Mineral-soda-alumina (m-Na-Al) glass
Fifty-four (68.4 %) of the 79 tested Zanzibar beads are made of mineral-soda-alumina glass (Online Resources 1 and 4). Dussubieux et al. (2010) have identified five different subtypes of m-Na-Al glass in samples dating from about the fifth century BCE to the nineteenth century CE. 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. 2008), but that attribution may be questionable as will be shown.
Dussubieux et al. (2010) differentiated the m-Na-Al 1 and 2 subtypes on the basis of the concentrations of four characterizing elements: Sr, Zr, Ba, and U. 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. (2010) m-Na-Al 1 and 2 subtypes, we used compositional data, kindly provided by Dussubieux. This included beads from South and West India as well as Kenya. Our Sri Lankan data came from Dussubieux (2001). 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 Sri Lankan data is from Giribawa, a third-century BCE to second-century CE glass- and beadmaking center.
When subjected to principal component analysis (PCA), the compositions of these beads (expressed in ppm of the elements and then standardized) formed two clusters representative of the compositional features of most of the samples of each m-Na-Al subtype (see blue and pink ovals of Fig. 4). The majority of the Zanzibari, Sri Lankan, and South Indian beads form a fairly compact group that suggests they are closely related, but the numerous outliers from this core suggest that there may be other subtypes that have not yet been identified. The overlapping between the two subtypes is a consequence of samples that lie between the concentration ranges as reported by Dussubieux et al. (2010).
The PCA extracted five principal components, of which only the first two (PC1 and PC2) were considered significant on the basis of the scree plot. These explain about 70 % of the variance. Most of the Zanzibar beads (red points of Fig. 4) form a compact cluster within the area of Dussubieux et al.’s m-Na-Al 1 subtype, while the samples reported in Table 2 appear external to or borderline to this group. In particular, samples UU003, UU097, UU144, and UU163 fall in the area of overlap between the two subtypes. The analysis of variable loading (Fig. 4 inset) shows that along PC1 uranium and the other four elements as a group are inversely correlated: the former (with highly negative loading) essentially favors the m-Na-Al 2 cluster, while the latter load on the m-Na-Al 1 side. The variable loadings of the second PCA component show that the positions of the single samples reported in Table 2 (except UU003, 097, 144, and 163) are due mainly to the loading of the Al, Sr, and U variables.
The blue and pink ovals of Fig. 4 statistically circumscribe about 70 % of the m-Na-Al 1 and m-Na-Al 2 samples on the basis of the Dussubieux data. Therefore, the borderline samples (UU013, 057, 059, 212, 218, and 228) and those just outside the border (UU204, 209, and 213) may be part of the m-Na-Al 1 subtype. 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. 4) indicates the contradictory or uncertain assignments of these samples when the data for each element are considered separately. This underscores the limits of the Dussubieux et al. (2010) criteria when attempting to determine to which subtype some individual samples belong. 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 (2015, personal communication).
Unfortunately, due to their similarity, beads of the two m-Na-Al subtypes cannot be separated based on morphology. It is interesting, however, that the majority of the chemically unusual beads that do not fit comfortably in either category come from disturbed contexts. As Table 3 shows, 8 of the 13 beads were found in trench 10 contexts 005, 006, and 009—all of these are from the fill of a pit dug into the trench.
Glass of the m-Na-Al 1 subtype is known from Sri Lankan, South Indian, and Southeast Asian sites dating between about the fifth century BCE and the tenth century CE (Dussubieux et al. 2009, p. 159, 2010; Dussubieux 2001; Dussubieux and Gratuze 2013). Archaeological and associated chemical evidence indicates that one manufacturing center for this glass was located at Giribawa in Sri Lanka (Bopearachchi 1999, 2002; Dussubieux 2001), 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. No other sites manufacturing this glass type have been located, but there likely were other such glassmaking centers in Sri Lanka; for example, Francis (2013) concluded that Mantai was both a glassmaking and beadmaking center. However, as Dussubieux and Gratuze (2013, p. 404) note, South India may have produced m-Na-Al 1 glass as well. The orange beads made of this glass (a rather bright pumpkin orange [Munsell 5YR 5/10]) are potentially significant markers in that this is the first time they have been recorded in eastern Africa, although they do occur in the late occupation phase (fourth to sixth centuries CE) at Berenike, a Red Sea port in Egypt (Francis 2002, p. 228, note 21). They were common at Mantai where Francis (2002) 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. 2008, p. 814) and the other from Mahilaka in northwest Madagascar (Robertshaw et al. 2006).
