Archaeological and Anthropological Sciences. A selected sample of the beads recovered was analyzed by laser ablation-inductively coupled plasma-mass spectrometry LA-ICP-MS to determine the origins of the glass used to make the beads and potential trade relationships are considered. The data show that two major glass types can be identified: mineral-soda glass, m-Na-Al, produced in Sri Lanka and possibly South India and plant ash soda glass. The latter comprises three subtypes: two with low alumina concentrations and different quantities of lime here designated v-Na-Ca subtypes A and B and one with high alumina designated v-Na-Al.
The v-Na-Ca subtype A beads are chemically similar to Sasanian type 1 glass as well as Zhizo beads found in southern Africa, while v-Na-Ca subtype B compares reasonably well with glasses from Syria and the Levant. In contrast, during this period, all beads imported into southern Africa were made of Middle Eastern glass from east of the Euphrates v-Na-Ca subtype A and appear to have arrived on ships from Oman and the Persian Gulf.
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These data suggest that the two sections of the African coast were engaged in different Indian Ocean trade circuits. Most of the beads recovered are not morphologically distinct; they are small and monochrome and are often types that were made over the span of several thousand years. We perform elemental analyses to trace the origins of the different glasses used to produce the beads. Since glass was widely traded in antiquity, however, we also examine the method by which the beads were made.
We draw on these datasets to begin to reconstruct the trade routes that brought the beads to the African shore.
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We also compare the Zanzibar bead assemblages and the trade connections they inform with those in southern Africa during the same period to attempt to discern whether or not the two ends of the eastern African coast were involved in overlapping trade circuits. Unguja Ukuu is located on a narrow coral-rag peninsula between a resource-rich bay and a small, shallow inlet. Fukuchani is situated on a long beach protected by the island of Tumbatu, which sits directly opposite the bay. Our excavations targeted areas where subsurface deposits appeared intact.
The artifacts analyzed in this study derive from four trenches at Unguja Ukuu trenches 10—13 and three at Fukuchani trenches 10— The glass beads were found throughout the full depositional sequence at both sites in association with quantities of other imported goods, including Chinese and Near Eastern ceramics e. A suite of radiocarbon dates obtained from both sites support this chronology Crowther et al. Trench 11 at Fukuchani primarily contained a human burial. The selection of the beads for chemical analysis was not random but rather a range of each type and color was chosen to try to include all possible glass types present.
Thus, 68 beads from Unguja Ukuu and 11 beads from Fukuchani were chosen for analysis. A small blue cuboidal glass object from Unguja Ukuu UU measuring 5.
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Five hundred shots were fired per spot. The raw ICP-MS data acquired in counts per second during the last shots were averaged for each of the 53 isotopes. Subsequently, the raw ICP-MS data were subjected to quantification with the so-called sum normalization calibration method previously described van Elteren et al. Glass standards from the Corning Museum of Glass CMG , which mimic ancient compositions, are the most suitable for accurate quantification of ancient glasses, especially for elements not present at trace levels, such as in lead glasses.
Since ancient glass samples often show signs of surface degradation, the laser was used in drilling mode to enable the measurement of the actual elemental bulk composition underlying any degraded layer. Most of the samples presented leached and corroded surfaces with an enrichment of magnesia, potash, lime, or alumina compared to the pristine glass, due to the precipitation of mineral phases from the soil onto the glass surface. Online Resource 2 presents all the data from the analysis of 80 artifacts but including 84 analyses since several beads are multicolored and each color was tested comprising the contents of 52 elemental oxides.
Online Resource 3 presents reduced compositions of all samples based on the method of Brill , pp. Online Resource 4 presents mean concentrations and standard deviations of all oxides by glass type and color, and Online Resource 1 provides morphological characteristics of all samples, their site contexts, and glass type.
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Plot of lime vs alumina for all analyzed Zanzibar beads reduced compositions calculated using the method of Panighello et al. Fifty-four Dussubieux et al.
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Two of these m Na-Al 1 and m-Na-Al 2 are found at sites in Africa, though the time span during which this glass occurred at sites sampled to date within Africa is more restricted. One of these glass subtypes, m-Na-Al 1, is present in the Zanzibar beads along with a few outliers that do not comfortably fit any known subtype.
The m-Na-Al 1 glass was formerly known as low uranium-high barium or lU-hBa glass. At first, we thought it possible that some of the outliers belonged to the m-Na-Al 2 subtype, a high uranium-low barium glass formerly known as hU-lBa Dussubieux et al.
