Scientific study of the etching process used on ancient carnelian beads

Abstract

• Etched carnelian beads are a striking example of the very high level of technological skills developed by ancient civilisations to create ornament and jewellery. This study combines the characterisation of etched areas of archaeological beads from the British Museum collection with the experimental etching of modern carnelian in laboratory conditions, based on ethnographic descriptions from the 1930s by Mackay and Majumdar, in order to understand the manufacturing technology of etched carnelian beads. The characterisation of etched archaeological beads was carried out using optical/digital microscopy and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, combined with X-ray diffraction and Raman spectroscopy. Experimental etching started with the making of the etching agent, substituting the traditionally used but unavailable Capparis Aphylla juice with sodium alginate and mixing it with an alkali in the form of washing soda. The experiment continued with the application of this etching agent on the surface of modern unpolished carnelian samples. These were fired in an electric kiln under a range of conditions in order to optimise firing temperature and duration. The results from this scientific study suggest that the white designs etched on the surfaces of the beads are produced thanks to the dissolution of silica by the etching agent, which creates a network of fine pores that scatters the light. Experiments also revealed that the etching process was successful using sodium alginate and optimised around 450°C for 5 to 15 minutes.