelting ages. Hatched bars show 14C dates, shaded bars TL dates. The uncalibrated 14C ages are shown numerically. The burnt house daub chronology extends beyond 4700 years. The short vertical lines through the burnt house daub and Kintampo ceramics TL ranges show the position of the youngest TL date in each set. Ranges of 2 are shown throughout. The radiocarbon 2 calBP ranges were obtained using the calibration curve of Stuiver et al. [55]. The vertical gray bar marks the position of a 210 year gap (at 95% confidence) between the Kintampo occupation and the Iron Age at Birimi.
wholly consistent with each other, our chronology offers no support for suggestions made by Davies [11,12], or Shinnie and Kense [53], among others, that the Early Iron Age in Ghana is a continuous development of the Kintampo complex. On the contrary, the Kintampo and Iron Age components at Birimi fail to overlap even at the 99% confidence interval (three standard deviations) of the available chronological data. The smelter TL ages presented here offer important and independent support for the presence of Early Iron Age in northern Ghana, as suggested by absolute dates at other sites ([3,13,42,53,56,63] and others). Still, we believe that anthropogenic site disturbance may be responsible for a number of spurious associations of iron artifacts and very early radiocarbon ages. Extensive and deep pits uncovered in the excavations in east Birimi contained mixed deposits that included artifacts diagnostic of both Kintampo and Middle Stone Age toolkits. As there is reason to believe that Iron Age people were at least as adept at disturbing the soil, we urge extreme caution in interpreting the on-site presence of iron and Kintampo (or Kintampo-like) ceramics and other artifacts as contemporaneous. For this reason, investigations into the chronology of iron smelting technology should focus on materials directly associated
with the activity, such as the remains of smelters or the slags themselves. 9. Conclusions The TL dates presented here demonstrate a fullblown smelting technology in Northern Ghana during the Early Iron Age. Iron smelting at Birimi was established 1600 years ago, and continued until w1080 years ago. This time period predates any previous direct ages for iron smelting on the Gambaga Escarpment by 500–1000 years, and demonstrates that this technology flourished in the Northern Region of Ghana centuries earlier than previously suspected. The appearance of smelting technology at Birimi post-dates by 670 years the latest evidence for the Kintampo complex at the site. Further chronological work is needed to determine if Early to Late Iron Age iron production on the Gambaga Escarpment was episodic or continuous. Based on these results, we feel that the methods chosen for chronological investigations of the Iron Age in future should focus more on materials directly associated with this industrial process. One of such methods is luminescence dating. Optical dating (OSL), although originally developed for the dating of
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unheated sediments [22] can equally be applied to date heated materials, since the OSL signal is completely erased by heating to a high temperature. The recently developed SAR technique [34] offers an optical dating alternative to the classical TL method. Luminescence dating (TL and OSL) may be applied not only to smelters themselves, such as we have done here, but also to other smelting paraphenelia, such as fragments of tuyeres, slag-coated pottery sherds, and many other remains clearly associated with smelting activity. Because the products of smelting (slag and bloom) retain temperatures in excess of 1000 (C during processing and for some time afterwards, we suggest that luminescence may even be applied to the surfaces of hollowed out depressions in the soil into which slag had been directed while still molten. At these temperatures and volumes of slag [35,47] sufficient heat will have been transferred to the underlying earth to have baked a layer at least a few centimeter thick to a temperature in excess of 500 (C. This temperature is sufficient to completely erase any pre-existing luminescence signal in the minerals of interest, and allow the heating event to be dated. We postulate therefore that luminescence dating may be applied to chambers for slag collection, provided that the in situ association of slag with the underlying sediment can clearly be demonstrated. Such proof may take the form of reddened and hardened crust of earth directly underlying the slag, matching cast-and-mould shapes of slag and earth depression, or adhesion of the chamber’s upper surface to the slag. The additional benefit of dating smelting activity by luminescence is that the old wood effect, a potentially confounding and difficult to assess accurately issue inherent in the radiocarbon dating of smelter residues and slags, is avoided [32]. Indeed, with careful selection of material unquestionably associated with smelting activity, luminescence dating may be useful in confirming the very early dates for iron metallurgy in Nigeria, and finally validating the existing radiocarbon chronology. Since smelter remains and associated paraphenelia represent a closed physical system with respect to the last firing event even if the site suffers some disturbance, questions based on suspicion of recent carbon contamination or raised due to doubtful contextual grounds (see, for example, Ref. [33, p. 466]), may also be resolved by this method.
Acknowledgements The TOSL Laboratory is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through equipment and research grants to D.I.G.S. The excavations at Birimi were supported by a Social Sciences and Humanities Research Council of Canada (SSHRC) research grant to J.L.C., while the
archaeometric studies were supported through a SSHRC research grant to D.I.G.S. Technical assistance of Mrs P. Scallion and Mr K.B. Vaughan in the TOSL Laboratory is warmly acknowledged.
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