Degradation of archaeological horn silver artefacts in burials
© Marchand et al.; licensee Chemistry Central Ltd. 2014
Received: 6 September 2013
Accepted: 12 February 2014
Published: 24 February 2014
Archaeological silver objects from burial sites are of a grey-lavender aspect. The formation of AgCl leads to an increase in the volume of the objects, which may undergo a complete transformation into corrosion products. This degradation process has been little studied to date. In this paper, eight horn silver objects were studied by SEM and XRD.
Analyses showed a corrosion system composed of bi-layers: a compact inner layer and a porous outer layer. Corrosion products were mainly silver chlorides (AgCl), chlorargyrite. Some objects displayed copper inclusions both in the metallic core and in the inner layer. Highly mineralized objects contained other oxygen-rich phases (with Si, Ca) in the inner layer. Soil markers were detected in the outer layer.
Based on these results, we put forward a corrosion mechanism for horn silver objects. Silver chloride is formed by the disappearance of the initial silver oxide layer. This AgCl layer is not protective and supports ion transport. Thus an outer layer of silver chloride, incorporating soil markers, is formed.
Silver was one of the first metals to be used by humans, and is found in many archaeological objects such as coins, jewellery and ornaments . Silver was often alloyed with copper to enhance its mechanical properties. The best known Ag-Cu alloy is sterling silver (92.5wt% Ag / 7.5wt% Cu), which was commonly used for coins. Its degradation process is well known. Because of low copper solubility in silver, copper-silver alloys are composed of a silver-rich phase and a copper-rich phase . Alteration of this copper-rich phase causes the formation of green corrosion products on the silver object [2–4]. We find pure silver in many precious objects, like jewellery, which are often stored indoors in locations such as museums or churches. The degradation of these objects depends on the presence of pollutants in the atmosphere, a phenomenon which has been widely studied. The atmospheric corrosion of old silver causes a thin film of tarnish to appear [1, 5–7]: these black corrosion products are formed by sulfur pollutants such as hydrogen sulfide (H2S) and carbonyl sulfide (OCS). The corrosion layer is mainly composed of silver sulfide (Ag2S).
The deterioration of archaeological objects from marine environments leads to the formation of thick layers of corrosion products and concretions . The mixture of corrosion products frequently separates into two layers. The inner layer extends from the residual metal core to the original surface and does not contain calcareous or seabed materials. The outer layer extends from the original surface out onto the surrounding media. Concretion (a mixture of organic and inorganic calcite and aragonite (CaCO3)) is often only weakly bonded to the corrosion product layer . In marine silver artefacts, the main corrosion products are silver chloride AgCl, silver sulphide Ag2S and mixed silver chloride and bromide AgCl/Br. In aerobic conditions, concretions tend to protect silver artefacts. In anaerobic conditions, the main corrosion product is silver sulphide .
When silver objects have remained underground for a long period of time, a mineralization process also occurs. Generally, silver is corrosion resistant. Silver adsorbs molecules of oxygen which are partially converted into oxygen ions (O2–). Combination between metal cations and oxygen ions should result in the formation of an oxide (generally Ag2O) . Silver oxide is outstandingly protective. When humidity is high, more than two layers of adsorbed water are present and electrochemical reactions take place. According to the Pourbaix diagram , Ag2O is stable in mildly acid, neutral and basic solutions. However, chloride and sulfide ions tend to enlarge crystalline defects . Chloride, sulphur and oxygen were detected in a Roman silver artefact buried in soil . In a chloride rich environment, the objects can be completely transformed into AgCl. The presence of AgCl is always associated with horn silver. Horn silver objects are stable and are of a greyish aspect [2, 11]. In the presence of sulfate-reducing bacteria, silver objects contain silver sulfide (Ag2S) and chlorides (AgCl) [12, 13]. North et al.  explained this phenomenon by the alternative exposure of an object in a chloride rich environment to an aerobic (AgCl) and anaerobic (Ag2S) environment. In the case of typical archaeological silver objects found in burial sites, the main corrosion products are silver chlorides [2, 12, 14–17]. Silver chloride forms a brittle, finely granular layer. However, unfavourable conditions will result in an object being completely converted into silver chloride. Certain soil components (Ca, K, Al, Si, P…) interact with metal corrosion products . The formation of chlorargyrite (AgCl) in burial environments is still poorly understood . The types and mechanisms of metallic silver embrittlement were explained by R. Wanhill [17, 19–22]. However, for horn silver corrosion there is only a German study from 1979 .
The objective of this work is to examine the corrosion products of archaeological horn silver objects from burial sites. Several objects were available for this study. The metallic core and the corrosion layer were characterized by using a scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). Based on these observations, the corrosion mechanisms of horn silver will be presented.
Materials and methods
Description of samples
Horn silver brooch
Horn silver jewel
Corroded object, no metallic core
OM, EDS-SEM, μXRD
Horn silver jewel
Metallic core is partially present
Horn silver jewel
Corroded object, no metallic core
Horn silver earring
Presence of green corrosion products
Horn silver earring
Presence of a metallic core
Horn silver earring
Presence of a metallic core
Horn silver small spiral rod
Presence of a metallic core
Samples were embedded in an epoxy cold mounting resin, and polished with series of silicon carbide papers up to 4000 grit and 1μm diamond paste. After polishing, samples were stored in a desiccator.