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. 2008; 2010; Robertshaw et al. 2003, 2006, 2010b). 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 2013, p. 404) but no details of the contexts in which the beads were found are provided. 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 2001a, b; Kanungo 2004; Sode and Kock 2001). It appears unlikely that any of the Zanzibar beads were made from this glass type. As Table 2 and Fig. 4 demonstrate, none of the beads fit unequivocally into that type’s parameters. 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. 2010b). 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. 2009, p. 159, 2010; Dussubieux 2001; Dussubieux and Gratuze 2013), while in southern Africa, no m-Na-Al glass beads have been recorded that predate the tenth century (Robertshaw et al. 2010b; Wood 2011).
Plant ash soda glasses
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.
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 (UU225), 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 (UU225). Two outliers, UU091 and UU173 with their higher soda concentrations and mostly lower concentrations of other major oxides (as well as trace element concentration differences), do not fit any of the identified groups; these are discussed in the section “The outliers” below.
The distinction between the proposed subtypes is tentative. Lime and phosphorus pentoxide contents show quite good separation between the subtypes (Fig. 5), but samples cannot be easily assigned to a subtype based on the quantities of numerous other elements, though some elements, such as chromium and boron, appear more promising as discriminants (e.g., Fig. 6).
v-Na-Ca subtype A
Subtype A includes 12 beads, the blue and white glasses of the eye bead (UU225) and the glass weight but produced 17 analyses because three beads are multicolored and each color was tested; the multicolored beads are discussed more fully below. Colors that are found in subtype A beads include cobalt blue, blue-green colored with copper, white colored with tin and one colorless sample (UU114) with a notable, if predictable, absence of any coloring agent. Four of the seven cobalt blue beads, including the two striped beads, have been reheated on a flat surface (see “Bead types in the overall 2011 Zanzibar assemblages” for a description and Online Resource 1 for details). Among the blue-green beads, UU090 was pinched from a large tube with a large perforation and then reheated on a flat surface. FK017 is similar and may have been produced in the same manner. The remaining two, UU112 and UU235, are smaller but still have large perforations. Their walls are very thin so the ends have deteriorated making it impossible to determine whether they were reheated at all. The colorless bead (UU114) is uniformly rounded on both ends. The remaining v-Na-Ca subtype A samples are the white and blue “eyes” within the eye bead which is the only wound bead in the Zanzibar assemblages; it is described in the following section.
In the Zanzibar assemblages, cobalt blue glass is found only in beads (and the weight) made of v-Na-Ca glass and specifically only within subtype A; those with cobalt concentrations of over 1000 ppm (n = 9) will be discussed here. Cobalt in glass is often associated with various elements incorporated in the raw cobalt-bearing materials. Gratuze et al. (1992) note that cobalt can come from cobaltite (CoAsS), skutterudite (Co,Ni) As3-x, or trieuite (2Co2O·CuO·6H2O), or it can be associated with Fe, Ni, and As or Pb, Zn, In, etc. One of the three main cobalt mineral groups identified by Gratuze et al. is cobalt-zinc, in which these two elements are correlated with lead and indium. The cobalt in the Zanzibar beads is correlated with zinc and copper but not with other elements; they do include variable amounts of lead (0.1 to 5 wt%) and iron (0.9 to 2.1 wt%) but these are not correlated with the cobalt, and our indium levels are unfortunately not reliable due to the interferences between 118Sn and 115In isotopes in the ICP-MS system. Figure 7 shows that there is a strong Co-Zn linear correlation for all of our blue samples except UU225 (the eye bead), which has a Co/Zn ratio of 0.8 compared to the other samples with a ratio of 3. This different glass is the blue “pupil” of the eye bead, and given that the morphology and construction technology of this bead is so different from all the others, it might not be surprising that a glass colored with cobalt from a different source was used in its manufacture. Unfortunately, there is a dearth of information on the chemistry of cobalt sources in the Middle East during the early Islamic period.
