Slag Investigations from Bronze Age Copper Smelting Sites

Abstract

During the Bronze Age intensive mining and smelting activities for copper production took place in the Eastern Alps. To get information about the copper smelting process, the elemental compositions of slags are marked in equilibrium phase diagrams (e.g. FeO-CaO-SiO2) and so the melting properties can be estimated. Doing so you have to keep in mind that slags have complex compositions and phase diagrams are available for three compounds only. For the analytical measurements it has to be ensured that only molten parts of the slag are measured and not contamination of other ambient material. Spot and area measurements by SEM-EDX are useful to get realistic data. In this case a complete correlation between the image of the analyzed area, the microstructure and the chemical composition of the sample is necessary. For marking spots in the phase diagram the calculation method has to be described exactly. For our results we calculated the ratio FeO-SiO2-CaO(+MgO+Al2O3). From the morphology of the observed phases, their chemical composition and the data from the phase diagram a solidification sequence can be suggested. We recommend this method because measurements by e.g. XRF provide rather general composition values. If the slag samples are inhomogeneous, unrealistic melting points are read from the phase diagram. Inhomogeneities can be caused by soil contaminations, which are not part of the molten slag, or by corrosion, when some phases were attacked and changed during storage in soil.