The sulfidation process of sterling silver in different corrosive environments: impact of the process on the surface films formed and consequences for the conservation-restoration community
© Storme et al. 2015
Received: 29 January 2015
Accepted: 7 July 2015
Published: 12 August 2015
Precious objects made of silver and/or its alloys tarnish and become black when exposed to ambient atmospheres containing moisture and ppb-amounts of H2S. Such objects usually contain small but variable amounts of copper as alloying constituent and this affects the corrosion process due to a preferential oxidation of copper. However the visual appearance of the formed tarnishing layers on different types of silver alloys is very similar. Therefore, conservators-restorers are confronted with the problem that in some cases certain cleaning techniques are very effective while in other similar cases the removal of tarnishing layers is unsatisfactory. Since cleaning experiments are not allowed on genuine objects, many investigations use artificially corroded dummies instead. In order to evaluate the representativity and reproducibility of this often used methodology, differences in morphology, microstructure and composition of the sulfide layers on sterling silver generated by different sulfidation methods were analysed.
Sterling silver samples were artificially aged in five different environments. The samples exposed to uncontrolled ambient air at different locations (e.g. residential and laboratory environments) showed different corrosion rates and corrosion colours. Three accelerated ageing methods were executed in a gaseous or liquid environment under controlled conditions. These tests showed different results in morphology, microstructure, composition, thickness and the interface between bulk and corrosion layer. A first accelerated sulfidation procedure in a Na2S solution alternated with exposure to air, resulted in a fast corrosion rate and an even corrosion layer formation with several S-species. A second series of sulfidation in a controlled gas environment of H2S and SO2 developed a thin but uneven corrosion layer, mainly consisting of oxides. A third corrosion methodology used was based on the thioacetamide method. This resulted in an even and relative thick corrosion layer, comparable to the Na2S/aeration sulfidation system. However, the interface between the corrosion layer and the bulk is importantly different, showing severe voids.
The corrosion layers generated by five different experimental sulfidation series on identical prepared sterling silver coupons were clearly different from each other. Analyses demonstrated that the composition and microstructure of the corrosion layers were strongly dependent on the sulfidation method used and copper was found to be an important element present in all sulfide layers analysed. Therefore, artificially corroded sterling silver is not necessarily representative for naturally tarnished historical objects and the extrapolation of the cleaning results obtained on dummies to historical objects must be performed with care.
Many historic silver objects are not made of pure silver but rather of silver alloyed with small but variable amounts of copper. For example, a typical alloy used in jewellery is sterling silver (Ag: 92.5 w%; Cu: 7.5 w%), which consists of an Ag-rich matrix containing Cu-rich inclusions. It is well known that small amounts of Cu play a crucial role in the corrosion process of silver alloys due to the preferential oxidation of Cu . As a result of this, the composition, microstructure and physical properties of tarnish layers formed on top of pure silver is substantially different from the ones formed on silver alloys, although all advanced forms of tarnish layers have a very similar visual appearance: black and dull.
Unsatisfactory cleaning performances It is known that electrolytic techniques , laser  or low temperature atmospheric plasma afterglow cleaning techniques [4, 5] exhibit certain difficulties on the removal of sulfide films on sterling silver contrary to pure silver on which the corrosion products can easily be reduced.
Unpredictable cleaning performance Usually, conservators-restorers do not have access to a full chemical analysis of the metal surface and have to rely on the visual appearance of the surface for the selection of the most appropriate cleaning technique. As visually similar surfaces may have a different chemical makeup, it is very difficult to predict exactly the cleaning performance, especially with low-invasive cleaning techniques. This might give the impression that a selected cleaning technique is unreliable or insufficient;
Unpredictable future corrosion When using a new cleaning technique; it is not possible to predict the future behavior of the surface without performing chemical analysis or without extensive practical experience.
In order to solve the problems described in the list above, cleaning experiments are needed. However, the Venice Charter of 1964 clearly states in article 10 that experimental techniques can only be used when the efficacy has been shown and proven by experience  or the ECCO Guidelines in article 9 that ‘The Conservator-Restorer shall strive to use only products, materials and procedures which, according to the current level of knowledge, will not harm the cultural heritage, the environment or people. The action itself and the materials used should not interfere, if at all possible, with any future examination, treatment or analysis. They should also be compatible with the materials of the cultural heritage and be as easily and completely reversible as possible’ .
