Remember when I told you about the mystery of incuse coins? It goes as follows: how did the ancient Hellenes mint coins which shows the same image on the front and back, but with the image on the back sunk into the metal so that it appears as a negative or incuse version of the front? Researchers at Macquarie University's Australian Centre for Ancient Numismatic Studies (ACANS) have joined forces with scientists from the Australian Nuclear Science and Technology Organisation (ANSTO), on a joint research program to solve a twenty-five century-old mystery behind the technology, and there are results to report.

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, thus reports the Archaeology News Network.

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. Temperature is an important parameter of any metal deformation processes because it can cause the grain atomic lattices to realign themselves differently.

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.

The pole figure for a silver Greek coin from the 4th century BC showed a 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.