REQUEST TO CONDUCT LASER ABLATION ON FIFTY HAND DRAWN GLASS BEADS

1 GLASS BEADS TRACE ELEMENT ANALYSES BY LASER ABLATION
1.1 Introduction
PGS Heritage (Pty) Ltd (PGS) was appointed by the Nkomati Anthracite Mine to manage the Phase 2 archaeological mitigation work required for the mitigation of several identified archaeological sites and to investigate features that may be graves. The planned expansion of mining activities in the area known as “Block L” will affect these archaeological sites and features.

PGS obtained an excavation permit from the SAHRA (Case ID: 18339) to undertake archaeological mitigation, test excavations, and ground penetrating radar scans to extensively excavate the archaeological sites and features and to collect representative samples of the archaeological material for analysis to determine the temporal localisation, cultural affiliation, and possible social structure and layout of the settlements. The mitigation work must also identify unmarked burials and ensure, as far as possible, that no unmarked burials are left in the area after the completion of the mitigation work.

The chemical analysis of archaeological materials can potentially determine how, when, and where objects were made and possibly indicate their authenticity and origin. Trace element analysis by Laser Ablation is a non-invasive method which ensures that the integrity of an object stays preserved (Knaf et al. 2017). Glass beads recovered from archaeological sites are a result of previous trade with other communities. These beads originated from a different source, and trace element analysis can assist in identifying that source. At the time of writing, we are awaiting a permit to conduct this analysis.
1.2 Background
Over 2000 glass beads were recovered from most of the excavated sites. These included yellow/lime green, blue, red, white, weathered beads and beads with a residue which results in an indeterminate colour. The origin of the opaque yellow hand-drawn glass cylinder beads found has yet to be determined. It is the most abundant bead type recovered but does not seem to fit in any of the described and known bead series.

These beads are similar to the yellow glass beads recovered at Simunye, an archaeological site approximately 50km further south in Eswatini; Marilee Wood, a prominent glass bead expert, has analysed them. She could not determine the origin of the beads but mentioned that they resemble beads from Sofala, Mozambique, in the Van Riet-Louw collection (Ohinata 2002).

We plan to subject 50 of the Nkomati yellow/lime green hand drawn beads to a trace element analysis by Laser Ablation to identify their composition and compare them to other known analysed beads and collections. This could assist in identifying the beads' source or origin, and what material was used to create the beads. The laser ablation will be done at the Spectrum Analytical Facility at the University of Johannesburg, where similar analyses on other glass beads have been conducted.

1.3 Analysis Methodology
Henriette Ueckermann will assist in the analysis and explains how the analysis will be done. Trace element analysis will be performed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) using a 193nm ArF RESOlution SE Excimer Laser (Applied Spectra) coupled with a Thermo Fisher iCAP RQ ICP-MS, based at the Spectrum Analytical Facility, University of Johannesburg. The laser settings which will be used will be a spot size of 100 µm diameter or smaller, a beam energy of 6 mJ and a beam attenuation of 50% at a repetition rate of 10 Hz. Helium will be used as a carrier gas to the ICP. The NIST612 standard glass (Jochum et al. 2011) will be used as a bracketing external standard. Samples will be analysed bracketed at regular intervals using the NIST610 standard glass and the BCR2G basaltic glass (Jochum et al. 2005) as independent controls on reproducibility and instrument stability. The 29Si isotope will be used as the internal standard. Data will be processed using the GLITTER software (Van Achterbergh et al. 2001).

The size of the ablation spot inflicted on the sample will be 100 µm diameter and a depth of 30 µm at most, resulting in a volume of 825 000 µm3, or 8.25 x 10-4 mm3, being removed and analysed. This spot will not be visible to the naked eye and is barely visible when using a 10x magnification hand lens.
 
2 BIBLIOGRAPHY
KNAF, A. S. C.J.; KOORNNEEF, M. & DAVIES, G. R., 2017. “Non-invasive” portable laser ablation sampling of art and archaeological materials with subsequent Sr–Nd isotope analysis by TIMS using 1013 U amplifiers. J. Anal At. Spectrom, Vol 32, pp. 2210 – 2216.

JOCHUM, K. P., WEIS, U., STOLL, B., KUZMIN, D., YANG, Q., RACZEK, I., JACOB, D. E., STRACKE, A., BIRBAUM, K., FRICK, D. A., GUNTHER, D., & ENZWEILER, J. (2011). Determination of Reference Values for NIST SRM 610-617 Glasses Following ISO Guidelines. Geostandards and Geoanalytical Research, 35(4), 397–429. https://doi.org/10.1111/j.1751-908X.2011.00120.x

JOCHUM, K. P., WILLBOLD, M., RACZEK, I., STOLL, B., & HERWIG, K. (2005). Chemical characterisation of the USGS reference glasses GSA-1G, GSC-1G, GSD-1G, GSE-1G, BCR-2G, BHVO-2G and BIR-1G using EPMA, ID-TIMS, ID-ICP-MS and LA-ICP-MS. Geostandards and Geoanalytical Research, 29(3), 285–302. https://doi.org/10.1111/j.1751-908x.2005.tb00901.x

OHINATA, F., 2002. The beginning of ‘Tsonga’ archaeology: excavations at Simunye, north-eastern Swaziland. Southern African Humanities Vol. 14 Pages 23–50 Pietermaritzburg December, 2002.

VAN ACHTERBERGH, E., RYAN, C. G., JACKSON, S. E., & GRIFFIN, W. L. (2001). Data reduction software for LA-ICP-MS. In P. J. Sylvester (Ed.), Laser ablation-ICP mass spectrometry in the earth sciences: principles and applications. (Vol. 29, pp. 239–243). Mining Association of Canada.