Volume 59, Issue 12

So-called “plate slags” from the archaeological Bronze Age copper smelting site Acqua Fredda were examined. What was common to all the examined slags was that their compositions, once converted and plotted on the FeO-SiO 2 -CaO phase diagram, were located at the phase boundary between olivine and wollastonite, resulting in melting temperatures of approx. 1100 °C for the slags. Slags with a composition which fall within the olivine region exhibit long, thin olivine crystals in a glassy matrix. Slags in the wollastonite region have a very fine-grained structure, making phase identification impossible. The copper-containing inclusions in the slag mainly consist of Fe-poor Cu 2 S, but FeO and SiO 2 may also be present. General assertions about the local copper production in Acqua Fredda should not be made based on the slag analyses conducted in this study.

The Cu ingot from Velem contains approximately 10 wt.% Sb, 4.5 wt.% As, 1.2 wt.% S and Ag, as well as other trace elements in an amount below 0.2 percent by weight. A Cu 2 S band running virtually across the entire width of the ingot suggests that it was formed by the reaction of molten Cu with antimonite (Sb 2 S 3 ). Remarkably, the generated cutting surface was not copper-colored but had a silvery appearance which, however, can be attributed to the high Sb content. The metallographic examinations revealed a very inhomogeneous structural composition of the ingot. Large areas consist of dendritically solidified Cu, while the interdendritic areas are characterized by mixtures of different phases of the elements Cu-Sb-As-S.

The Drassburg Bronze Age hoard find (approx. 25 kg) also comprises some sickles and sickle fragments. One of these sickle fragments was provided for metallographic examinations. A XRF analysis revealed 94.5 wt.% Cu, 4.31 wt.% Sn, 0.61 wt.% Pb, and 0.12 wt.% S in the alloy. It can therefore be attributed to tin bronzes. The material has a uniform dendritic microstructure. Higher concentrations of Sn, S, and Pb were measured in the interdendritic areas. This suggests a precipitation of the phases Pb and Cu 2 S from the residual melt. Klemm II etching also revealed the dendritic areas which exhibited different orientations. Deformations were observed in thin edge areas of the sickle fragments. They were introduced in the course of mechanical postprocessing of the bronze or during its use. The surface of the sickle exhibits a patina with a layered structure and a thickness of up to 500 μm. The Sn contents measured in the outer layers are somewhat higher than in the inner layers.

The metallographic preparation of samples from classical antiquity requires the use of very gentle preparation techniques. On the one hand, this is necessary since the samples provided are often very small and on the other hand, because the remaining sample material can be inserted back into the original object. Both sample preparation and microstructural evaluation may offer insights into the manufacture of the objects, for example whether casting, heat treatment or reshaping had taken place. In addition, corrosion products on the sample surfaces can be characterized. The results of the metallographic preparation of three different ancient copper objects are presented: 1. A bronze vessel from Gordion. 2. Bronze roof tiles from Caligula’s Nemi ships. 3. Roman copper nails from Carnuntum.