Characterization of Glass, Shell, and Fishbone Beads on Ibo Island (Northern Mozambique) in the Context of the Indian Ocean Trade

Abstract

A set of beads made of glass, gastropod mollusk shell, and fishbone from a Swahili occupation level on Ibo Island (northern Mozambique) is dated to the eleventh and twelfth centuries AD. The chemical composition of the glass beads and their chromophores, and the shell and fishbone materials, are studied to understand the local and trading provenance of these items. Representative samples of each material, including two flat glass shards, were characterized using optical microscopy, field emission scanning electron microscopy (FESEM) with energy dispersive X-ray (EDS) microanalysis attached, UV-Vis-NIR spectrophotometry, and X-ray diffraction (XRD). Glass beads represent 60% of the bead assemblage. The shell and fishbone beads are 38.75% and 1.25%, respectively. Three groups of glasses were identified: (1) mineral-soda alumina glass beads associated with the recently identified m-Na-Al 6 group that come from Western India; (2) vegetal-soda alumina, represented by one flat glass shard probably from the Middle East, and beads of v-Na-Al glass of type A that came from Central Asia; and (3) conventional soda-lime silicate glasses, of modern chronology and probable European origin. Groups 1 and 2 show a variety of chromophores, both ionic and colloidal. A ruby-color bead was also identified as belonging to the Na2O-ZnO-SiO2 system and colored with cadmium sulfoselenide colloids, but this was of modern chronology. Shell beads were made from Lambis lambis gastropod mollusk shells and were locally produced. The fishbone beads may have been from a cartilaginous fish of the Chondrichthyes class, but it is difficult to specify the taxonomical level. The results of the analysis suggest that Ibo Island was integrated very early into the Swahili trade networks of the Indian Ocean.