Laser ablation helps to trace the origin of medieval metals

Archaeologists have long wondered why the people of Anglo Saxon England began using more silver coins and fewer gold coins between 660 and 750 AD. Researchers in Europe now say they have developed a method to help find the answer. This technology combines laser ablation with traditional trace element analysis to match the isotopic abundance of silver bars in coins with known sources of metal ores from that period.

Isotope ratios in historical coins
Previous generations of archaeologists classified coins and other artifacts based on their shape, inscriptions, and other easily visible details, while today's researchers explore these objects at the atomic level.

A team from the University of Oxford and Cambridge in the UK, as well as the Free University of Amsterdam (VU) in the Netherlands, selected 49 coins from the collection at the Fitzwilliam Museum in Cambridge. These coins were discovered in the Anglo Saxon regions of England, Frisia, and France along the North Sea coast. All of these can be traced back to around 670-816 AD, which historians sometimes refer to as the "long eighth century.". The team members are searching for lead isotope ratios in silver and concentrations of gold, bismuth, and other trace elements, both to understand the origin of silver and to gain insight into the details of medieval refining and smelting processes.

Using laser ablation
Firstly, the researchers placed these coins in a laboratory system called Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICPMS) at the University of Cambridge. The team used a laser energy of 6 J/cm2 and a 10 Hz pulse to ablate each coin for 60 seconds, and quantified the presence of 18 isotopes, including silver, gold, chromium, and zinc.

Jane Kershaw from the University of Oxford stated that the accuracy of a typical laser ablation device is about 10 times lower than that of collecting physical samples, dissolving samples with wet chemistry, and running them through a laboratory mass spectrometer. However, archaeologists hope to achieve the highest level of accuracy while minimizing damage to precious historical relics.

This is why portable laser ablation systems and their PTFE filters make archaeologists happy. "Through this approach, we can obtain high-precision minimally invasive sampling (equivalent to wet chemistry)," Keshaw said. "In addition, there are no chambers, so the size of objects we can analyze is not limited. This is a very new technology that has been applied to pigments in museum artworks, but we were the first to adopt this technology in archaeological silverware. There are only a few portable lasers around. You cannot purchase these machines, you must build them, and we are fortunate to be able to use the machines built by the Free University of Amsterdam."

The Origin of Silver
The research team found that the isotopic characteristics of the 29 earliest coins (dating back to before 750 AD) matched the silver extracted by the Byzantine Empire from the 3rd to 7th centuries AD. Later silver coins contained a small amount of gold, which was a characteristic of the silver mined in M é le (modern western France).

Keshaw said, "There is nothing else in archaeological records that can convince us of the existence of such a large amount of Byzantine silver in England, to the extent that it may have been the source of the first batch of post Roman silver coins." "The idea of the Eastern Mediterranean driving economic recovery in Northwest Europe is quite shocking. The results of the later coins (after 750 AD) are more in line with our ideas. Nevertheless, it still emphasizes England's dependence on silver from the European continent."

Keshaw has been studying medieval silverware for the past five years. "We use laser methods and direct physical sampling, always combining lead isotope and trace element data (especially important for gold and bismuth)," she said. "In the past, people were concerned that mixing and refining (excluding) silver would make the interpretation of results difficult or even impossible. However, we have been able to draw conclusions about Islamic silver, Viking silver, and now Anglo Saxon silver. We can model mixing, but refining results is not as common. These methods are powerful - paradigm effective!"

Next, the team will bring the recently developed portable laser ablation system from the University of Victoria to the Fitzwilliam Museum to measure lead isotopes in coins. The device consists of a 532 nm wavelength pulsed diode pumped solid-state laser, which is connected to a fiber optic and then connected to 100 nm μ A m wide aspherical lens focuses the beam of light on each coin. The PTFE filter collected trace amounts of ablative material, and the research team analyzed it using mass spectrometry.

Source: Laser Net