2 Methodology and analyses
Samples were taken from two axes (fig. 3), one from Museum Het Valkhof in Nijmegen (The Netherlands; labelled AC20) excavated at Ubbergen near Nijmegen and one from the Gallo-Romeins Museum in Tongeren (Belgium; labelled BH76), which is one of the axes from the Geistingen hoard.
Fig. 3 Sampling locations on axes AC20 from Museum Het Valkhof (left, courtesy: Rijksuniversiteit Groningen) and BH76 from the Gallo-Romeins Museum (right).
2.1 Samplesnext section
Both axes have a clear casting seam (figs. 2 and 3), indicating that both objects have been cast in a bi-valve mould. Seven samples were taken using a jeweller’s saw. These were taken from each half of the axe and from the casting seam. One sample from axe AC20 is heavily oxidised and is thus excluded from this analysis. The other six specimens are triangular in shape and have dimensions of about 2 x 1 x 1 mm3, so that a cross-section of the wall both lengthwise and crosswise is present. All samples used in this research originate from the upper part of the axe (i.e. the part closest to the original socket mouth, see fig. 3).
2.2 Analytical techniques
Compositional measurements on the same axes were recently taken by Postma et al. by means of Neutron Resonance Capture Analysis (NRCA; Postma et al. 2005a; Postma et al. 2011/in press). NRCA is able to detect elements within the ppm to 10-4 range if the elements show neutron resonances at energies in the range 1-500 eV. Elements that do not satisfy this criterion are for example lead, nickel and iron, which have resonances in the keV region and therefore require an energy resolution beyond the one presently used to distinguish their resonances from those of other elements in the object. Lead, nickel and iron can be determined in the per cent region. Compounds like oxides and sulphides can only be detected by diffraction experiments. NRCA is a bulk technique, it measures the composition of the entire object, including the patina. (Postma & Schillebeeckx 2005b)
This study provides two additional and complementary micro-scale techniques to determine the composition: X-Ray Fluorescence (XRF) and Electron Probe MicroAnalysis (EPMA). The detection limits of XRF generally are in the order of a few tenths of a per cent. The XRF instrument used has a silver tube to generate x-rays, which leads to a detection limit for silver of 1-2 wt%. EPMA can only measure a limited number of elements due to limited number of detection crystals and the detection limits range between 10-3 and 10-1. Whereas the composition per phase has been measured with EPMA, the fraction of each phase present has been deduced from electron micrographs. Combining this data leads to the overall composition that will be indicated as measured with EPMA.
The microstructure is analysed on the basis of Scanning Electron Microscopy (SEM), detailing phase fractions, dimensions and morphologies of the primary copper phases and inclusions. The microstructure is the crystalline structure of a metal that can be seen under a microscope. It is formed during the production process of the metal, more specifically during the cooling of the melt. During this process the atoms will order themselves in a crystalline arrangement. The resulting microstructure depends on a number of factors, including the chemical composition of the metal, the initial temperature, the cooling rate and the mould. The microstructure is also responsible for the final properties of the product. Consequently, by studying the microstructure of the Geistingen axes, a part of the production process can be reconstructed and the properties of the material can be discussed.