9th Experimental Archaeology Conference abstract – Fregni
Inverse segregation: It’s not just for artists any more
E. Giovanna Fregni
University of Sheffield
Surface analysis of ancient bronze objects has always been problematic. Techniques such as portable XRFs are ideal for non-destructive analysis of metal objects, but they have acknowledged limitations. Readings frequently display elevated levels of tin and lead in Late Bronze Age bronze alloys. This has variously been attributed to metal depletion or enrichment due to the burial environment or corrosion processes. Other explanations include deliberate surface treatment to give an object a different appearance (Tylecote, 1979, 351, Scott, 1991, 43-44, Robbiola et al., 1998). However, a major contributing factor that is well known to modern bronze sculptors, but not well recognised by archaeologists is the process of inverse segregation.
As alloys cool in a mould, the heated surface of the interior of the mould will delay the hardening of the alloys with the lower melting temperature, thus enriching the surface with elements such as tin and lead, elements typically part of Late Bronze Age alloys.
After conducting experiments casting bronze tools in closed moulds and also casting ingots in open moulds, XRF analysis showed that the tools had a surface enriched with tin and lead, while ingots that were cast into open moulds did not.
The use of experimental archaeology, supported by chemical analysis enables new insights into Bronze Age metalworking practices. The recognition of inverse segregation introduces the possibility of further experiments to explore mould temperature as a means of altering the surface colour of bronze objects. While there are metal objects from the Bronze Age that could have been given surface treatments to change the colour, the possibility of controlling the temperature and cooling rates of moulded castings opens new potential for how we interpret
metal-smithing practice in the Bronze Age.
ROBBIOLA, L., BLENGINO, J. M. & FIAUD, C. 1998. Morphology and mechanisms of formation of natural patinas on archaeological Cu–Sn alloys. Corrosion Science, 40, 2083-2111
SCOTT, D. 1991. Metallography and microstructure of Ancient and Historic Metals, Los Angeles, The Getty Conservation Institute
TYLECOTE, R. F. 1979. The effect of soil conditions on the long-term corrosion of buried tin-bronzes and copper. Journal of Archaeological Science, 6, 345-368