Unguja Ukuu produced three multicolored beads, all from trench 10 below the rubbish pit mentioned earlier (section “m-Na-Al 2”). Two of these beads are made of v-Na-Ca subtype A glass, while the third (UU225) consists of a base glass assigned here to v-Na-Ca subtype B glass and eyes of blue and white glass that appear to be of subtype A. The fact that both v-Na-Ca subtypes appear to have been used in the manufacture of a single bead highlights the tentative nature of the identification of two subtypes; however, it has been shown that glass workshops in the early Islamic period sometimes used raw glass from different primary sources (Freestone et al. 2002, p. 270; Robertshaw et al. 2010b: Table 7), so it is well within the bounds of possibility that two different (sub)types of glass could have been used in the manufacture of a single multicolored bead.
Studies of these multicolored beads and others like them from sites in Sri Lanka, Thailand, and Scandinavia have enabled their manufacture to be placed in a time frame roughly between the late eighth and mid-ninth centuries (Wood, unpublished data). Two of these (UU231 and UU232) are cobalt blue with white stripes that run parallel to the perforation. The third bead (UU225) is known as a stratified eye bead (see Fig. 17 for images). It is wound (all others in the entire Zanzibar assemblage are drawn) with a black body and eyes made of a circle of white glass topped by a smaller one of cobalt blue. Eye beads are fairly common with a very long history, but the type discussed here is specific enough to have been named the “Takua Pa eye bead” after an eighth- to eleventh-century archaeological site (sometimes called Takua Pa but actually named Thung Tuk) on the Andaman coast of peninsular Thailand (Chaisuwan 2011) where a large number were found and where Francis (2002, p. 97) proposed they may have been made. We compared the chemistry of UU225 with three other “Takua Pa” eye beads from Thung Tuk (data courtesy of Jim Lankton), an eye bead from al-Basra in Morocco (PR1019FM; Robertshaw et al. 2010a) and two eye beads from Chibuene in Mozambique (CHB0045 and CHB0047; Wood et al. 2012). All of these sites have eighth- to ninth-century components.
The al-Basra and Chibuene eye beads are morphologically unlike the “Takua Pa” eye beads but all are made of v-Na-Ca glasses, as is UU225. As Fig. 8 demonstrates, UU225 is very similar in terms of its major oxide concentrations and ratios to the Thung Tuk beads, being characterized by a high combined concentration of magnesia and potash and a low alumina to lime ratio. By contrast, the Chibuene eye beads differ in their alumina to lime ratios, while the al-Basra bead has less magnesia and potash than the Zanzibar and Thung Tuk specimens. The distinction between the Zanzibar eye bead and those from Thung Tuk on the one hand and the al-Basra eye bead on the other seems to be borne out by examination of the apparent coloring agents in the white glasses of these beads as well as of the blue and white striped beads from Unguja Ukuu and Tumbe, discussed next. The al-Basra white glasses, but not the others, are notable for their relatively high concentrations of lead and arsenic, as well as tin (Table 4), suggesting that the white color in the al-Basra glass might derive from a mixture of cassiterite and lead arsenate, whereas the others, including the Chibuene eye beads, seem to have been colored primarily with cassiterite. It is perhaps worth noting that the lead/arsenic ratios of the al-Basra white glasses are very different from those of the other white glasses discussed here. Moreover, with the exception of al-Basra, the quantities of arsenic, tin, and lead in the white glasses from the other sites are not correlated.
