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 (10th– 16th centuries AD) and southern Africa (13th - 15th centuries AD), Central Asia (9th– 14th centuries AD) and southeast Asia (12th– 13th 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, Na2O, 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 Na2O 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-Al glass was manufactured in Central Asia and possibly worked into vessels and beads there.
Highlights
In recent years, large scale studies of African glass artefacts have been conducted using scientific methods such as LA-ICP-MS
We have used the electron probe microanalysis (EPMA) results for the major and some minor element oxides (Na2O, SiO2, CaO, MgO, K2O and Al2O3) and the LA-ICP-MS results for other minor and for trace elements
The glass vessels from Malindi and the glass beads from Mambrui can be confirmed as plant ash high alumina (v-Na-Al) glass, which has been found in Central Asia [29], southeast Asia, Mtwapa in Kenya and southern Africa, with rare examples occurring in the west [30]
Summary
Large scale studies of African glass artefacts have been conducted using scientific methods such as LA-ICP-MS These include the study of glass beads from southern Africa and east Africa, the studies by Marilee Wood and her colleagues of glass beads from Chibuene, southern Mozambique and from Zanzibar [1, 2] and the monumental study of southern African glass beads by Peter Robertshaw and his colleagues [3, 4]. Dussubieux and her colleagues [5, 6] published reviews of mineral soda alumina glass found in Africa and east Asia. The low magnesia levels (< 1.5%) in m-Na-Al glass suggests that a mineral alkaline flux (e.g. reh) was used to make the glass, and the high alumina contents (c. 5% to 15%) suggests the use of a low quality sand [6]
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