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Nuclear techniques produce first data on Greek coins

There is a mystery associated with one of the world’s first coinages, incuse Greek coins from the 6th century BC that may be solved by the combined efforts of numismatists (currency experts) from Macquarie University and researchers from ANSTO.

Nuclear techniques produce first data on Greek coins
Incuse coins are characterised by a relief design on the obverse (front) of the coin, 
which is repeated intaglio on the reverse side. They were produced in the Greek colonies
 of Southern Italy (modern day Basilicata and Calabria) in the 5th 
and 4th centuries [Credit: ACANS]
These ancient coins, which were produced for about 150 years in Greek colonies in Southern Italy, are less thick and are more finely crafted than coins produced in other locations at the same time, and Greek coins that were made later.

Researchers at the Australian Centre for Ancient Numismatic Studies (ACANS) at Macquarie University, led by Director Associate Professor Kenneth Sheedy, want to know how they were made, why they were made and why production stopped suddenly after only a century and a half.

ACANS approached ANSTO’s Bragg Institute to collaborate on a joint research program using highly sensitive neutron diffraction and imaging techniques to gain an understanding of the processes used to make incuse coins (see box)  and evidence for theories to explain their existence.

As there are no surviving contemporary accounts of Ancient Greek coin manufacture, no illustrations, and only three or four surviving dies that were used to imprint the coins with images, letters and numbers, the numismatists at ACANS needed a way to acquire quantifiable data while keeping the coins intact.

“How the coins were minted must be derived from the coins themselves without damaging them,” said instrument scientist Vladimir Luzin, who is a contributor to an article on the research to be published in Studies in Mediterranean Archaeology (SIMA) this year.

Ancient silver coins represent objects unique and of high value, hence any investigation must be nondestructive; that is, no original sample material should be removed and the object itself should not be modified in any way during examination (1).

The research team from the Bragg Institute has just completed a comparative study which set a benchmark for the measurements of ancient incuse and non-incuse Greek coins using neutron analysis techniques. Luzin would like to see this data used to develop a definitive source of quantitative information for reference purposes.

The production of coins involved locating a source of metals, refining the metals by smelting, and then producing blanks, or flans, in moulds. Metal dies were made and used to stamp images (types) and/or words (legends) into the flans. In striking a coin, two dies were needed; one die was positioned for the obverse side and the other for the reverse side. In incuse coins, the relief design on the reverse retained the shape of the image on the obverse side.  The flan was reheated before a hammer was used to set the image with a strike (2).

Nuclear techniques produce first data on Greek coins
The Metapontum Coinage Project jointly undertaken by the Australian Centre 
for Ancient Numismatic Studies and ANSTO [Credit: Chris Stacey]
The Numismatic Centre provided scientists at ANSTO with 34 Ancient Greek coins, consisting of 30 incuse coins from four different cities (Metapontum, Kroton, Taras and Caulonia) and four non-incuse coins dated from the 6th to the 4th century BC for investigation. In addition four other non-incuse silver coins were studied that are dated from medieval times and are originated from different regions of the non-Greek world.

The selected coins were studied by using three different neutron analysis techniques. All the coins were studied by crystallographic texture analysis using the Kowari instrument; twelve coins were imaged with neutron tomography using the Dingo instrument, and two coins compared by neutron powder diffraction using the Echidna instrument.

To further enhance result conclusiveness, a control groups of silver coins were produced under strictly-controlled laboratory conditions or stamped at a local metalworking facility using a modern coinage technique.

Instrument scientist Max Avdeev took measurements using the Echidna high-resolution powder diffractometer that accurately determined the composition of a coin that turned out to be a fake.

The two coins appeared to be very similar visually but an analysis of diffraction patterns and the identification of phases indicated the composition of the two coins was different. One of the coins had a bronze-copper core and silver outer plating, which was revealed without damaging it while the other was almost pure silver.

Another technique, neutron texture analysis on the residual stress and texture diffractometer Kowari was more helpful is answering questions about the ancient technologies used to produce the coins.

Using Kowari, the same neutron diffraction pattern can be systematically measured in many different directions by using the same Bragg’s Law to determine the so-called preferred orientation or crystallographic texture of the underlying crystal lattice of metal grains.

A graphical representation of the orientation distribution of the crystallites is known as a pole figure and it can be measured in the texture experiment.

“When metals are worked by forging or hammering, these actions cause the atomic lattices of the metal grains to realign themselves, producing a characteristic pattern of grain orientations that we call texture, which can be experimentally studied.

The physical conditions of the coinage process, temperature, amount of plastic deformation and heat treatment can be forensically reconstructed since the texture patterns are preserved in the metal,” explained Luzin.