For this purpose, cleaning experiments are usually performed on artificially sulfidized silver or sterling silver coupons. According to literature rather extreme corrosion conditions were used to generate sulfidized coupons which were considered by the authors as an imitation of the natural corrosion of silver such as immersing silver in hot aqueous solution of 0.1 M CuCl2, hot (50–60°C) Na2S·xH2O 2.5 g/l or low concentration H2S vapours (10 ppm) for 7 days [8, 9]. This suggests that some authors assume that the underlying corrosion mechanism for artificial corrosion is identical to the natural corrosion process and that the only difference would be the speed at which the process takes place. However, artificially tarnished dummies are not necessarily representative for natural tarnishing layers: (1) morphology, microstructure and composition of the corrosion layer might be dependent on the corrosion method used, and (2) there is no reason to assume that natural tarnishing layers can be described as a single corrosion state and the presence of a set of natural corrosion states must not be excluded. Moreover, objects may have different microstructures and copper distribution at the surface, may have underwent different use and storage conditions leading to a variation of corrosion layers, etc. [8–10].
The work presented here focuses on the influence of different tarnishing procedures to the obtained surface states on identical polished sterling silver coupons. The results and the knowledge gained from of this study will be beneficial for the testing of new cleaning techniques, for the understanding of the results obtained by certain cleaning techniques, and for the development of proper mitigation actions to reduce future corrosion.
The numerous analyses conducted over decades about corrosion rate, underlying mechanisms and the subsequently formed corrosion products, demonstrate that the corrosion process of silver alloys is affected by several parameters such as temperature, moist content, concentration of H2S [11–28]. Compared to pure silver, alloys have several additional properties (e.g. silver-rich and copper-rich phases, grain boundaries precipitation) that affect the corrosion process from the very first steps in the making of an object. For example, production methods such as casting, hammering, coinage, rolling or annealing have a considerable impact on the distribution of copper rich phases in the bulk and at the surface and influence the corrosion process [1, 8, 29].
Another parameter that must be emphasized is the finishing procedures which has been applied during alloy manufacture such as silvering, copper depletion, burnishing as a polishing technique, etc. [1, 29–31]. Such procedures had the intention to generate a silver-rich top layer just beneath the surface. Therefore, the copper content at the surface of a silver object does not always have a direct relation to the copper amount of the bulk. As an example, pickling is a well-known technique where the previously heated and oxidized object is immersed in a hot sulfuric acid solution in order to dissolve the formed oxide-products at the surface. After the pickling, the silver rich surface is burnished to obtain a surface with high gloss. In the use of the object, along with the wear and tear, it also has to be remembered that repeated cleaning alters the surface finish and removes also possible earlier formed copper corrosion products from the surface, since Cu-inclusions corrode preferentially. This results in a silver-rich top layer. However, extensive cleaning can also again remove this silver-rich layer. Therefore, the presence of silver-rich top layers or the amount of Cu-particles in a top layer can hardly be predicted on various objects.
For this research, in order to obtain a set of coupons with a very similar microstructure and copper distribution at the surface, all samples were cut from the same plate and polished in such a way that the surface state can be considered nearly identical to each other.
Sterling silver sheets with a composition of Ag: 92.5 w%; Cu: 7.5 w% and dimensions of 100 mm × 200 mm × 1.0 mm were purchased at Schöne Metals Corporation. Smaller coupons with a surface area of 20 mm × 50 mm were cut from a single sheet using a parallel metal cutting guillotine. In order to standardize the corrosion tests and to study the influence of copper rich inclusions at the surface, the top layer of the sterling silver coupons was grinded and polished before weathering. Polishing steps were performed with a soft cotton rotating wheel (1,850 rpm, radius of the wheel was 60 mm) with 3 M Finesse-IT water soluble liquid polishing paste followed by 3 M Imperial lapping water soluble liquid. Each polishing step took about 1 min. This method complies with historical abrasive polishing techniques. The cleaning of the polished surface was performed according to NBN EN ISO 8891:2000 by immersing the coupon in an ultrasonic bath with ethanol for 2 min. Afterwards, the coupons are rinsed with deionised water and dried with oil- and water-free compressed air. The visual appearance of the surface is very similar to that of silver objects, finished with a traditional mechanical and abrasive polishing technique as described in technical handbooks [32, 33].
