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4 September 2015

SEM analysis at Birmingham University

One of the aims of my time here with the Staffordshire Hoard at Birmingham is to try and determine the method used by the Anglo-Saxon goldsmiths to enrich the gold (see my introduction blog). The previous analysis did not reveal how thick the enrichment layer was. Researchers were also struggling to find evidence of the solder that was used to attach components together as surface analysis could not differentiate the solder from the surrounding metal.

The decision was made that metallographic analysis of a small number of hoard objects should be carried out to answer these two research questions. This type of analysis needs access to equipment and instruments not available in Birmingham Museum and Art Gallery’s conservation studio. The School of Metallurgy and Materials at the University Birmingham have very kindly offered their facilities and expertise to the Staffordshire Hoard project. The Metallurgy and Materials Science department is a world class teaching and research facility. They are considered to be one of the leading schools for many areas of metallurgical research.

This new analysis project requires small samples to be taken from the objects, to reveal the cross section. However as the Hoard objects have been broken and dismantled in the Anglo-Saxon period it is possible to remove samples (if they had been undamaged sampling would not be feasible). Two samples of enriched sheet and a cross section through a sheet and wire were taken for this initial study (Figure 1).

Figure 1. Cross section through a wire and sheet (left) and a sample from the sheet of a hilt-plate (right).

Before analysis can start the samples need to be mounted in resin, and then ground and polished until they have a mirror like finish. Gold is tricky to prepare as it is soft and can smear or become contaminated during this process. I used the sample preparation facilities in the Birmingham metallurgy department which has a variety of equipment. As sample preparation is a messy business I wore the classical white lab coat (Figure 2).

Figure 2. Preparing the mounted samples for analysis.

Once prepared the samples were then ready to be analysed. The School of Metallurgy and Materials has eight different SEM-EDX (scanning electron microscope with energy dispersive x-ray spectroscopy) systems to choose from, each have their own specialities although all could do the required analysis for this project. With the help of a Gareth a PhD student at the university (Figure 3) I was trained on the Tescan Mira FEG-SEM (Figure 4). The acronym FEG stands for field emission and means that the machine delivers clear and sharp imaging, just what I needed for the tiny samples.

Figure 3. Gareth showing Ellie how to use the machine.

Figure 4. The Tescan Mira FEG-SEM.

The results from this work are potentially exciting but it is still early days. However the data still needs to be processed and compared to other research studies of solder and enrichment, and also to the gold-working recipes written by historical authors.

Figure 5. Sample in the SEM chamber.

Figure 6. Using the SEM-EDX to look at changes in composition at the surface of the sheets.

Dr Ellie Blakelock,
Staffordshire Hoard Scientist