In summer 2005, a silver coin hoard was discovered on the Athenian Agora just a few inches below the ground in a building previously identified as the Strategeion, the headquarters of the generals. The building was being re-excavated because its identification had recently been called into question. The coins, corroded together from hundreds of years of burial, were recovered in one lump by the archaeologists, apart from 46 coins that were excavated separately.
Once out of the ground, the hoard was brought directly to the conservation laboratory. Here, each of the 46 loose coins was weighed, and it was noted that although they are not similar in shape they are remarkably similar in weight. The total weight of the lump was then divided by the average weight of the loose coins and it was estimated that the hoard consists of approximately 400 coins.
The coins are covered with a lavender-gray layer of corrosion acquired during burial, but they are still quite legible: they all bear the goddess Athena’s head on one side and her owl on the other. Within each group, owl or head, there is evidence that different “Athena” and “owl” dies were used to strike the coins. Based on the size and weight of the coins, and on stylistic criteria, our numismatist Irini Marathaki was able to determine that the coins are tetradrachms, struck in Athens, and most probably date to the second half of the fourth century B.C.
A selection of three coins before treatment. The coins, Athenian tetradrachmas dating to the 4th century B.C. show quite clearly the head of the goddess Athena (obverse-top) and an owl (reverse-bottom).
But even more can be learned from this exceptional find: The Agora’s numismatist plans to carry out a study of the dies from which the coins were struck. One aspect involves closely examining the coins’ surfaces, which are hidden below the corrosion, for thin raised lines indicating the die used to strike the coin was cracked. Such cracks continued to propagate with successive strikings, creating larger and larger lines on the surface of the coins, until the die eventually broke and a new one was brought into use. From this study the archaeologists hope to gain a better understanding of the manufacturing techniques and circulation of the coins, which in turn may help address unanswered questions about the ancient building in which they were found. Therefore, the conservation lab was asked to reveal the original surface of all 400 coins.
Silver coin before and after conservation treatment.
Visual examination of the coins using microscopes suggested the corrosion is composed of a particular type best treated with a chemical method that loosens the corrosion layer and reduces the corroded silver surface back to metallic silver. To be sure that the proposed treatment was appropriate, it was necessary to confirm the composition of the corrosion crust as well as the alloy content of the silver before proceeding. A selection of coins was chosen for micro X-ray fluorescence (μ-XRF) analysis, an entirely nondestructive technique that uses an exciting X-ray beam to produce characteristic X-rays of the individual elements that compose the analyzed area. To prepare the coins for analysis, each one was placed in a water tank and ultrasonic waves were used to eliminate the loose dirt. Harder crusts, formed on the surface of the coins during burial, were removed with a scalpel and pin tools under a microscope. Scientists from the Institute of Nuclear Physics at Greece’s National Center of Scientific Research “Demokritos” kindly performed the μ-XRF analysis. The results indicated the coins are composed of very pure silver and the corrosion layers are composed mostly of silver compounds for which the chemical reduction treatment is appropriate.
At the beginning of 2009, 124 coins from the hoard had been cleaned by the reduction method, including the first coins removed from the lump. Each coin requires from two to five hours’ treatment time, not including the immersion times of the coin in the initial wash bath, the reduction bath, and the final rinse bath. But it is certainly worth it: The treatment so far has produced a brilliant silver surface and clarified minute details of the coins’ relief motifs.