PNNL

Abstract

European Bronze and Iron Age vitrified hillforts have been known since the 1800s, but archaeological interpretations regarding their function and use are still debated. We carried out a series of experiments to constrain conditions used to melt the inner wall stones in the hillfort at Broborg, Sweden. Potential source rocks were collected locally and heat treated in the laboratory, varying maximum temperature, cooling rate, and starting particle size. Crystalline and amorphous phases were quantified using X-ray diffraction on quenched samples (ex situ) and while heating and cooling (in situ). Textures, phases, and compositions were compared with archaeological rock samples and equilibrium crystallization calculations. ‘Dark glass’ and its associated minerals formed from amphibolite or dolerite rocks melted at ~1000-1200°C under reducing atmosphere then slow cooled. ‘Clear glass’ formed from non-equilibrium melting of feldspar in granitoid rocks. Comparative melting behaviors for amphibolite, dolerite, granite, and basalt suggest that differences in local lithology contributed little to feasibility of vitrification, only to extent.