Reaction of Thiourea with Silver Sulfide Tarnish Layers on Silver Surfaces

Silver objects react with atmospheric gases, in particular S-containing species such as H2S, which results in formation of a disfiguring dark tarnish layer of Ag2S. Silver and its alloys have been greatly used in historical objects and the development of tarnish detracts from the physical appearance of objects. Tarnish layers are removed using thiourea containing cleaning solutions, an effective ingredient in the removal of tarnish, but the mechanism of removal is unclear. The main purpose of this project was to characterize the chemical reactions taking place between thiourea and Ag and Ag2S tarnish layers. In order to replicate the Ag2S corrosion layers, pure polished silver samples were tarnished in the laboratory using H2S gas and Na2S solutions. The chemical composition and the topography of pure polished and tarnished silver surfaces were examined using X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM) and Inductively-Coupled Plasma Mass Spectrometry (ICP-MS). The corrosion layers developed by H2S(g) and Na2S(aq) did not produce identical tarnish in terms of morphology and composition. All of the corrosion surfaces contained some unreacted metallic silver within the XPS sampling depth. However, corrosion by H2S(g) produced the Ag2S layer with the lowest Ag:S ratio. The role of individual ingredients of acidified thiourea in removal of silver was examined. Treatment with water and the detergent resulted in no significant changes on Ag and Ag2S corrosion layers. However, the presence or absence of acid produced a change in the species observed on the surfaces. The results suggested that thiourea was the active ingredient and played a major role in removal of silver from Ag2S surfaces. However, neither the separate ingredients nor the combined acidified thiourea mixture removed silver from pure silver but are only active on surfaces containing Ag2S. H2S(g) corrosion layers produced the highest concentration of Ag+ after treatment with acidified thiourea solution. Composition analysis indicated that Ag2S tarnished samples treated with acidified thiourea solution exhibited an overall loss of S-containing species. However, a large amount of S remained on the surfaces. Similarly, a noticeable increase in N levels was also observed on tarnish surfaces after thiourea treatment. The mechanism of action of the thiourea has not been elaborated but it appears to involve decomposition of the thiourea molecule, leaving some N-containing product bound to the silver sulfide surface. Our study strongly suggests that there is a specific mechanism which enables thiourea to react with Ag2S but not with Ag. This specificity could be due to binding of thiourea through its sulfur atom to sulfur in Ag2S forming a structure such as Ag-S-S-C(NH2).