Sampling and Analysis
Modern analytical techniques are increasingly sensitive and now can provide reliable and accurate data from very small sample sizes. A single 0.6mm diameter hole drilled into the cylindrical edge of a coin can provide a 12mg sample and this is sufficient for the measurement of fourteen chemical elements and lead isotope analysis.
Approximately 10mg of the drilled sample is accurately weighed into a sample vial and then dissolved with a mixture of hydrochloric and nitric acids (aqua regia) after gentle warming the solution is made up to 10ml volume. The acid mixture is required to ensure that all the elements of interest are dissolved, however, the hydrochloric acid combines with the silver to form an insoluble precipitate of silver chloride, turning the solution milky. Consequently, the solutions then have to be centrifuged and the silver-free supernatant liquid is then taken off for analysis. A second solution is therefore made by dissolving approximately 2mg of sample in only nitric acid which ensures that the silver is in solution (but not gold and tin) and can be measured.
The analysis of the main (aqua regia) solution is by inductively-coupled plasma atomic emission spectroscopy (ICP-AES) with the analysis of the silver being by atomic absorption spectrometry (AAS).
Major constituents: silver + copper + lead. But ‘silver bullion’ that was alloyed at the mint would have not been pure elemental silver, as measured by analysis, but would have been contaminated with significant traces of gold, bismuth and lead.
Trace elements: gold, bismuth, arsenic, cobalt, chromium, iron, manganese, nickel, antimony, tin and zinc.
Lead isotope ratios (208/204 + 207/206 + 206/204 + 208/204) have been measured by multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) by Vanessa Pashley at the NERC isotope geochemistry laboratory (NIGL).
Metallography of cut sections of several coins has also been undertaken by both optical and scanning electron microscopy (SEM-EDS).