CHAPTER 3 – METHODOLOGY
3.5. Analytical study of the mounted specimens
3.5.4. Scanning Electron Microscopy
Two types of electronic beam instruments were used, a scanning electron microscope and an electron probe micro-analysis. The first instrument, mostly dedicated to the specimens from the Oberstockstall site, was a Philips XL30 Environmental SEM (ESEM) with an Oxford Instruments INCA spectrometer package and equipped with both secondary electron (SE) and back-scattered electron (BSE) detectors. For this type of analysis, the requirement for the samples was to be conductive; they were thus carbon coated, using a high vacuum evaporation and sputtering system. Pictures were usually taken both in SE and BSE modes (Fig. 3.6) at the same magnifications as the optical microscope to allow comparison (Fig. 3.7) between the images, and at higher magnification when necessary. The acceleration voltage applied to all analyses was 20 kV, the analytical distance 10 mm, the acquisition time 150 seconds, and the beam current set to reach a 30-40% dead time
Fig. 3.6. Photomicrographs of a matte cake in BSE mode (left) and in SE mode
(right) (500x), showing the difference in atomic weight (the brighter, the heavier the element) in the former case and the topography in the latter. Topography in polished samples is typically low, and a residual BSE signal dominates the SE image.
Fig. 3.7. Photomicrographs of a matte cake in plane polarised light (left) and in
backscattered electron mode in the SEM (right) (500x, long axis ~250μm).
The electron probe micro analyser used was a JEOL JXA 8600 with an integrated operating system for the WDS analysis and a separate Oxford Instruments INCA software for the acquisition and processing of the EDS data, similar to the one fitted to the ESEM. This instrument was also run at an acceleration voltage of 20 kV, with a beam current of 10 nA giving a dead time of 30-40% for a process time of 5, and an acquisition time of 150 seconds,. All the samples from the Angertal smelting site were analysed with this equipment. In order to allow for comparability of results between both electron beam instruments, several samples from this site were analysed in both instruments and compared in terms of the concentrations of elements identified, to check that the values given by both microscopes and their
precision were in the same range. Several standards were also run in both to further ensure the validity of comparing the data. The results of these overlapping analyses have demonstrated a good agreement between instruments.
3.5.4.1. Scanning Electron Microscopy - Energy Dispersive Spectrometry (SEM-EDS)
The SEM-EDS was used for detailed structural inspection and local chemical composition analysis both of the bulk and of single phases. The principle of this instrument relies on the excitation of the specimen placed in a high vacuum chamber by an electron beam. The excited sample emits distinct signals at different energy levels, such as secondary electrons (SE), back-scattered electrons (BSE), transmitted electrons, X-rays, heat, etc. (Reed 1996; Watt 1997). The SE emission is indicative of the topography of the sample while the BSE signal gives an image of the chemical and phase composition. The energy dispersive spectrometer (EDS) detects X-rays emitted by the sample, usually by means of a semiconductor crystal. These characteristic X-rays result from the excitation of the specimen by the incident electron beam. For each element, these X-rays are always of a specific energy and characteristic of the elements present in the sample; they therefore allow their identification. By measuring the intensity of the characteristic energies for each element that has been detected, it is possible to determine their concentration. The association of these two types of data, the high-magnification imaging achieved and the high spatial resolution of the chemical information, made the SEM-EDS ideal for this research.
3.5.4.2. Electron Probe Micro Analysis - Wavelength Dispersive Spectrometry (EPMA-WDS)
WDS was available on the ESEM but the set-up of the WDS mode on this particular microscope and the duration of each analysis were too time-consuming for the rather large set of samples to be studied. In addition, the information gathered from the EDS proved to be sufficient in most cases. However, for a relatively large number of the Angertal samples, WDS analysis proved useful to resolve energy overlaps between important elements of the materials analysed, such as lead, arsenic and sulphur. WDS was readily available on the JEOL JXA 8600 and allowed relatively quick analyses compared to the ESEM. Wavelength dispersive
wavelength diffracted by a crystal. The wavelength of the characteristic X-ray and the crystal lattice spacings are related by Bragg's law and produce constructive interference if they fit the criteria of this law. Unlike EDS, WDS reads or counts only the X-rays of a single wavelength, not counting a relatively broad spectrum of wavelengths or energies. This generally means that WDS is able to identify quantitatively elements, whose energies closely overlap and which are in lower concentrations in typical energy dispersive spectra. A customised WDS method including twenty of the most relevant elements detected beforehand by EDS in various areas of the samples was therefore used for the Angertal specimens. The elements selected for this method included sodium, magnesium, arsenic, aluminium, silicon, phosphorus, sulphur, silver, potassium, tin, antimony, calcium, titanium, manganese, iron, nickel, copper, zinc, gold, lead. The three crystals were thallium acid phtalate (TAP), pentaerythritol (PET), and lithium fluoride (LIF); TAP was used to detect the Kα lines of the lighter elements (Na, Mg, Al, Si) and the Lα line of
arsenic; PET for the Kα lines of P, S, K, Ca, and Ti and the Lα lines of Ag, Sn, Sb;
LIF was used for the Kα line of Mn, Fe, Ni, Cu, and Zn and for the Lα lines of Au and Pb.
This scientific methodology, which includes the morphological description of the samples, their microstructural examination and chemical analysis, was used to study the assemblages of Oberstockstall and the Angertal, and to some extent samples from the Bockhart and the Erzwies. The results of this science-based investigation were then combined with a critical review of contemporary written sources, to compare the theory and practice of the metallurgy of precious metals. The interpretation of this data firstly aimed at identifying as much of the metallurgical chemical processes represented by the various finds as possible. These were then further discussed with the purpose of understanding the skills, intentions and ideas of the people performing the processes, and placing their activity into the wider socio- economic framework outlined in previous chapters (cf. Chapter 2).