Glass Beads, Markers of Ancient Trade in Sub-Saharan Africa: Methodology, State of the Art and Perspectives
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heritage Review Glass Beads, Markers of Ancient Trade in Sub-Saharan Africa: Methodology, State of the Art and Perspectives Farahnaz Koleini 1 , Philippe Colomban 2,* , Innocent Pikirayi 1 and Linda C. Prinsloo 3,4 1 Department of Anthropology and Archaeology, Faculty of Humanities, University of Pretoria, Pretoria, Hatfield 0028, South Africa 2 Sorbonne Université, CNRS, MONARIS UMR8233, 4 Place Jussieu, 75005 Paris, France 3 School of Earth and Environmental Sciences, University of Wollongong, Wollongong, NSW 2522, Australia 4 Evolutionary Studies Institute, University of the Witwatersrand, PO Wits 2050, South Africa * Correspondence: [email protected] Received: 5 May 2019; Accepted: 2 August 2019; Published: 6 August 2019 Abstract: 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. Keywords: trade; beads; glass; pigments; provenance; Africa; Asia; Mediterranean world 1. Introduction: Overview of Glass Production History We present here the first review of research conducted on trade beads from African sites covered in the literature and discuss about all identification methods for morphology and colour, elemental and molecular composition, pigments and secondary phases. We first give a very brief historical overview of glass production, then review the analytical methods, particularly non-invasive techniques adapted to identify glassy silicates and colouring agents/pigments/opacifiers. The classification history of beads with emphasis on sub-Saharan bead sites is considered before reviewing the results obtained and the discussion of special cases such as local productions, European copies and heterogeneous glass. Additionally, some perspectives are also discussed. The first long distance trade in obsidian, a natural hard and brittle volcanic glass, which fractures with very sharp edges and is used to make cutting and piercing tools, was established during the Neolithic times [1,2]. The trade in synthetic glass objects and especially glass beads is not that old but has covered long distances, especially around and through the Indian Ocean. Since the beginning of the first millennium AD, the Indian Ocean was organized into a place around which the first world trade system was built, mostly with the assistance of monsoon winds [3,4], connecting the Near/Middle East, India, South Asia, China and Africa [3–5]. European merchants had commenced trading beads to Heritage 2019, 2, 2343–2369; doi:10.3390/heritage2030144 www.mdpi.com/journal/heritage Heritage 2019, 2 2344 Africa from the 15th century AD onwards. European glass beads were also exported to America as early as the 16th century AD [6–8]. The production of most historic glasses was obtained by reducing the melting temperature of silica (sand) through adding fluxing (sodium-, potassium- or lead-rich compounds) and stabilizing (calcium or aluminium based compounds) agents to form a non-crystalline solid that is often transparent. This technique was first mastered to make glazed artefacts around 5000–4000 BC [9] and the first synthetic glass, or more precisely a coloured incompletely molten material was made in Egypt, Mesopotamia and the Indus Valley circa 2000 BC to replace semi-precious stones and gems [10–12]. From about 1500 BC, transparent glass artefacts had been produced in the Levant, Crete and Egypt with a colour palette which was gradually broadened until 1200 BC when political unrest led to a dramatic break in glass production [13]. New achievements in glass making had to wait until the 9th century BC, when the production of an almost colourless glass commenced in Syria and Cyprus [14,15] and an upgraded moulding technique made it possible to achieve new complex shapes and designs [16]. Around 200 BC, Roman glassmakers developed the blowing technique and the production of glass artefacts became a heavy industry around 50 BC [11,15]. Alexandria (Egypt), Sidon (Lebanon), Belus River (Levant), Campania and Rome (Italy) as well as Gaul (France) are mentioned as the most important industrial centres in Pliny the Elder’s texts [17]. The most famous artefacts are enamelled glasses such as those from the Begram Ptolemaic Treasure [18] dated from the Roman to the early Byzantine period [19]. Then, the Normandy Court in Sicilia continued the production of complex enamelled objects [20] which developed during the Fatimid and Mamluk dynasties [21]. Enamelling and gilding declined in the Near Eastern countries at the end of the Middle-Ages as a direct consequence of the decline of Islamic influences following the sack of Damascus by the Mongols and of Byzantium by the Crusaders [22]. The golden age of Venetian glass thus started [23,24] after the Byzantine craftsmen had developed Venetian glass production [11]. The expertise of Venetian glassmakers reached its height in the 16th century when Murano workshops were the first to produce not only cristallo (rock crystal-like) glass, but also more complex lattimo (milky glass), aventurine glass and millefiori (glass with complex coloured design) [9,11]. A decentralisation of glass production centres occurred during the Middle-Ages in western and northern European countries. Cologne, Leuven, Namur, Amiens and Beauvais then became important sites [25]. By the 16th century, Façon de Venise glass objects with the same transparency as original Venetian cristallo started to be produced in Holland [9]. Then, new types of clear, lead-based glasses were introduced during the 17th century. In England, Ravenscroft discovered the advantages of lead-based glass (crystal and flint glass) with a high optical index and pliability. The so-called crystal and flint glasses were made of high purity raw materials [26,27]. At the same time, Bohemian glassmakers prepared another kind of lead-based glass, richer in potassium and calcium which made the enamelling process easier [28]. Lead glass became also popular with Dutch glass engravers. Finally, the implementation of press-moulding in the USA (ca. 1825), transfer painting and acid-etching as well as new colouring agents broadened the variety of glass [26,29]. Glass production in southern India, Thailand, Vietnam, the Philippines and China seems to have started 3000 years ago [30–33]. Arikamedu is considered as the main and oldest Indian bead making site dating to a few centuries BC [5,34]. Its old name Virampatnam has been identified with the Pôdouké Emporion mentioned in the ancient texts of Ptolemy and the Periplus Maris Erythraei (namely “Trek in Red See”, an ancient travel Greek book) [5]. Other ancient production sites are Mantai (old name Modutti) in Sri Lanka, Oc éo (Kattigara, Funan Kingdom, now southern Vietnam) as also mentioned in Ptolemy’s Geography, Khlong Thom in Thailand and Kuala Selinsing in Malaysia. In China, the glass paste/glassy faience production dates from the Western Zhou Dynasty (~1000 BC), but glass production was mainly developed during the Warring States Period and then the Eastern Han Dynasty (around the 4th century BC) in the provinces of Xingjian, Shandong, Henan, Guangdong and along the Yangtze River [33]. Two types of lead-based glass were produced using barium, potassium or sodium as the other fluxing agents [35]. Glass production in central Vietnam is associated with Heritage 2019, 2 2345 Heritage 2019, 3 FOR PEER REVIEW 3 theproduction Sa Huynh in culture Europe (the and first millenniumthe Mediterranean BC) [5] dueworld to the is development archaeologically of firing well techniques documented [36]. Although[9,11,15,22,26], ancient it isglass still a production work in progress in Europe for Asian and the countries Mediterranean [35,37]. world is archaeologically well documented [9,11,15,22,26], it is still a work in progress for Asian countries [35,37]. 2. Glass Bead Research 2. Glass Bead Research Glass objects differ in their shape because of aesthetic and cultural/commercial constraints. Consequently,Glass objects the dimorphologyffer in their shapeof a glass because object of aestheticcan be a and criterion cultural for/commercial dating and constraints. sourcing. 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