Raman signatures of the modern pigment (Zn,Cd)S1−xSex and glass matrix of a red bead from Magoro Hill, an archaeological site in Limpopo Province, South Africa, recalibrate the settlement chronology
Highlights
► Two beads excavated on Magoro Hill were spectroscopically analyzed. ► The pigment coloring the red bead was identified as cadmium sulfoselenide. ► The pigment was only used during the 20th century. ► This recalibrates the settlement chronology obtained from oral tradition.
Introduction
Recently it was illustrated that Raman spectroscopy is a very useful method to study glass trade beads as information about the glass structure, as well as the pigments used to color the glass, is encapsulated in one spectrum [1], [2], [3]. Identification of the pigments used to color the beads can also be used to broadly date a specific bead if, as in the case of synthetic pigments, the first manufacturing date is known. In southern Africa glass trade beads are found at many archaeological sites of the past two millennia and testify to international trade. Information about the origin of the beads and their date of production can in many instances help in the historical reconstruction of a site’s occupation [1], [2], [3].
In sub-Saharan Africa this is very important as only a very few written records exist of the pre-Portuguese era and records thereafter often tend to privilege the history of European colonists in their interaction with Africans. The documentary record on African societies remains shallow even late into the 19th century, which implies that oral tradition and archaeology are often the main sources of information on underrepresented or marginalized groups. Oral chronologies are fickle, though, since traditions can be affected by memory loss, feedback, selection, reinterpretation, lengthening and telescoping. Fortunately they can be verified in some instances through physical archaeological evidence. In this regard, glass beads serve as useful temporal markers as long as their primary stratigraphic context is well established. Since they are small objects that can easily become lost or displaced, care should be taken to ensure that the matrix from which the beads are retrieved is truly associated with the occupational level that is to be dated. It should also be recognized that glass beads could have been treasured as heirlooms, and thus could pre-date an occupation.
Section snippets
The archaeological context
Current historical archaeological research at Magoro Hill (nearest town Louis Trichardt), an erstwhile Venda stronghold in South Africa’s Limpopo Province, aims to unravel the site’s complicated settlement sequence. The Magoro Hill complex was located on the fringes of colonial settlement and seldom visited by literate observers during the 19th century. Extant oral traditions relate that at least four chiefs reigned from the stronghold during this period of time [4] and that each successive
Samples
The two beads analyzed can be seen in Fig. 1. The first is a bright red (Fig. 1a), barrel-shaped bead, in pristine condition and manufactured by using a mold. The second is a bright yellow hand-drawn cylinder (Fig. 1b), which appears quite weathered.
Experimental detail
The Raman and room temperature photoluminescence spectra were recorded with a T64000 micro-Raman spectrometer from HORIBA Scientific, Jobin Yvon Technology (Villeneuve d’Ascq, France). The Raman spectra were excited with either the 514.5 or 488 nm lines of a Coherent Innova® 70C Series Ion Laser System and the 100× or 50× objectives of an Olympus microscope was used to focus the laser beam (spot size ∼2–12 μm) on the samples and also collected the backscattered Raman signal. An integrated triple
Pigments coloring the beads
Glass coloring is usually obtained by doping the glass with 3d/4f ions or dispersing colored crystalline phases (pigments or metal nanoparticles) in the glass matrix [7]. Coloring obtained by doping disperses ions (e.g. Co) throughout the silicate matrix and in many instances no special Raman signal can be observed [7]. Small crystallites of a pigment are sometimes distinguishable under a microscope objective (especially in ancient glasses), which makes it possible to focus on the pigment
Conclusion
The identification of lead tin yellow Type II as the pigment coloring the yellow bead dates it as much older than the red bead. The appearance of the yellow bead, high corrosion level and glass composition fit in with this classification and the bead was most probably passed onto the owner by ancestors and kept as an heirloom. The pigment coloring the red bead was identified as nano-(Zn,Cd)S1−xSex mixed crystals, a pigment that was only widely used in the early 20th century. This date casts
Acknowledgements
Jan Boeyens and Maria van der Ryst wish to thank their Unisa colleagues and students for assistance during fieldwork at Magoro Hill. Special thanks are due to Wim Biemond, who first drew our attention to the archaeological significance of the red beads, and to Pieter Snyman, who led the team that excavated the brick-built structure.
We gratefully acknowledge the financial support of the NRF, UNISA and the University of Pretoria.
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