Chemical characterisation of argillaceous sediments used for traditional pottery around Port St Johns, Eastern Cape Province, South Africa

Abstract

As part of a process to better understand aspects of contemporary zero electricity usage pottery-making and firing technology, in the Port St Johns region of the Eastern Cape Province, atomic absorption spectrometry (AAS), inductively coupled mass spectrometry (ICP-MS), and Fourier transform infrared (FTIR) spectroscopy were used to chemically characterise argillaceous sediments used in such traditional pottery. The results were compared with those obtained for commercial pottery in the nearby East London region. The major oxides and the following trace elements: Rb, Sr, Ba, Sc, V, Cr, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Pb and Th were determined in the sediments used for both traditional and commercial pottery. The results depicted moderately altered and weathered sediments having chemical index of alteration (CIA) values of 56.99–82.14 and chemical index of weathering (CIW) values of 63.65–96.17. The K₂O/Na₂O values were 0.19–5.00 and the SiO₂/Al₂O₃ values were 3.14–5.43. Concentrations of trace elements and particularly heavy metals were very low and unlikely to directly pose any human health risks. Diagnostic peaks in the FTIR spectra reflected Al-OH, Al-O and Si-O functional groups in the high frequency stretching and low frequency bending modes, which correspond to kaolinite, and quartz, smectite and muscovite interferences. Based on the results of the chemical analyses of the argillaceous sediments used for traditional pottery compared to those used for commercial pottery, it was found that there were no particular differences in their chemical attributes, thus revealing the soundness of indigenous knowledge pertaining to clay selected for ceramics praxis in the region.

Keywords:

• chemical index of alteration
• clay
• health
• kaolinite
• quartz
• trace elements
• Mpondo ceramics praxis

ACKNOWLEDGEMENTS

The authors are grateful to Alice Gqa Nongebeza and Debora Nomathamsanqa Ntloya for welcoming us to their homesteads and ceramic studios, and for sharing their knowledge so freely, and thank Siziwe Sotewu for help with interpretations of events and translation of discussions. The Directorate of Research Development at Walter Sisulu University is also appreciated for financial assistance.