Morphology observation and composition analysis
First, samples (S2 – S8) were examined by a Zeiss optical microscope with a 50× magnification objective lens. A Scanning Electron Microscope (SEM) (JEOL JSM5800LV – 15 kV or Stereoscan 440 – 20 kV) was used to observe the morphology of corrosion products, and Energy Dispersive X-ray Spectroscopy (EDS) was used to elucidate elemental composition.
Corrosion products of the brooch (S1) were analyzed by X-ray Diffraction (XRD) with diffractometer (X’Pert pro PANalytical) using Cu Kα1 and Kα2 (40 V and 40 mA) in 2Ө configuration. Sample S2 was analyzed at C2RMF (Centre de Recherche et de Restauration des Musées de France, Louvre Laboratory) by μ-XRD with a Rigaku microma×002 tube, a Kirchpatrick-Baez optical and a 2D R-Axis IV detector (45 kV, 660 μA for an incident beam of 200 μm).
Sample S1: corrosion products
Sample S5: silver alloys
Green corrosion products are present on sample S5. SEM-EDS analyses show the presence of 3 phases in the metallic core: a copper-rich phase (85at% Cu, 10at% Zn, 5at% Ag), a silver-rich phase (85at% Ag, 12at% Cu) and an oxygen-rich phase (57at% O, 10at% Zn, 5at% Ag). These green corrosion products are probably due to copper corrosion. As sample S5 is an Ag/Cu alloy, it will not be used in the study of horn silver corrosion.
Samples S2, S3 and S4: samples without metallic core
Samples S3, S6, S7, S8: samples with metallic core
Summary of results
Proposal for the corrosion mechanism
The formation of AgCl has been investigated by many scientific papers, but in other media and under different conditions. The formation of AgCl on silver electrodes has been specifically studied in chloride media [24–26]. Formation of the first layer follows an adsorption-desorption mechanism, then a thin film of AgCl grows due to diffusion. As the AgCl film is discontinuous, ion transport takes place via spaces between grains of AgCl. As the film thickens, the spaces between the AgCl grains close up and the ion transport occurs mainly via micro-channels running through the AgCl grains . In these studies [24–26], the thickness of the AgCl layer is of the order of tens of μm, and Ag+ ions are produced by the anodic potential. The formation of AgCl on archaeological silver objects occurs under different conditions. Accelerated corrosion tests were conducted to study the corrosion film observed on archaeological objects [14, 23], but these standard test samples also had a fine layer of corrosion products. Removing the protective oxide film is necessary to support the formation of silver chloride. In our analysis, we did not identify the presence of Ag2O. Jaro M.  suggested that in burial conditions (e.g. in crypts or graves), ammonia dissolves silver oxides, and silver ions react to form silver chloride, chlorargyrite (AgCl). Organic decomposition products (ammonia, nitrates, acetic acid, H2S) favour the initial corrosion process . Silver chloride does not create a protective layer, thus the metal can be completely transformed into silver chloride.
Horn silver has bilayer films that correspond to the corrosion process as identified by Payer et al. . A generalized model for this corrosion system is presented for metals in chloride solutions. Corrosion products form 2 layers: a porous outer and a compact inner layer. According to Payer’s model, corrosion is initiated by the protective layer becoming non-protective. In the case of horn silver we examine, it is thought that ammonia may have destroyed the silver oxides in the burial environment . This new non-protective layer then becomes an inner layer. A thicker outer layer then develops through the precipitation of soluble species (e.g. sediments) and further reaction of corrosion products (AgCl). The porous outer layer can permit the penetration of water and the diffusion of Cl. Cations are produced by the oxidation of metal at the metal/inner layer interface. High ion transport in the inner layer allows for the growth of corrosion films: Ag+ cations diffuse to the outer layer, and Cl- anions diffuse to the inner layer.
Copper inclusions do not seem to modify the corrosion mechanisms of horn silver. When copper is an alloying element (Sample S5), copper corrosion products are found in the corrosion layers. Otherwise copper inclusions are found only in the inner layer (and in the dark phase). Copper may play a local protective role through galvanic coupling. However, pure silver samples, such as S8, or samples with 2-5at% copper are subject to the same corrosion system: a compact inner layer and a porous outer layer of AgCl.
Horn silver artefacts have a specific corrosion system: the surface is deformed by dull grey corrosion products. The objects are weakened and sometimes entirely mineralized. Artefacts displaying this corrosion system are few and far between because the corrosion process requires specific conditions. The opportunity to analyze and observe several horn silver objects is exceptional. We studied the different layers of corrosion products and formulated a corrosion process. The inner layer is formed with chloride by the disappearance of initial silver oxide layer. This AgCl layer is not protective and there is a high level of ion transport. Below the original surface, the inner layer is compact and contains other oxygen-rich phases. Above the original surface, silver chloride precipitates in the porous and larger outer layer; it incorporates soil markers. The difference in porosity between the outer and inner layers ensures good cleavage on the original surface.
The authors would like to express their gratitude to archaeologists, Daniel Prigent and Marie-Laure Hervé, for providing horn silver artifacts. We also wish to thank Francois Mirambet for his μ-XRD analysis. We are very grateful to Clémence Cauvin, the student who participated in this work.
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