Challenges in Silver Conservation: Characterizing the Composition and Sources of Unusual Tarnish on Seleucid Silver Coins Using SEM-EDS

Due to its status as a noble metal, one might expect that silver, a material abundant in antiquity and cultural heritage, would require little or no conservation, particularly in controlled museum environments.While silver demonstrates greater resistance to oxidation compared with other ancient metals like copper, iron, lead, and tin [1], it possesses a vulnerability that complicates its long-term preservation.Silver exhibits an electrochemical affinity with sulfur, a contaminant that is naturally found in unregulated indoor environments, primarily in the form of hydrogen sulfide (H 2 S) and carbonyl sulfide (COS) [2].This process, commonly known as 'silver tarnishing', is well-documented and characterized by the growth of silver sulfide (acanthite, Ag 2 S) on the metal's surface [3].Remarkably, airborne sulfur concentrations as minute as 0.2 ppb have been reported as sufficient to trigger silver sulfidation, with increased levels of moisture and sulfur accelerating the tarnishing rate [4].Prolonged exposure to ambient indoor conditions can result in tarnish layers beyond 100 nm thick [5], giving the originally sleek and shiny metal a black/grey discoloration and dull appearance [6,7,8,9] which, by many curators and patrons, may be considered aesthetically displeasing.Consequently, conservators may be tasked with removing the tarnish chemically or mechanically using particles to abrade the surface, as necessary to reduce the crystalline silver sulfide [10].At the same time, unless the underlying environmental factors contributing to tarnishing are addressed, the silver artifacts will continue to react with the sulfur, requiring continuous treatment to maintain their appearance.This tarnishing/treatment cycle will cause the progressive removal of surface atoms, resulting in the loss of surface details that characterize the cultural heritage of the object.As such, the long-term preservation of silver artifacts necessitates the maximally achievable elimination of tarnish-inducing agents from their storage and/or exhibition environments.The exact sources responsible for the accelerated development of silver tarnish, however, are not always evident.Featured in this work, is a case study of two extensively tarnished silver coins originating from the Seleucid dynasty of the Hellenistic period (117-118 BCE).Having been on display for nearly four decades, these coins had developed significant surface tarnish, as inferred from visual inspection.Contrary to the typical description of advanced silver tarnish in literature, their physical appearance, as shown in Figure 1, is neither entirely 'black/grey' nor 'dull'.Notably, localized areas exhibiting vivid and distinct colouration alongside a 'glossy' almost 'oily' surface quality prompted an inquiry into the contaminants responsible for the clearly non-silver appearance of the coins.To identify the causes behind the coins' pronounced tarnishing and 'glossiness', as well as to formulate targeted remediations for eliminating the tarnish-inducing contaminants in and around the display case, elemental characterization of the coin surfaces was conducted using non-destructive analytical techniques.Micrographs and elemental spectra were captured using a Hitachi SU-7000 Schottky Field Emission Scanning Electron Microscope (SEM) combined with an Oxford Ultim Max SDD for Energy Dispersive X-ray Spectroscopy (EDS), operating with an acceleration voltage of 20kV.Prior to SEM imaging, the coins were gently swabbed with ethanol to eliminate loose surface particulates and excess carbon contamination.As illustrated in Figures 2 and3, EDS analysis of both tarnished coins revealed the presence of sulfur, chlorine, and carbon within the uniformly distributed surface tarnish.While the detection of sulfur was anticipated, the significant presence of chlorine was surprising.Although conservation literature acknowledges silver as prone to corrosion attacks by chlorine [3] and identifies chlorine as a common constituent of silver tarnish [7], its study in the context of silver tarnish is limited, with experimental simulations predominantly focused on mechanisms of the sulfidation process.The accumulation of sulfur on the coins was mainly attributed to airborne emissions from gastrointestinal and metabolic processes (humans) within the gallery, whereas chlorine was linked to potential off-gassing from polymer-based materials, like polyvinyl chloride (PVC) components within the display case and/or the microclimate unit feeding its air.Given the observed 'glossy' surface texture of the coins, it was hypothesized that an organic film composed of hydrocarbons might be overlaying the silver tarnish.However, solely relying on EDS spectra, which reported significant carbon signals, made it challenging to definitively classify it as an organic coating rather than copious amounts of carbon contamination, as expected due to limitations with sample cleaning.To provide supplementary evidence for the presence of a hydrocarbon film, an SEM technique for visualizing the response of organics to a focused electron beam was employed, with results depicted in Figure 4.This methodology, utilizing a low-energy beam at an accelerating voltage of 3kV and a magnification of up to 45,000x, enabled the observation of the surface actively excited into motion by the beam.This surface activity is attributed to radiolysis, which involves the breaking of weak covalent bonds [11].Given that silver sulphide is an inorganic crystalline corrosion product, the observation of radiolysis on the surface of the coins, combined with the significant amounts of carbon detected by EDS, suggests the presence of an organic coating.This 'glossy' film is attributed to hydrocarbons (oil and grease) sourced from a kitchen environment [3] which likely shares ventilation and piping systems with the Greek gallery that supplies the air into the display case.

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