The two striped beads from Unguja Ukuu (UU231 and UU232) are morphologically very similar to one (PR788) from the site of Tumbe on the north end of Pemba Island in Tanzania (Fleisher and LaViolette 2013). Similar beads are also present at Thung Tuk. All these beads are also chemically very similar, with the exception of the blue glass of the Tumbe bead (Table 5 and Fig. 9), but this particular glass is clearly corroded, as is evident from both its high silica and low soda levels. In all the beads, the blue color can be ascribed to additive levels of both copper and cobalt, while the white glass exhibits high levels of tin, corresponding to tin oxide used as a white opacifier. The ratios of lead and tin to arsenic are strikingly similar in the white glasses of UU232 and the Tumbe bead (Table 4). Although all of these sites have eighth- to ninth-century components, this type of striped bead may also be present outside this time frame.
v-Na-Ca subtype B
Of the eight subtype B beads, five are an opaque brick-red glass that is filled with bubbles, giving the ends a sponge-like appearance (see Fig. 17 FK019a and b): the three from Fukuchani (FK019a, b, and c) are medium sized (5.5 to 6 mm diameter), while the two from Unguja Ukuu (UU007 and UU115) are large (9.6 diameter) and appear to have been pinched from a glass tube rather than cut from tubes like the smaller beads. All were reheated on a flat surface to slightly round the cut ends which is an unusual treatment (see section “Bead types in the overall 2011 Zanzibar assemblages”). Two subtype B beads are translucent blue-green and large (the more complete one measures 11 mm in diameter), and they were made by a process known as segmenting. Both the brick-red and blue-green subtype B beads were colored with high concentrations of copper. The final bead of this subtype comprises the black glass that forms the base of the eye bead (UU225) discussed above.
The origins of the increased concentrations of CaO in the subtype B glass are not entirely clear. Henderson (2013, p. 284) reports that most lime in plant ash glass comes from the ash itself. However, the lime content of some halophytic plants was insufficient to make Islamic glass, so a third primary raw material, such as feldspar which is rich in both calcium and alumina, could have been added along with the sand which, especially with desert and stream sands, is often low in calcium (Duckworth et al. 2015). Unfortunately, our data cannot differentiate between potential calcium sources. The mean ratios between the lime, alumina, and phosphorus oxide contents in subtypes A and B are similar, but other oxides that characterize the ashes, such as sodium, potassium, and magnesium, are rather different. In the end, these differences cannot account for the differences in lime levels between the two subtypes, thus suggesting at the very least the use of ashes from different plants.
Comparison of v-Na-Ca subtypes to related datasets
African comparisons: There are two datasets comprising chemical compositions of beads of plant ash glass from the late first millennium CE that are appropriate for comparison with the Zanzibar v-Na-Ca subtypes. The first comprises the plant ash glass beads from the port of Chibuene in Mozambique where three subtypes of plant ash glass were identified (Wood et al. 2012). The first of these, Chibuene “v-Na 1” glass, is represented by beads, vessel shards, blobs, and cullet, with the beads identified as belonging to the Zhizo series (Wood 2011). Chibuene “v-Na 2” glass, with relatively high concentrations of chromium, nickel, and zirconium, consists of green vessel shards, cullet, and blobs but no beads, while Chibuene “v-Na 3” glass is represented only by beads which were assigned to a new morphological type, the Chibuene series (Wood et al. 2012). The second comparative African dataset used here comprises the results of chemical analysis of 16 beads of the Zhizo series from various southern African sites (Robertshaw et al. 2010b). These Zhizo series beads are chemically very similar to those of the Chibuene v-Na 1 series. It may be noted that glass beads with chemistries very similar to the Chibuene v-Na 1 and Zhizo series beads have also been recovered from several West African sites, including Igbo-Ukwu in Nigeria (Robertshaw, unpublished data), Gao in Mali (Cissé 2011; Cissé et al. 2013), and Kissi in Burkina Faso (Robertshaw et al. 2009).
