Elemental characterization of surface and bulk of copper-based coins from the Byzantine-period by means of spectroscopic techniques

Abstract

Thirty-three Byzantine copper-based coins (so-called anonymous follis) from 970 to 1085 A.D. were investigated in order to determine the elemental composition both of the bulk and of the surface. The ancient coins analysed − belonging to the Thomas Bertelè Collection, now located in the Bottacin Museum of Padova (Italy) − were studied by way of physical, chemical and chemometric testing techniques. This study aims to obtain information on socio-economic aspects of the Byzantine Empire using chemical and physical data of currency as indirect indicators. First, the weight, diameter and thickness of each piece were measured, then a micro-Energy Dispersive X-Ray Fluorescence (µ-EDXRF) technique was applied on a cleaned and polished area of the surface of the coin to evaluate the presence of minor elements in a non-destructive fashion. A micro-destructive sampling method for both surface and bulk samples, based on mechanically drilling the edge, was finally employed, and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) was used for the quantitative 2 determination of Cu (the major alloying element), Pb, As, Fe, Zn, Ag, Ni, Sn and Mn (minor elements) in the powder which resulted from drilling each coin, previously dissolved via aqua regia dissolution technique overnight at room temperature.The average copper concentration of the folles was 95.9±3.1% for the bulk and 91.7±3.5% for the surface. The Pb-Cu ratio appears to be an index which can be used to discriminate between different coins, in particular for Class B (from 1030 to 1042 A.D.), as confirmed by exploratory chemometric data analysis. According to our results, all the anonymous Byzantine folles under examination were minted in much the same way and using the same mineral raw materials. Using the chemical data measured on the bulk, we can grasp that the monetary choice of the period under study excluded silver, gold and bronze (concentrations of Pb is low, while Sn resulted undetectable or unquantifiable) to turn, substantially, to a copper-only coinage (deducted the impurities given by the gangue of the starting minerals). Moreover, along the observed period of 120 years, the mass of the coins was significantly reduced (with an overall loss of material of approximately 60%).

Sasanian copper and billon coins from the collections of the Royal Museums of Art and History, Brussels, Belgium—insights using semi-quantitative analysis by μXRF

“…Conversely, tin is enriched on the surface when it is present in conjunction with zinc, whereas in high-tin bronzes it is usually depleted in the corrosion layer (Gaudenzi Asinelli and Martinón-Torres 2016). Crosera et al (2019) found that Cu was slightly depleted on the surface (91.7 ± 3.1% determined by μXRF analysis) compared to the bulk (95.9 ± 3.1% determined by ICP-AES); however, taking into account their uncertainties, both results actually overlap.…”

What You Clean Is What You Get: A Novel Chemical Cleaning Technique and the Interpretation of Corrosion Products Found in Late Roman Copper Alloy Coins Retrieved from the Sea

“…In order to document the cleaning procedure's effect and trace the particularities of the corrosive processes that altered the MMB coins, different analytical methods and measurements were applied. Given the coins' fragility, these methods were required to be sensitive and non-destructive, as is anyway preferable with archaeological objects [26,27]. Our analytical procedure consisted of the following:…”

Archaeometric Characterisation and Assessment of Conservation State of Coins: The Case-Study of a Selection of Antoniniani from the Hoard of Cumae (Campania Region, Southern Italy)

“…This type of patina consists of copper, chlorine and traces of oxygen, aluminium and phosphorous (Figure 8b,c; Table 3) but, unfortunately, it was not identified by Raman spectroscopy. The formation of chlorides on the surface of a buried metal product is due to a humid or saline underground environment [18,73]. The chlorine ion attacks the copper from its oxides or carbonates transforming it into cuprous chloride and for this reason they often stratify on the surface of the coin [74].…”

“…The necessary condition to have a reliable investigation is the homogeneity of the surface, which is difficult to have on objects that have been buried for a long time and which must not be further altered by diagnostic analyses. For this reason, a semi-quantitative significance is often assigned to portable X-ray fluorescence (pXRF) surface analyses [18]. Several authors have tried to use the pXRF technique for the chemical analysis of the alloy composition, in part because it consists of portable instrument that can be easily transported into the museums where ancient monetary collections are kept [19][20][21][22][23].…”