Identical sterling silver coupons with a polished surface were exposed to five different sulfidation environments. Based on the discoloration of the surface, the corrosive environments were arranged from slow to fast proceeding corrosion
Slow corrosion process ⇔ Fast corrosion processes
Liquid + aeration
Uncontrolled and variable conditions
Uncontrolled and variable conditions
Controlled and constant conditions
Controlled and constant conditions
Controlled and constant conditions
Several coupons exposed to natural corrosion
Several coupons exposed to natural corrosion
Several coupons exposed to acid TAA gas atmosphere (NBN EN ISO 4538:1995)
Real-time corrosion in controlled relative humidity mixed with acidifying gases (H2S, SO2)
Several coupons immersed in alkaline 0.1 M Na2S solution + aeration (NBN EN ISO 8891:2000)
Maximum exposure time
Method 1 A series of polished sterling silver coupons was exposed to an ambient atmospheric indoor environment at a private residence just outside the historical center of Antwerp (Belgium) with successive prolonged exposure times for each coupon. The average indoor conditions were 17–22°C and 40–60% relative humidity.
Method 2 A similar experiment as in method 1 was performed in a chemical laboratory environment in the historical center of Antwerp (Belgium) with average conditions of 22°C ± 2 and 35–70% RH. The laboratory is fitted with a continuous ventilation system without filtration. For both methods 1 and 2, H2S concentration in indoor situations ranges from 86 to 600 ppt , which is below the detection limits of most sensors. Therefore, these values could not be recorded and the corrosion conditions must be considered as (partially) unknown.
Method 3 A series of samples were exposed to a H2S containing environment following the NBN EN ISO 4538:1995 thioacetamide (TAA) procedure: a saturated Na2SO4 solution in the desiccator generated a constant relative humidity at 91% while the TAA powder distributed across the horizontal plate in de recipient slowly decomposes to H2S due to presence of moist. Three coupons were sulfidized with exposure times of 24, 72 and 168 h respectively.
Method 4 In this method, a single sample was exposed in a gas tight weathering chamber to a well-defined atmosphere. A gas mixing unit provided the corrosive atmosphere, where synthetic air is mixed with any amount of relative humidity (0–95% RH) and acidifying gas (H2S, SO2) to simulate real world atmospheric corrosion conditions [34, 35]. In this method, a polished Ag925 coupon was exposed to a controlled gaseous atmosphere consisting of synthetic air, 90% RH, 500 ppb H2S and 500 ppb SO2. For the first 24 h of weathering the samples were exposed to 90% RH and 500 ppb H2S followed by 90% RH and 500 ppb SO2 for another 48 h, leading to a total weathering time in sulfur containing environment of 72 h.
Method 5 A series of polished sterling silver coupons were corroded by immersion in a 0.1 mol/L Na2S solution, combined with aeration following the norm NBN EN ISO 8891:2000. More specific, every cycle of 1 min consisted of an immersion of 12 s followed by an aeration of 48 s. The number of cycles needed to obtain a black tarnish layer was set to 60 (i.e., 60 min).
Techniques for analysis
Characterization of tarnishing layers
The surfaces of the untreated and treated coupons were characterized using a multi-analytical approach.
UV–vis spectra were collected with an Avantes spectrophotometer consisting of an AvaLight-DH-S-Bal light source combining a halogen-tungsten lamp for IR and visible light and a deuterium lamp for UV-light and the Avaspec-2048 spectrometer. Light source and detectors were coupled to a 50 mm integrating AvaSphere using fibre-optic cables. The surface was illuminated with an angle of incidence of 8°, while the partially reflected light was measured at 90°. This instrument was also used to determine the surface colour from a zone of 10 mm diameter by measuring the L*a*b*-values. The Avasoft software allows selecting a D65 illuminant and a CIE standard observer of 2° in order to calculate the L*a*b*-values from the collected UV–Vis spectrum. Colour variations were evaluated by calculating ΔE = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2. Measurements were taken as a function of time, except for method 4 since this is not possible with the specific experimental setup.
