**2. Technological Background to Research**

The study of ancient silver coins includes aspects associated with minting skills, technological abilities, and political and economic considerations of a particular period, and can assist in understanding the connection between political and economic events [5–7]. Moreover, it can assist in understanding the economic choices of the minting authorities during times of crisis according to conflicts, as well as during times of reduced supply

of metal [8]. For example, throughout history, during periods of high inflation, the silver content of coins is lower than the standard. Hence, the concentration of silver during a specific period can be used as an indicator for the estimation of economic, trade, and social conditions [9].

During antiquity, silver was typically produced from silver-rich galena lead sulfide (PbS) ore containing approximately 1–2 wt% Ag [10]. The cupellation method was a multistage process engaging three separate hearths. The first hearth was used for enriching smelted impure lead that contained silver (bullion). Wood fuel was used in order to remelt the lead bullion at high temperature. The lead was oxidized to litharge (PbO), with a melting temperature of 880 ◦C, inside a bellows-powered tuyères. Additional bullion was added until an appropriate amount of silver-enriched lead was acquired. Next, the silverenriched lead was moved to a second hearth and oxidized again; however, at this stage, the litharge was removed by sinking iron rods into the hearth to create coated litharge cones on top of the rods. The rods were then removed, the litharge cones were thrown away, and the rods were dipped again, finally leaving silver globules inside the hearth. In the third hearth, several globules were melted and refined to obtain silver ingots and the remaining lead oxide was absorbed in the porous container wall (cupel). The cupellation method is a very effective process for producing silver metal with more than 95% purity [11].

A partial cupellation process resulted in silver alloy with lead presence. Hence, low quantities or absence of lead in the silver alloy points towards a successful silver refining process [12–14]. In addition, high content of lead transforms the silver alloy into a brittle material after a long burial period. Therefore, a good state of preservation of ancient silver objects is often connected to the absence of lead in the alloy.

Pure silver is a shiny white–grey metal with aesthetic appearance; it is a very soft metal that has excellent ductility and malleability [15,16]. Copper was a main alloying element in ancient silver coins. Ancient silver objects, including coins, are commonly available as silver–copper alloys with various ratios of silver and copper [17]. The addition of more than 3 wt% Cu to silver was frequently made to improve the mechanical properties of silver objects and also act as a melting-point depressant. Hence, the presence of more than 3 wt% Cu usually suggests that the copper was intentionally added [12,18–20].

Non-invasive and non-destructive testing (NDT) analyses of coins can be rather challenging due to various factors that change the surface metal composition, including long period corrosion processes and the presence of various corrosion products, tarnish and oxide layers, silver enrichment of the surface, cleaning residues, and conservation treatments [19,21,22]. The patina of ancient objects made of silver alloys may contain Ag2O, Ag2S, and/or AgCl, which can result in uncertainties concerning the obtained elemental data [9]. Therefore, the detected chemical composition obtained from the surface of the coin can be fundamentally different from the chemical composition over the entire volume of the coin [23]. Moreover, even when bulk material composition of ancient silver coins is obtained by destructive testing methods, there may be uncertainties concerning elemental analysis results that assume homogenous elemental distribution, which is often incorrect [21]. Therefore, measuring the composition in several different areas for each coin is recommended [24].

Surface-enrichment can be performed deliberately for technological and/or economic considerations or naturally due to segregation of the metals during casting and cooling stages, and because of corrosion processes [25]. For example, surface-enrichment, making silver–copper coins look like pure silver (silver depletion), was common during the Roman period. When silver–copper coin blanks were cast, they were kept at red heat condition to oxidize the copper on the external surface. Following the copper oxidation, the blanks were dripped into an organic acid bath which removed the copper from the surface, leaving a silver-enriched layer of up to a few hundred microns deep (usually up to a depth of approximately 200 μm). This process was employed by the Romans on silver–copper alloys with a composition of up to 80 wt% Cu, allowing the treated coins to leave the mint looking as if they were made of pure silver [26,27].

Copper located near the external surface of ancient silver coins can be oxidized and form corrosion products after a long burial period in aggressive environments; for example, cuprite (Cu2O) and tenorite (CuO) can be formed on the coin surface. When these minerals are removed from the surface, an Ag-rich exterior is achieved depleted in Cu [7]. Nevertheless, ancient silver coins have relatively high durability. In order to examine whether there had been a silver-enrichment of the external surface, Ashkenazi et al. (2017) [19] grounded the surface of a fourth-century BCE silver ring from the Nablus Hoard (Ring B) and found that the Cu wt% concentration at the surface of the ring (before grinding) was similar to the Cu wt% concentration of the bulk alloy (after grinding). This led to the conclusion that the SEM-EDS measurements of the well-preserved fourth-century BCE shiny silver metallic areas of the objects accurately represented the bulk concentration of the silver jewelry from the Samaria and Nablus Hoards. X-ray fluorescence (XRF) and inductively coupled plasma with atomic emission spectrometry (ICP-AES) analyses of southern Palestinian Persian period silver coins also supported this conclusion [19].
