Towards refining the classification of glass trade beads imported into Southern Africa from the 8th to the 16th century AD

Glass beads have been produced and traded for millennia all over the world for use as everyday items of adornment, ceremonial costumes or objects of barter. The preservation of glass beads is good and large hoards have been found in archaeological sites across the world. The variety of shape, size and colour as well as the composition and production technologies of glass beads led to the motivation to use them as markers of exchange pathways covering the Indian Ocean, Africa, Asia, Middle East, the Mediterranean world, Europe and America and also as chronological milestones. This review addresses the history of glass production, the methodology of identification (morphology, colour, elemental composition, glass nanostructure, colouring and opacifying agents and secondary phases) by means of laboratory based instruments (LA-ICP-MS, SEM-EDS, XRF, NAA, Raman microspectroscopy) as well as the mobile instruments (pXRF, Raman) used to study glass beads excavated from sub-Saharan African sites. Attention is paid to the problems neglected such as the heterogeneity of glass (recycled and locally reprocessed glass). The review addresses the potential information that could be extracted using advanced portable methods of analysis.

Excavations conducted between 2010 and 2012 at Magoro Hill, a site in South Africa's Limpopo Province frequented or intermittently occupied by African farming communities since the first millennium AD, yielded a substantial glass bead assemblage. A selection of the beads was studied non-destructively by classifying them according to morphological attributes, supplemented by Raman analyses and XRF measurements. It became evident that a morphological classification of beads recovered from sites that include imports into Africa after the seventeenth century AD could be problematic due to apparent morphological similarities between earlier and later beads. This paper demonstrates the use and archaeological application of Raman and XRF measurements to separate earlier imported beads from later counterparts by identifying glass nanostructure, as well as pigments and opacifiers, which were not used in bead series pre-dating the seventeenth century AD. Results obtained from Raman and XRF measurements indicate that although some beads retrieved from Magoro Hill pre-date the seventeenth century and belong to the Indo-Pacific (K2, East Coast, Khami) and Zimbabwe series, the largest number of beads is from a later European origin. This ties in with the settlement history of the site, which suggests that it primarily served as a rendezvous for episodic rainmaking rituals before it became the stronghold and capital of a Venda chiefdom, headed by the Magoro dynasty, in the second half of the eighteenth century AD. The comparative analysis of the long bead sequence sheds new light on changing patterns in the availability, range, consumption and origin of glass trade beads imported into the northern interior of South Africa over a period of about 1000 years.

Abstract

Twenty-eight representative beads found at the thirteenth-fourteenth century AD site of Mutamba in southern Africa were classified morphologically and then analysed with pXRF. Eighteen beads were selected from four identified series of K2-IP, EC-IP, Mapungubwe Oblate and European for Raman spectroscopy. The results show corrosion has a great effect on the composition and nanostructure of Mapungubwe series with lixiviation of alkalis. In corroded K2-IP beads elevated amounts of potassium recorded on the surface layer compared to less corroded IP beads. Despite these changes pXRF and Raman spectroscopy were successful in discriminating of the bead series by considering the concentration of trace elements (Zr, U), detected pigments (lead tin yellow (II), red lead), opacifiers (SnO 2 , calcium antimoniate) and colorants (Fe, Cu, Co) and their concentrations in the beads.