A SEM-EDX FEI Quanta 250 was used for the measurements on the samples of methods 1 and 2. The instrument is equipped with an Everhart–Thornley secondary and backscattered electron detector and an Oxford EDX detector (for semi-quantitative element analytics). For methods 3, 4 and 5, a Focused Ion Beam (FEI Quanta 200 3D) equipped with detectors for secondary and back scattered electrons and an EDX detector was used to obtain cross sections of the thin corrosion layers applying Ga-ions for the bombardment. The obtained FIB crater size was approximately 30 × 10 µm.
For surface topography images (secondary electron detection), elemental distribution images (backscattered electron detection) and cross-section analyses SEM-EDX measurements were performed applying a Philips XL 30 ESEM-FEG equipped with a detectors for secondary electrons, backscattered electrons, a large field detector for secondary electron measurements in low vacuum conditions, an electron backscatter diffraction (EBSD) detector (soft. OIMDC 5.1 fa. EDAX) and an energy dispersive X-ray detector.
Method 1, 2: natural but uncontrolled tarnishing in ambient air conditions
Ag925 coupons were exposed to two different uncontrolled indoor environments (method 1 and 2) for 6 months. Colour measurements were performed every month to track the change over time. During exposure in residential environment (method 1), visual changes could hardly be observed, while exposure of the coupons in laboratory environment (method 2), lead to a visually brownish tarnish layer which gradually darkened over time.
Method 3: thioacetamide (TAA) gas sulfidation method
Summary of quantitative results obtained from SEM-EDX spectra collected from the surfaces after sulfidation according to the respective methods 1–5, values in wt%. Since the penetration depth of electrons might be deeper than the thickness of the sulfide layer, interference with the substrate cannot be avoided
Non-treated samples average
Area 6 × 6 µm with corrosion grains (0.1–0.8 µm)
Area 6 × 6 µm with corrosion grains 0.5–3.0 µm)
Area with corrosion needles
Areas between agglomerated grains
Corrosion agglomerates (0.1–3.0 µm)
Area with corrosion grains (0.3 µm)
Method 4: controlled exposure to H2S, SO2 and moist
Here a corrosion process is executed under well controlled laboratory conditions (synthetic air, 90% RH, 500 ppb H2S and 500 ppb SO2) but using higher pollutant concentrations compared to most normal ambient air circumstances (c. 1 ppb). The calculated ΔE value after 72 h of weathering was 18.9.
Method 5: Na2S immersion-aeration method
For the corrosion of sterling silver in ambient atmospheric conditions (method 1 and 2), the laboratory environment resulted in a much faster corrosion than for the residential environment. These corrosion processes are regarded to be too slow for the practical use of sulfidizing coupons to perform cleaning experiments. Short production times, controlled conditions and reproducible samples are important issues and therefore, artificial corrosion methods 3, 4 and 5 seem to be more convenient to produce sulfidized sterling silver coupons. However, the sulphidation environments for the 5 methods are completely different: (1) the TAA method 3 is governed by the acid gas H2S while the Na2S immersion method 5 occur in a strong alkaline solution of pH = 14, or (2) some methods occur in a gaseous environment other ones in a liquid environment.
For the artificial corrosion methods 3, 4 and 5, the surface of the resulting sulfide layers appear with the naked eye very similar. However, at a microscopic level both surface and cross section show significant differences. The surface topography shows important differences between the different sulfidation environments with a needle-like structure for method 3 (Figure 6c), small grains on the surface (Figure 7a) and coarse grains, obscuring all surface features for method 5 (Figure 11d). The corrosion state is highly dependent on the exposure time. Also, the morphology and thicknesses of the formed corrosion surface layers formed by the method 3, 4 and 5 show significant differences in cross-section. This means that when corrosion conditions are not well controlled, dummies with a variation in corrosion states are obtained.
In addition, the chemical composition of the corrosion layer differs between the methods used. As shown in method 1, 2 and 4, the corrosion process can involve a strong formation of oxides. The chemical composition of the corrosion layers also shows a substantial Cu enrichment. This means that cleaning evaluations should not be focused on Ag2S removal only.