The wide range of coins studied enabled a qualitative comparison of the incuse coins against similar incuse silver coins of the same period from different cities, silver non-incuse coins of the same period and silver non-incuse coins of later periods.

The incuse coins or non-incuse coins (e.g. a medieval silver penny) can reveal similarities or differences in texture pattern suggesting similarities or differences in the mechanical processes used to produce them.

The KOWARI data demonstrated that all incuse coins of the same kind were very similar in their texture characteristics and depicted a distinct pattern (symmetry and parameters of distribution) that is characteristic of a forging process.

“The pole figure for the incuse coins show characteristics for forging probably at an elevated but not hot temperature,” explained Luzin. Temperature is an important parameter of any metal deformation processes because it can cause the grain atomic lattices to realign themselves differently.

The incuse coin shared a circular pole figure pattern found in the crystalline structure of English pennies from the 13th century AD.

The medieval pennies are known to have a strongly compressive deformation which was caused by hammering sheets of silver to a thickness of about 0.5mm. Circular blanks were cut out of the sheets.

A blank was positioned between two dies and then struck with a heavy hammer without pre-heating the silver.

The pole figure for a silver Greek coin from the 4th century BC showed a similar texture pattern with weaker features that is characteristic of deformation caused by high temperature.

Coins from Naxos, which were minted at the same time, demonstrated a very different texture that indicated far less forging but rather casting or metalworking at a temperature close to the melting point of silver.

In 2009 the federal government awarded ANSTO $37M to build 4 new neutron beam instruments. One of these is the recently commissioned neutron radiography/tomography/imaging instrument Dingo.

Ulf Garbe, the scientist who built Dingo decided to test whether useful information about the coins could be obtained using tomography. Using Dingo in its highest resolution Garbe was able to resolve structural features within the coin down to 0.03mm.

“Reconstructed 3D virtual coins can be analysed in detail to reveal structural features related to the manufacturing processes, such as the presence of voids, inclusions, creases, corrugations and other defects, “ said Floriana Salvemini, an instrument scientist for Dingo  and who, coincidently, happened to be born in the South of Italy.

Among the interesting findings was the confirmation that one of the coins had a copper based alloy or iron core, which correlated the neutron powder diffraction measurement.

“The clear and contiguous surface at the interface between the core and the plate, together with an absence of joins, suggest the coin was crafted by applying thin malleable silver foil over the core. This may have then been welded together by thermal treatment,” said Salvemini.

One of the samples was characterised by high porosity (presence of voids in the bulk matrix) that could be due to rapid solidification in the mould or caused by gas that forms during casting.

The homogenous structure of another incuse coin which was free of inclusions and voids, suggests that the metal blank was subjected to further refinement before it was coined.

“Analytical techniques based on neutron beams that have been developed in the last decade have proved to be very useful in studies of ancient metal objects,” said Scott Olsen, who is the coordinator of the coin project at ANSTO.

He explained that they are often combined with other methods of analysis to provide information ranging from conservation status to the forensic reconstruction of smelting and metalworking processes.

The only neutron research facility in Australia is at the Open Pool Australian Lightwater reactor (OPAL) at ANSTO in Sydney.

The Bragg Institute at ANSTO operates 13 neutron beam instruments for academic and industrial research. Three of the instruments are either being commissioned or are under construction.

These techniques are important in the study of cultural heritage objects because they are non-destructive, insensitive to surface conditions and extremely useful for bulk objects.

As the neutrons can penetrate deeply into matter, they can provide information about the interior of bulky and metallic objects, unlike X-ray crystallography or analyses using electrons.

Incuse coins used in the research, held at Macquarie Universities Numismatic Centre, are part of a larger collection of 1267 South Italian coins donated by the late Dr W L Gale.  In addition to coins from the Greek cities of South Italy, the collection includes Roman Republic coinage, and Roman Imperial coinage from the time of Hadrian.

The use of neutron diffraction to study ancient coin manufacture is among the first in the world and is expected to attract world-wide interest from archeologists and nuclear physicists.

The data has been provided to ACANS, whose experts will infer the production steps based on their knowledge of ancient materials and technology available at the time.

“They have a theory to explain the uniqueness of the incuse coins and will use the quantitative evidence to confirm or refute it. The mystery may be solved, or not, but we have established the value of neutron diffraction and imaging for ancient coinage,” said Olsen.

References

(1) Flament, C., Marchetti, P (2004) ‘Analysis of Ancient Silver Coins,’ Nuclear Instruments and Methods in Physics Research B, Vol.226, pp.179-184.

(2) M Wickens, J M, The Production of Ancient Coins

Credit: ANSTO [April 10, 2015]
TANN

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