Silver isotope analysis
Application
The isotope ratios of chemical elements in natural rocks and cultural-historical artefacts can vary. One reason for this is the radioactive decay of unstable isotopes, for example rubidium, uranium or rhenium, which leads to variable isotope compositions of strontium, lead or osmium. On the other hand, stable isotopes are also separated from each other due to their mass differences in the course of physical (diffusion, vaporisation), chemical (smelting, oxidation, reduction) and biological processes. Such processes are of great importance both in the production of cultural-historical objects (metals, alloys, slags, ceramics, glass) and during the formation of their source materials (ores, minerals).
They give the reaction products a specific isotopic fingerprint. This may contain two pieces of information: firstly about the nature of the element source and secondly about the transfer process from the source to the reaction product. The isotopic fingerprint in the respective materials can then be used to answer questions regarding origin, production technique or authenticity. The measurement of stable isotope ratios to answer archaeological questions is in its infancy, and details of isotope systematics still need to be researched.
A single isotope system often does not provide a clear fingerprint for determining the origin, as deposits themselves can exhibit large variations in isotope compositions. Hypotheses can be tested by integrating additional data from other isotope systems, other geochemical methods or from the historical sciences. CEZA has state-of-the-art analytical equipment, extensive databases and natural scientists at its disposal to advise on and interpret the analytical data.
Basics
Silver has only two isotopes, so the materials examined must be characterised using the isotope ratio 109Ag/107Ag. This application of the isotope ratios of silver to archaeological and geological problems is in its infancy and only a few systematic investigations have been carried out to date. In nature, silver occurs solid and often in combination as sulphide (e.g. argentite), arsenide (e.g. proustite) or antimonide (e.g. pyrargyrite). Important silver carriers, in which silver is an important secondary component, are pale ores, galena and gold. These minerals are of hydrothermal origin and such deposits are supplemented by secondary compounds formed during weathering.
This diversity suggests a wide variation in the isotopic composition of silver in nature. Silver and gold are often transported together in hydrothermal systems and surface waters. Silver isotope data in natural gold can therefore be used to find the source rocks of secondary gold and to investigate the physico-chemical processes that lead to gold deposition. The above-mentioned minerals can also be the source of archaeological silver and gold artefacts (coins, jewellery, relics) that may reflect the silver isotopic signature of the source. It is also very likely that statements can be made about manufacturing techniques and their changes over time. This could make it possible, for example, to distinguish modern from historical processes and thus genuine from fake artefacts.
CEZA offers to determine the isotope ratios of silver in solid and archaeological gold as well as in silver artefacts and smelting products. Details of the analytical method and an example of the application of the silver isotope composition can be found in the following scientific publication:
Brügmann, G.; Brauns, M. & Maas, R., 2019, Silver isotope analysis of gold nuggets: An appraisal of instrumental isotope fractionation effects and potential for high-resolution tracing of placer gold. Chemical Geology, 516, 59–67, https://doi.org/10.1016/j.chemgeo.2019.03.015.
Sample composition
The sample size depends on the element concentrations in the sample, which can be very variable! For metal samples (bronze, silver, gold, tin) or cassiterite, we recommend a sample size of 50 mg. From a purely technical point of view, 1 mg or less may be sufficient. However, this small quantity raises the question of the extent to which it is representative of the total sample and whether the analysis results can be interpreted meaningfully. Representative sample quantities of copper, tin and silver ores should be at least 0.5 g, as such materials are very heterogeneous by nature.