Purple haze: Combined geochemical and Pb-Sr isotope constraints on colourants in Celtic glass

Publication date: May 2017Source:Journal of Archaeological Science, Volume 81
Author(s): D.J. Huisman, J. van der Laan, G.R. Davies, B.J.H. van Os, N. Roymans, B. Fermin, M. Karwowski
The composition of 2977 Late Prehistoric glass objects was investigated to derive information on the nature of the colourants used. 2673 Late Iron Age Celtic (La Tène) bracelet fragments from the Netherlands and Austria and 51 Early Iron Age beads from the Netherlands were analysed. Hand-held XRF analyses demonstrated that all glass objects were of the soda-silica-lime type, which has a presumed origin in the Eastern Mediterranean. Copper was used as colourant, in the form of copper filings, in most of the Early Iron Age glass beads to give recycled glass a blue-green colour.The vast majority (98%) of the translucent Iron Age glass, was coloured using cobalt (blue), manganese (purple; colourless), antimony (colourless) and iron (green). Manganese, however, was added to all glass, contributing additional amounts of elements like copper, cobalt and iron. Opaque decorations were produced using antimony, or a combination of tin and lead.REE analyses on a selection of representative objects indicate that the manganese ores in translucent glass are of hydrogenetic-diagenetic (Early Iron Age) or hydrothermally influenced diagenetic (Late Iron Age) types. Strontium isotope ratios show mixing between a calcite-related seawater source (0.709) and manganese ores with isotope ratios of ∼0.70766. Lead isotope ratios are dominated by colourant-derived Pb. The isotope ratios of the manganese used to produce translucent glass and lead used for opaque glass decorations fall in the same range. The most likely general provenance of both lies on Lavrion or the Western Cycladic Islands, although an origin in the central Taurus or the Sinai mountains cannot be excluded. The conclusion is that manganese ore used for colourants contributes significantly to the REE concentration in the glass, including Nd, as well as to Sr and Pb. This needs to be taken into account when using concentrations or isotope ratios of these elements for provenancing other raw materials like sand and calcium carbonate. This appears to be the case for all antique soda-lime-silica glass.