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23 October 2015

Silver Analysis

For today’s blog I will provide a brief update on what I have been up to since starting the Staffordshire Hoard metals analysis here at Birmingham Museum and Art Gallery.

The gold has been the focus of analysis for the last two years, but there are also plenty of silver objects in the Hoard. So a project has been developed to analysis a selection of the silver objects. The first stage of many research projects is to develop a methodology for the study and identify the types of questions that could be answered. A small number of hilt-plates were selected for a pilot study (Figure 1).

Figure 1a. Hilt-plates analysed during the study – Front of hilt plate K593.

Figure 1b. Hilt-plates analysed during the study – Front of hilt plate K138.

The main technique I will be using to analyse the silver is X-ray fluorescence (XRF). Each element gives off a different energy and each of these energies is recorded as separate peaks which form a spectrum for the area being analysed (Figure 2). The XRF instruments software uses the spectrum produced to determine what elements are present (Figure 3). The amount of each element present is calculated by measuring the size of peaks for pure elements, and comparing it to the size of the peaks in the sample.

Figure 2. Spectrum showing gold, silver and copper as separate coloured peaks.

Figure 3. Spectrum from a gold object from the Staffordshire Hoard.

So far the analysis of the silver has revealed the presence of gilding on the surface. This can often be seen on the surface of the object but occasionally it is not very obvious. The analysis of the silver objects has confirmed that many of the objects have been mercury gilded (Figure 4). Mercury gilding uses a gold-mercury amalgam produced by adding fine particles of gold to heated mercury. This is applied to the surface of the silver, and when heated the mercury turns to a gas which leaves a film of gold on the surface. Often some of the mercury remains in the gold on the surface, and this can be detected using XRF. This process is would have been very unsafe, as the mercury gas released would have been toxic if inhaled.

Figure 4. Compares spectrum for a gilded silver object (red) with that of a silver object (blue). Both gold and mercury (Hg) have been detected by the XRF on the gilded object, thus none of the silver underneath the gilding layer has been detected.

Now the pilot study is complete a much larger study of more silver objects from the Hoard is being carried out. This will hopefully reveal more about the alloy choice made by the goldsmith and may also reveal more about the manufacture of the silver objects in this period.

Dr Eleanor Blakelock,
Metals Specialist