Authentication of the Silver Coins Related to Achaemenid Era Based on Ancient Archaeometallurgical Studies

Despite the brilliant history of Iran, as one of the first promoters of coinage and the worthy collections of coins remained from ancient times, the cognitive study as an independent kind of science or as the historian supplement studies, was at less attention in Iran. In this research, 10 samples of silver coins related to the Achaemenian dynasty, were selected from a big collection of 57 pieces, which is kept in Pars Museum of Shiraz. This collection were recorded from the thieves of cultural relics in Marvdasht of Fars province in 2009, and was used for the elemental analysis at this article. As these coins are precious museum objects, sampling is very restricted, and the analytical method must be as non-destructive as possible. In different laboratories around the world, extensive methods of analysis are used in order to quantitative analysis of the elements, including particle induced X-ray emission spectroscopy (PIXE). This facility is available in Iran, so due to limitations in sampling and in using the other destructive methods in this study, PIXE analytical method was selected to be used in the analyzing process. Since the amount of metal elements plays an important role in determining the authentication of the Achaemenid coins, the percent of following elements were measured in the coin samples: Aluminum (Al), Sulfur (S), Chlorine (Cl), Calcium (Ca), Titanium (Ti), Manganese (Mn), Iron (Fe), Copper (Cu), Zinc (Zn), Silver (Ag), Lead (Pb), and, Gold (Au). This assessment was based on the assumption that the trace elements present in the sample, especially gold, could be considered as an indicator of the authenticity of the coins. To confirm the results obtained, more comprehensive studies, trying to the other sophisticated non-destructive methods, study of the phases, and metallurgical studies, are recommended.
Keywords: Authentication, PIXE, Silver Coins, Shekel, Achaemenid.
 
Introduction
Most of the museums and even the collectors of artistic works are nowadays faced with fake and fraudulent works, while dealing with the crisis of counterfeiting, fraud and harassment is very important for the understanding of the events of the past based on the works of art. Museums and art collections in Iran, since most of their work in the past has been obtained via archaeological excavations, did not suffer much from fake works. But todays, as the destructive aspect of history and culture of the country, they face a huge amount of forging of ancient works in the contemporary sense, which has a history of about 100-150 years. These works have been gathered from the thieves in museums, and the continuation of this trend will have concerns about the authenticity of works in the museums. Therefore, the exact identification and recognition of these art works by using scientific and laboratory studies, before entering into the museums, is very important. The cognitive study is considered as a noticeable sample of interdisciplinary kind of study, including some areas of studies such as typology, linguistics, chronometry, geometry, chemistry and physics, as the related sciences. Moreover, the design information on the works plays an important role in recognition of the politic, economic and social status of the ancient societies.
Because of the metal resistance of the coins, they will be less affected by the environmental degradation factors comparing to the other historic and ancient artifacts; and in many cases, they remain in almost good quality. This advantage sometimes causes bugs in authenticity of some collections which were not obtained from the scientific archaeological excavations. Thus, it was necessary to deal with this issue, due to the limited interdisciplinary studies in this field.
Reviewing the related literature has revealed that there has not been any substantive and codified research on the authentication of coins assigned to the Achaemenid period. In the present research, 10 silver coins of the Achaemenid period, which were kept at the Pars Shiraz Museum and were recorded from the cultural thieves in the city of Marvdasht of Fars, in 2009. In order to identify their authenticity, their elemental composition were studied by using PIXE technique, and the results obtained were compared to that of Achaemenid genuine coins of National Museum of Iran. The physical properties of coins, including motifs and their weight, have also been studied.
 
Results and Discussion
In order to determine the authenticity of the sample coins, the determination of the amount of elements by the PIXE chemical analysis method, and the study of existing designs and their weights, along with the comparison of the results obtained to that of the research on the Achaemenid genuine coins in the National Museum of Iran were used.
In measuring the concentration of the elements in the samples by PIXE technique, certain X-rays are directly irradiated onto the samples, where they interact with the sample atoms, which consequently, this technique does not require the sample preparation with certain materials, and the coins studied must just be cleaned. Therefore, for the comparison of the XRS results of the samples with that of the Achaemenid genuine coins, the concentrations of four main metallic elements, Pb, Ag, Au, and Cu were considered.
In the results of the elemental analysis, high and almost identical silver (98-99%) was observed in the studied coins. As a result, it can be said that the coins of interest are coins of the Achaemenid period, often coined in the same mint.
The pattern of dispersion of the minor elements in the sample can also represent the same source of ore used for their preparation and coinage, indicating the authenticity of the coins studied. So, the amount of the selected minor elements has also be measured by PIXE technique.
From the dispersion diagrams of the sub-elements in silver coins, the copper and lead dispersion graph is important to study. This diagram has showed that the dispersion of lead and copper in silver coins has a good overlap with the dispersion of these elements in the Achaemenid genuine coins, which may indicate the same ore used in the preparation and coinage of these coins.
 
Conclusions
In this study, the elemental analysis of the samples was realized by PIXE technique and the results obtained were compared to that of the Achaemenid genuine coins of the National Museum of Iran, which have been analyzed by Olyaee and her colleagues in 2015.
The dispersion pattern of silver and gold elements in both sets of coins showed a similar and almost identical pattern, which shows that both groups of coins are similar in terms of primary ore and coinage conditions. The statistical analysis of the results showed that metal elements such as silver, lead, gold, copper and the other trace elements in the sample of this study have similar grade changes to that of obtained from the analysis of 27 Achemenid genuine coins, where this could be a sign of authenticity of them; show probably the same provenance of ore used, mining technologies, and the similar multiplication method.
The archaeological studies of designs and weight of the samples, also confirms their similarity to that of the genuine Achaemenid Coins. The survey of literature revealed that the Achaemenid silver coins have the weight limited to 5.5±0.2 g, where the measurements of the coins showed that all the studied samples obey this limit of Akhaemenid coins weight.