These four elements were probably chosen because they were the only elements which presented an acceptable separation between the concentration ranges in m-Na-Al 1 and 2 glasses, even though the first three of these elements partially overlap. An analysis of the whole dataset, however, indicates that alumina may also be useful in separating the two subtypes, so here we include it and thus employ five elements—Sr, Zr, Ba, U, and Al—to characterize and compare the m-Na-Al Zanzibar beads. In comparing the Zanzibar m-Na-Al glasses to the compositions of Dussubieux et al.
This included beads from South and West India as well as Kenya. Our Sri Lankan data came from Dussubieux The source of the South Indian data included two archaeological sites that appear to predate the current era.
The West Indian data is from samples ranging in date from the ninth to nineteenth centuries CE obtained at Chaul, a port site south of present-day Mumbai. The African samples are from four sites in Kenya. One of these has dates that span from the tenth to eighteenth centuries CE, and the others postdate the tenth century and cluster between the thirteenth and sixteenth centuries.
The black oval delineates samples from Sri Lanka, while the gray dashed-dotted rectangle outlines the South Indian ones. The red dashed line includes all of the Zanzibar samples. The blue circle delineates the Dussubieux et al.
Individual Zanzibar beads that do not fall comfortably within the parameters of either m-Na-Al 1 or 2 are identified by ID number and their relevant element concentrations and affinities are listed in Table 2. The blue and pink ovals of Fig. Therefore, the borderline samples UU, , , , , and and those just outside the border UU, , and may be part of the m-Na-Al 1 subtype.
Etude microstructurelle du transport par charriage de mélanges bidisperses à forte pente
As Table 2 illustrates, none of the beads in the area of overlap between the two subtypes fit unequivocally into the m-Na-Al 2 category. Finally, although the PCA analysis represents a statistical synthesis of the concentration distributions of five elements, a detailed analysis of the data for the samples individually reported in Table 2 and illustrated in Fig. This underscores the limits of the Dussubieux et al. In concurrence with this observation, new research by Dussubieux on other Zanzibar beads has confirmed the difficulty of assigning beads of m-Na-Al glass from the East African coastal region to only subtypes 1 and 2 , personal communication.
Archaeological and associated chemical evidence indicates that one manufacturing center for this glass was located at Giribawa in Sri Lanka Bopearachchi , ; Dussubieux , but that site is dated between the third century BCE and the second century CE and is thus too early to have been the source of the Zanzibar glass.
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No other sites manufacturing this glass type have been located, but there likely were other such glassmaking centers in Sri Lanka; for example, Francis concluded that Mantai was both a glassmaking and beadmaking center. However, as Dussubieux and Gratuze , p. They were common at Mantai where Francis proposed they were made. It is noteworthy that this is the first time m-Na-Al 1 glass has been recorded in eastern or southern Africa, apart from two morphologically unusual beads: one from Ungwana on the Kenyan coast Dussubieux et al.
Glass of the m-Na-Al 2 subtype is widely distributed in eastern and southern African sites, as well as at sites in Madagascar and India and beyond, dating from about the mid-tenth to seventeenth centuries CE Dussubieux et al. In East Africa, some reports indicate that this glass type appears in sites that have date ranges that include the ninth century Dussubieux and Gratuze , p. In addition, the earlier East African dating evidence is probably less secure than that for southern Africa.
All available evidence indicates that m-Na-Al 2 glass was probably manufactured at a number of locations in India widely distributed from the Uttar Pradesh region southwards from a mineral soda known, at least in recent times, as reh Brill a , b ; Kanungo ; Sode and Kock It appears unlikely that any of the Zanzibar beads were made from this glass type. As Table 2 and Fig. In addition, the AMS dates for the trenches in which potential examples were recovered predate the tenth century, whereas this glass type has not been recorded earlier than the tenth century. The most important conclusion that can be drawn from the analysis of the m-Na-Al beads found in Zanzibar is the pronounced contrast between the prevalence of subtype 1 and probable absence of subtype 2 of this glass in the assemblage and the fact that virtually all the m-Na-Al glass from southern Africa belongs to subtype 2 Robertshaw et al.
These differences can be explained by temporal parameters: subtype 1 glass, as has been mentioned, was produced up to but not beyond the tenth century CE Dussubieux et al. Two distinct types of glass were identified among the 26 samples fluxed with plant ash. The first type v-Na-Ca comprises 23 samples including the presumed glass weight that can be tentatively divided into two subtypes and two outliers, while the second v-Na-Al , which contains elevated levels of alumina, is represented by only three beads; these will be discussed separately.
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All of the 23 v-Na-Ca samples have relatively small quantities of alumina. Twelve of the beads, along with the blue and white glasses of the eye bead UU , and the glass weight, form a subtype, here designated A, characterized by medium concentrations of lime, while subtype B, characterized by higher concentrations of lime, is represented by seven beads plus the black base glass of the eye bead UU Plot of lime vs phosphorus pentoxide for the v-Na-Ca A and B samples.