New light on plant ash glass found in Africa: Evidence for Indian Ocean Silk Road trade using major, minor, trace element and lead isotope analysis of glass from the 15th—16th century AD from Malindi and Mambrui, Kenya

Abstract

Seventeen glass vessels and twenty glass beads recovered from the excavations at the ancient city of Malindi and the archaeological site of Mambrui in Kenya, east Africa were analysed using electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The results show that all of the glass samples are soda-lime-silica glass. They belong to the high alumina-plant ash glass type, characterised by high alumina and relatively low calcium contents, widely distributed in eastern (10 th-16 th centuries AD) and southern Africa (13 th-15 th centuries AD), Central Asia (9 th-14 th centuries AD) and southeast Asia (12 th-13 th centuries AD), made with plant ashes and sands. This is an understudied glass type for which previous research has indicated there were three types. When compared with published research on such glasses using Zr, Ti, Ba, Cr, La, Li, Cs, Na 2 O, MgO and CaO we have identified at least four different compositional groups of v-Na-Al glass: Types A, B, C and D. By comparing the results with contemporary v-Na-Al glass vessels and beads from Central Asia, Africa, and southeast Asia we show that most of the Malindi and Mambrui glass share similar characteristics to the compositions of Mapungubwe Oblate and some of the Madagascar glass beads from southern Africa. They belong to Type A v-Na-Al glass which is characterised by an elevated level of Ti and Ba and a relatively high ratios of Cr/La, relatively low Zr concentrations and low ratios of Zr/ Ti. Differences in Zr, Li, MgO and Na 2 O concentrations in Type A glass indicates that there are subgroups which might derive from different glass workshop(s) specialising in Type A v-Na-Al glass production. Comparison with the chemical compositions of glass from Ghazni, Afghanistan and Termez, Uzbekistan, and by using lead isotope analysis, we suggest v-Na

On-Site Raman Spectroscopic Study of Beads from the Necropolis of Vohemar, Northern Madagascar (>13th C.)

“…Plant-ash (characterized by both high MgO > 2.5 %wt and high CaO content) and mineral-soda (characterized by a low MgO (< 1.5 %wt) and high Al 2 O 3 level,~12 %wt) are in evidence, both with a variety of colors and assigned to an origin from southern Asia. Similar soda-lime-silica glass beads have been found in Kenya [21]. They belong to the high alumina-plant ash glass type, characterised by high alumina and relatively low calcium contents, widely distributed in eastern (10th-16th centuries CE) and southern Africa (13th-15th centuries AD), Central Asia (9th-14th centuries CE) and Southeast Asia (12th-13th centuries AD), made with plant ash and sand.…”

Cobalt and Associated Impurities in Blue (and Green) Glass, Glaze and Enamel: Relationships between Raw Materials, Processing, Composition, Phases and International Trade

“…Cobalt sources in Asia are less well documented (at least in western language sources). It was reported that blue glass beads had been made in Central Asia using cobaltite from Rajasthan in India [86]. Other sources are in Indonesia, China, and Japan which will be considered further.…”

LA‐ICP‐MS analysis of glass beads from Tié (12th–14th centuries), Kanem, Chad: Evidence of trans‐Sudanic exchanges

“…In contrast, the lime (CaO) component has a mean of only about 4.3 wt%. Siu et al (2020) have proposed four possible compositional types (A-D) of v-Na-Al glass, of which type A seems to have the greatest variation in the proportions of major elements. Since the value range of their type A is close to that of the Tié beads (see Table S3 in the additional supporting information), we provisionally propose an assignment to this type.…”

“…For instance, only one bead of this glass type has been respectively identified at Gao-Gadei (McIntosh et al, 2020) and Essouk (Lankton et al, 2017). It has been argued by Siu et al (2020) that glass of this type was ultimately made in Central Asia, though its precise origin is still unknown. Be that as it may, Kanem is not only the first region in northern Central Africa where this glass type occurs in significant numbers, but it is to date also the only African region so far off the East African coast, where this glass type actually dominates the assemblage.…”

“…In contrast to beads, vessel and other glass products are rare finds in Sub‐Saharan Africa. Places south of the Sahara where such unusual items were found are primarily early cities or places connected in some way with long‐distance trade, such as Gao (Insoll, 1996, 2000; McIntosh et al, 2020; Takezawa & Cissé, 2016), Djenné‐Jeno (McIntosh, 1995: 246), Essouk‐Tadmekka (Nixon et al, 2017), Marandet (Magnavita et al, 2007), and sites linked to the Indian Ocean trade network on the Swahili coast (Dussubieux & Kusimba, 2012; Siu et al, 2020) (Figure 1). Although there are earlier finds, the majority of imported glass and glass beads in Sub‐Saharan Africa date from approximately the last 13 centuries and are at first likely related to the Arab–Islamic expansion into North Africa and along the East African coast, where a larger Indian Ocean trade network played a prominent role.…”