Table 6 presents the mean compositions of the major oxides and selected trace elements for the Zanzibar plant ash subtypes and the African comparative dataset. Perusal of this table and Fig. 10 confirms the distinctively different composition of the Chibuene v-Na 3 glass (Chibuene bead series), as reflected, for example, in the quantities of lime, potash, magnesia, chromium, rubidium, barium, and uranium. The second most distinctive glass is the Zanzibar subtype B glass with its high lime levels, as well as somewhat elevated levels of soda and chromium, though, as noted above, lime aside, it shows considerable compositional similarities to Zanzibar subtype A, as well as to the Chibuene v-Na 1 and Zhizo glasses. Chibuene v-Na 2 glass is also not entirely dissimilar to the other types, at least when the quantities of individual oxides are compared, though it has notably higher amounts of chromium, nickel, and zirconium and less rubidium. Chromium and nickel levels in the Chibuene v-Na 2 glass are highly correlated (Wood et al. 2012, p. 64), as they are as well in Zanzibar subtype B glass (r = 0.855, p = 0.007) but not in Zanzibar subtype A (r = 0.324, p = 0.204), though the overall quantities of these elements are generally higher in the Chibuene v-Na 2 glass.
Middle Eastern comparisons: Plant ash glasses found in eastern and southern Africa that are dated to the late first millennium AD and contain less than about 4 % alumina are widely considered to have been manufactured in the Middle East (Robertshaw et al. 2010b; Wood et al. 2012), though the beads found on Zanzibar that were made of Middle Eastern glass may well have been manufactured outside the Middle East (see discussion in section “East Africa”). The question here is whether we can source either of the Zanzibar plant ash glass subtypes to any particular region within the Middle East. Henderson (2013, pp. 290–300) has investigated the complex variation in glass compositions across the Middle East, noting the inevitable complexities that result where raw materials, as well as raw and scrap glass were widely traded, as was the case here, but nevertheless noting some potential avenues of inquiry.
Al-Raqqa in Syria is the only primary plant ash glass manufacturing site of the late first millennium AD that has been thoroughly investigated and for which considerable compositional data are available (Henderson et al. 2004). Henderson and colleagues identified three plant ash glass types (types 1, 2, and 4) at al-Raqqa, with type 4 glasses varying considerably in composition because they were perhaps the products of experiments in the production process. Figures 11 and 12 compare some of the major oxide compositions of the Zanzibar plant ash subtypes with the al-Raqqa types. The Zanzibar plant ash glasses fall within the broad compositional range of al-Raqqa type 4 glasses based on their alumina and magnesia compositions (Fig. 11), but their potash and lime concentrations tell a different story, with the Zanzibar subtype A glasses aligning with al-Raqqa type 1, rather than type 4, because of their high lime concentrations, though subtype A still broadly matches al-Raqqa type 4 (Fig. 12). Furthermore, a plot of lime and alumina concentrations in raw furnace glass from al-Raqqa (Henderson 2013, p. 285) shows that this glass is characterized by >6 % lime and <2 % alumina, which makes it a poor match for the Zanzibar plant ash glasses.
Looking further afield in the Middle East than al-Raqqa, Henderson (2013, pp. 294–295) has drawn attention to possible broader geographical patterns in the major oxide compositions of Islamic glass across the Middle East. Given that high lime concentrations are what defines our Zanzibar subtype B glass, it is noteworthy that comparable high lime glasses include the raw glass from the eleventh-century Serçe Limani shipwreck (Brill 2009), eighth–eleventh-century glass from Ramla in Israel (Freestone et al. 2000), glass from furnaces on the island of Tyre (Freestone et al. 2002), and glass from Fustat in Egypt (Brill 1999). Henderson (2013, p. 294) argues that the high lime concentrations in these glasses derive from shell fragments in beach sands, with the Levantine coast being the most likely source. This encourages us to suggest that the Zanzibar plant ash subtype B glasses most likely derive from the same Levantine region, though it is important to note that the Zanzibar subtype B glasses generally possess more alumina than do their Levantine counterparts, suggesting the use of sand rather than quartz as the silica source. We should also remember that the Levantine samples derive from raw glass and vessel glass rather than colored beads.