The cross-sections of the sulfidized samples demonstrated that the microstructure of the region just below the surface is more heterogeneous than expected. Within our studies we could clearly show that sterling silver samples treated by the accelerated tarnishing test methods 3 (TAA) and 5 (immersion method) show the formation of voids during the sulfidation process. It is known that copper is oxidized preferentially and oxidizes faster than silver. Therefore, the larger voids located in the vicinity of the interface (Figures 6, 10, 12) must be the result of a migration process (oxidation) of copper inclusions. These oxidized copper species diffuse towards the surface and react subsequently with the environment. The sulfide layer in the cross sectioned samples appear to be compact with a uniform thickness, suggesting that the corrosion products grow laterally over the surface. The residual gaps are in general not completely filled with bulk material or corrosion products within the monitored time frame although some solid material appeared to be present in these voids (see Figure 12). This shows that artificial corrosion procedures might affect the cleaning performances and evaluation.
It is a common practice that new cleaning techniques are tested and evaluated on artificial corroded dummies. For pure silver, several corrosion procedures have been used because they all appear to produce Ag2S. However, for sterling silver the tested corrosion procedures seem to generate variations in morphologies, compositions and voids below the surface. Therefore, it is possible that the cleaning performance might be affected by the corrosion procedure used. Moreover, several indications suggest that the natural corrosion state of sterling silver cannot be considered as a single corrosion state but merely a set of corrosion states. The variation between artificial corrosion procedures and within natural corrosion processes complicates the representativity of the cleaning tests for genuine historical objects.
Due to the complexity of the examined corrosion states of sterling silver in different sulfidation environments, there is no indication that the corrosion methods are identical. There were clearly differences between the corrosion methods used and between different exposure times of the same corrosion method. The most notable differences were: (1) the difference in morphology of the sulfide layer, (2) the absence or presence of sulfur in the tarnish layer, and (3) the presence or absence of voids.
From a conservation-restoration point of view, the results indicate that cleaning experiments on artificially sulfidized sterling silver coupons is indicative for historical results but should not be directly extrapolated to historical objects. For that reason, cleaning experiments on artificially sulfidized sterling silver coupons can be expected to be different in regard to genuine historical objects. The introduction of new cleaning methods (e.g. cold atmospheric plasma cleaning) goes along with a continuous learning process and gradual gain in experience where the complexity of dummies and genuine objects is increased step by step.
Precise measurements of the silver objects surface are necessary to identify elements and corrosion products prior to treatment, especially for the use of non- or low-invasive cleaning techniques, in order to increase the chances of attaining a predetermined target of treatment result. However, as shown in this article, surface measuring techniques (e.g. UV–Vis, SEM-EDX) are not always capable of revealing all of the relevant aspects that might influence the cleaning performance, such as voids or intermediate layers. To reveal the latter, cross sections should be taken. Moreover, the presence of metastable corrosion states and the presence of voids suggest that the corrosion process of sterling silver in insufficiently understood which also may hamper cleaning test evaluations.
PS: Acquisition and interpretation of data, drafting the manuscript, accountable for all aspects of the work. OS: Acquisition of data, analysis and interpretation of data, drafting the manuscript and revising it critically, accountable for all aspects of the work. RW: Acquisition of data, analysis and interpretation of data, drafting the manuscript and revising it critically. All authors read and approved the final manuscript.
The authors are grateful to the EU-project PANNA (FP7, no 282998) for funding, The Centre for Electrochemical Surface Technology CEST for the use of instrumentation and The Institute of Science and Technology in Art of the Academy of Fine Arts Vienna for the cooperation. More in particular, we thank the following persons: Christoph Kleber, Centre for Electrochemical Surface Technology, Viktor-Kaplan-Strasse 2, 2700 Wr. Neustadt, Austria; for the acquisition of data, analysis and interpretation of data. Manfred Schreiner, Institute of Science and Technology in Art, Academy of Fine Arts Vienna, Schillerplatz 3, 1010 Vienna, Austria; Supervision of a contributing research group. Joost Caen, University of Antwerp, Conservation Studies, Blindestraat 9, B-2000 Antwerp, Belgium; Supervision of the contributing Heritage & Sustainability research group. Karolien De Wael and Gert Nuyts. University of Antwerp, Chemistry Department, Groenenborgerlaan 171, 2020 Antwerp, Belgium; Supervision of the contributing AXES research group for SEM-EDX measurements.
Compliance with ethical guidelines
Competing interests The authors declare that they have no competing interests.
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