Hydrogen sulphide (H2S) is a natural component of the atmospheric biogeochemical sulphur cycle. The size of the budget, as well as the sources, sinks, and reactivity of H2S are discussed. Atmospheric concentration ranges of H2S, carbonyl sulphide (OCS) and organic acids (OA) for a variety of environments have been tabulated. The effects of H2S on people and artefacts are also summarised. It is also pointed out that in Europe/UK urban H2S levels are predicted to rise to 3-5 times their current levels over the next 15 years, and hence this is not a problem that will "go away".
Using anonymous case studies, all of which involved museums with silver collections, three scenarios are discussed. These included situations with a range (from 700 ppt (high) to l50 ppt (low)) of H2S levels but high (biannual silver cleaning) silver tarnish rates. It is concluded that in many cases H2S concentrations alone are insufficient to explain observed tarnishing rates. Other atmospheric chemical components working either separately (e.g. OCS) or synergistically with H2S (e.g. HCl, OA) combine to produce observed tarnish. The history of the artefact itself is also important as this will determine the number of reactive centres on the silver surface, and hence the tarnish rate. The other type of scenario examined was the accidental release of H2S leading to transient high concentrations (typically 10,000 ppt for < 24 hours). Depending on the furniture in the museum, this type of exposure can lead to very high concentrations (400 ppt) up to 6 weeks after the initial event. It is postulated that this is due to an adsorption/re-release mechanism on some woods and fabrics.
It is concluded that the process of silver tarnishing is not straightforward, nor necessarily the same in different museums. In terms of care of collections, whilst a survey of H2S concentrations is very important, continued monitoring is only appropriate in the context of mitigation measures and a museum policy on H2S levels.
Useful References & Further Reading
Watts, S.F.  The mass budgets of carbonyl sulphide, carbon disulphide and hydrogen sulphide. Atmos. Env. in press.
Watts, S.F. Roberts, C.N.  Catalytic Converters: implications for urban hydrogen sulphide and carbonyl sulphide concentrations. Atmos. Env. 33 pp169-170.
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