It is tempting to look eastwards, to Iraq and Iran, for an origin for the Zanzibar subtype A glass. Figures 13 and 14 compare the Zanzibar plant ash subtypes with a variety of Iranian and Iraqi glasses of similar age or, in the case of the Sasanian glass, earlier. While the high lime and alumina concentrations of most of the Zanzibar subtype B glasses show that they are not a good match with these eastern Middle Eastern glasses, those of Zanzibar subtype A are a better fit, particularly perhaps with the Sasanian type 1 glasses. Freestone (2006) noted that glasses found to the east of the Euphrates have higher potash and magnesia concentrations than those to the west (see also Rehren and Freestone 2015: Fig. 5), providing further support for an eastern origin for the Zanzibar subtype A glasses.
In summary, we suggest that the Zanzibar plant ash subtype A glasses may have been made east of the Euphrates while those of subtype B more likely originate in the Levantine region. These ascriptions are tentative. More reliable sourcing of the glasses within the Middle East will require much more detailed investigation of both trace element and isotopic data.
The two beads that are outliers (UU091 and UU173), which are made from v-Na-Ca glass but with rather different chemistries, remain to be discussed. UU173 is blue-green, colored with copper, while UU091 is blue but not dark cobalt blue like the other blues—it contains both copper and cobalt with a Cu/Co ratio of about 3 which results in a blue tending toward green tones rather than purple ones.
UU091 is, in terms of its major oxides, broadly comparable to Zanzibar subtype B but includes more soda and less of the other oxides. It also exhibits some differences in trace element concentrations from the Zanzibar subtype B glasses, for example, the vanadium, zinc, silver, tin, and barium concentrations. UU091 also possesses a very distinct rare earth element (REE) profile (Fig. 15), which distinguishes it from other Middle Eastern glasses. This REE pattern, with a high enrichment of heavy REEs, is typical of cobalt blue glasses derived from cobalt alum sources, as found, for example, in Late Bronze Age Egypt (Shortland et al. 2007).
UU173 has an unusual major oxide chemistry with low levels of both alumina and lime, which align the bead with the glass of the Chibuene series (Wood et al. 2012). However, the trace element concentrations in UU173 show both similarities to and marked differences from those of that series, with, for example, similar concentrations of lime and zirconium but different quantities of alumina, chromium, and rubidium (Fig. 15; Table 7), presumably indicating closely related but not identical glasses. Wood et al. (2012, p. 66) remarked that the chemistry of the Chibuene series beads is “strikingly different from any of the assemblages in the Middle East,” a conclusion that we can only echo for UU173.
The three plant ash glass beads with high alumina concentrations (v-Na-Al) were all found at Unguja Ukuu: UU001 is yellow, UU002 blue-green, and UU227 green. The differences between this v-Na-Al glass and the other plant ash glasses from Zanzibar are demonstrated in their REE profiles (Fig. 16). Subtypes A and B derive from very similar bedrock geology, whereas v-Na-Al is markedly different, notably with its large positive Eu anomaly, its lack of a negative Ce anomaly, and its overall higher concentrations of REE.
Two of these v-Na-Al beads came from trench 11 in contexts 001 and 002, which are made up of overburden mixed with modern intrusions, while the third, UU227, is from trench 13 context 006, which is over half way down the trench depth. These beads have a chemistry that is somewhat related to that of the Mapungubwe Oblate and Zimbabwe series beads which are found at thirteenth- to fifteenth-century sites in southern Africa and Madagascar (Robertshaw et al. 2006, 2010b), as well as to that of vessel glass from Mtwapa on the southern Kenyan coast (Dussubieux and Kusimba 2012), but they all vary enough to make direct correlations difficult at this point (Dussubieux, 2015, personal communication). Thus, based on our present knowledge of this glass type, these Zanzibar beads may represent an as yet unclassified variation of this glass type.