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

Background: 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. Results: 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. Conclusions: 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. © 2015 Storme et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Open Access *Correspondence: [email protected] 1 University of Antwerp, Conservation Studies, Blindestraat 9, 2000 Antwerp, Belgium Full list of author information is available at the end of the article Page 2 of 15 Storme et al. Herit Sci (2015) 3:25 Background 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 [1]. 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. Conservation strategies aim to reconstitute the original appearance (i.e. colour, gloss, texture, engravings, etc.) of an object while preserving as much as possible of the original information (e.g., traces of the original production techniques, hallmarks, wear traces, aesthetics) enclosed in objects. However, proven cleaning techniques which are accepted in the conservation community come short for some specific cases (e.g. silver combined with organic material parts). Some advanced low invasive cleaning techniques (e.g. plasma or laser cleaning) might be a solution but their performance is highly dependent on the amount of copper based compounds in the tarnish layer which cannot be determined just by the visual appearance of the surface state [2]. For new developed techniques to be approved by the conservation community, following issues should be addressed: • Unsatisfactory cleaning performances It is known that electrolytic techniques [3], laser [2] 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 [6] 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’ [7]. 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