3.2.2 Inclusions
Several types of inclusions can be found in the microstructures of the Geistingen axes.
Small silver and lead-antimony particles are sporadically seen throughout the structure of AC20. The most plausible explanation for their presence is that they are remnants of the ore since their presence is not uncommon in copper ores (Lindgren 1933). The melting point of pure silver (962 °C) is higher than that of bronze with a composition like AC20 (~950 °C) and of BH76 (~800 °C), and the silver particles thus remain solid in the liquid bronze. Lead has a very poor solid solubility in copper. Both cases result in these elements (either solid silver or liquid lead) being dragged along with the solidification front in the melt subsequently to be found in the interdendritic phase between the solid dendrites.
Both AC20 and BH76 display another distinct type of inclusion, namely copper-sulphide particles. Almost all of these particles in the two axes contain the same elements but their concentration differs: copper, sulphur, iron, oxygen and sometimes also tin, nickel and antimony. The majority of the particles can be identified as Cu2 S with the aforementioned elements in solution. A difference seen between the two axes lies in the morphology of these inclusions (see fig. 5). They are spherical and star-shaped in AC20, while BH76 only contains star-shaped particles. Substructures have been qualitatively identified in the spherical particles, but further research is needed to provide more information about the origin of these structures. The different inclusion morphologies indicate that for both axes the temperature of the melt has been around the melting temperature (1130 °C) of copper-sulphide. The spherical particles have been completely molten and solidified into spheres, while the star-shaped particles represent particles of which only the outside has been molten. This implies that the temperature of the melt of both axes ranges between 1100 °C and 1150 °C, and the absence of spherical Cu2 S-inclusions in BH76 indicates a somewhat lower temperature than for AC20. For both axes the applied temperature is higher than the melting temperatures of the main phases.
The copper-sulphide particles solidify during cooling at a higher temperature than the bronze does and can therefore end up between the dendrites, within the interdendritic phase as described above. This type of inclusion represents an important clue to the production of Geistingen axes since they can be identified as matte, a by-product of the smelting of copper-sulphide ore. It is therefore assumed that the presence of silver, lead-antimony and matte particles with their different morphologies can be attributed to the imperfect smelting of sulphidous copper ore, used for the initial melt that eventually formed the Geistingen axes after re-melting.