Tarnishing of Silver: A Short Review

The refurbishment of the Silver Galleries on the South Kensington V&A site has prompted us to re-examine our processes and procedures for protecting the silver collection. This review presents a synopsis of the literature survey that I have carried out over the past nine months.

Silver tarnishing takes place through a variety of mechanisms. In general, this leads to a disfiguring layer on the surface of the silver that is visually unacceptable. However, other corrosion processes are also taking place. We must concern ourselves with both the visible tarnishing and the less visible corrosion problems. To preserve our silver objects to the best standard, we must reduce both these forms of corrosion.

The main product of silver tarnishing is silver sulphide. The reaction mechanisms are:
8Ag + 4HS^- <---> 4Ag₂S + 2H₂ + 4e^-
0₂ + 2H₂O + 4e^- <---> 4OH^-

The first reaction is believed to occur in a thin film of water on the silver surface. In dry air, tarnishing does not take place. In the second reaction, oxygen acts as a cathodic species and consumes electrons as indicated in the equation. Higher concentrations of hydrogen sulphide increase tarnishing. Although the rate of tarnishing gradually declines with increased tarnish layer thickness, the reaction proceeds even on a heavily tarnished surface, since, owing to its coarse structure, the silver sulphide does not form a protective layer against surface corrosion¹. When relative humidity (rh) is between 5 and 50%, the amount of absorbed water on the surface is approximately constant and the reaction rate is steady. However, between 70 and 80% rh, surface moisture increases and accelerates the reaction rapidly. Considering the danger of capillary condensation of water, a low rh is preferable to reduce the tarnishing reaction level^2,3.

The concentration of organic sulphides in nature is lower than that of hydrogen sulphide, but they react more rapidly with silver and contribute to silver tarnishing ⁴. Other elements such as copper and iron contribute to the tarnish by producing their black sulphide compounds⁵and by electrochemical behaviour⁶.

The presence of nitrogen oxide and chlorine accelerates sulphidation reactions in higher concentrations of hydrogen sulphide gas ¹. Their synergistic effects are, however, small in typical concentrations of the gases. Silver chloride is also found as a corrosion product of silver⁷. Chlorine gas (from bleach) and sodium chloride (from human secretion) react with silver to produce silver chloride. Pure silver chloride is a white or transparent compound. Encrustation of soot and dust combined in this silver chloride corrosion layer can turn it into a darker colour.

Light reduction of ionised silver can also produce fine silver particles that appear black. Silver ions absorb ultraviolet (UV) radiation heavily. The absorption is strong at wavelengths of less than 240nm and weaker in an absorption band between 240 and 300nm. These shorter wavelengths effectively induce silver ion reduction to produce fine silver metal particles⁸. Some lacquers on silver objects can absorb ultraviolet light to reduce the above reaction rate but the lacquers are degraded in the process. Lower UV levels are preferable and can easily be achieved using filters.

Unlacquered silver objects have been known to tarnish in less than three months. Prevention of chemical tarnishing is achieved by using barriers such as lacquers to prevent the reactive chemical species contacting the silver. In the V&A, silver objects are routinely lacquered with a cellulose nitrate-based lacquer after cleaning. Corrosion inhibitors can be included in the lacquer to combine with the silver surface, forming a thin coating. Applications of inhibitors alone gives poor results owing to the difficulty of producing an even application and the physical weakness of the coating⁹.

Depending on the degree of corrosion, silver objects at the V&A are cleaned using a combination of:

• Goddard's Hotel Silver Dip^TM, Johnson Wax (a proprietary mix of mineral, acids, surfactant, sanitiser and organic complexing agent);
• Goddard's Long Term Foaming Silver Polish^TM, Johnson Wax (an aqueous dispersion of surfactants, jeweller's rouge, diatomaceous earth, tarnish inhibitor, perfume and approved preservative);
• Goddard's Long Term Silver Cloth^TM, Johnson Wax (impregnated with water, alcohol, silica, inhibitor, surfactants, thickener, dye, perfume and metal soap).

Each of the above contains different inhibitors¹⁰. The cleaning process selected may also affect subsequent tarnishing rates. Recording the effectiveness of cleaning and lacquering procedures used by the Metalwork Conservation Section of the Conservation Department will enable the best methods for reduction of tarnishing to be identified. Frigilene^TM(Wm. Canning Ltd.) is a cellulose nitrate based lacquer and is less stable than, for instance, Paraloid B72^TM(Rohm Haas)⁵. However, Frigilene is easier to apply evenly and has been used successfully to protect silver objects for over 8 years in old show cases in the Silver Gallery¹¹. We should, however, try to find a resin that is more stable than cellulose nitrate and which also has good working properties in order to prolong the time before cleaned silver objects require re-treatment.

Another barrier against tarnishing is the showcase. In the V&A, all new showcases are constructed to our own specification. The specification requires the elimination of harmful materials as far as possible, airtightness of the case to better than 0.1 air changes per day, and provision for adsorbent materials such as silica gel¹². The use of adsorbent materials for sulphide gases reduces the microenvironmental concentrations of pollutants coming from both the environment and the objects themselves. Adsorbents composed of materials such as zinc oxide¹³ and of activated carbons are commercially available. Using effective adsorbents alone will reduce silver tarnishing. When used with other procedures, such as lacquering of objects in showcases and good storage, there will be a marked improvement.

Having taken steps to create a microenvironment which inhibits tarnishing, it is also important to assess its performance¹⁴ .This makes monitoring, and specifically the use of real-time corrosion monitors, highly desirable.

Acknowledgements

I would like to give my thanks to the Japanese Ministry of Education for giving me a grant to study abroad.

References

1. Fiaud,C., Guinement, J., The effect of nitrogen dioxide and chlorine on the tarnishing of copper and silver in the presence of hydrogen sulfide, Proceedings Electrochemical Society, 1986, 86-6 (Corros. Eff. Acid Deposition Corros. Electron. Mater.), pp. 280-304.
2. Crossland, W.A., Knight, E., Wright, C.R., The accelerated tarnish testing of contacts and connectors employing silver alloy contacts., Elect. Contacts, 1973, Proc. Holm Weminar, pub. IIT Chicago, pp. 265-282.
3. Bennett, H.E., Peck, RL., Burge, D.K., Bennet, J.M., Formation and growth of tarnish on evaporated silver films, Journal of Applied Physics, 1969, 40 (8), pp. 3351-3360.
4. Sinclair, J.D., Tarnishing of silver by organic sulfur vapours: rates and film characteristics, Journal of the Electrochemical Society, 1982, 129(1), pp. 33-40.
5. Selwyn, L.S. Historical Silver: Storage, Display and Tarnish Removal, Journal of International Institute for Conservation of Historic and Artistic Works, Canadian Group, 1990, 15, pp. 12-22.
6. Pope, D., Gibbens, H.R., Moss, R.L., The tarnishing of silver at naturally occurring H2S and SO2 levels, Corrosion Science, 1968, 8 (12), pp. 883-887.
7. Martin, G., AMECP report of the study of PURAFIL SILVER COUPONS, V&A Science Group report, 17/93,1994.
8. Fang, J., Cai, Z., Tarnish protection for silver electrodeposits, Plating and Surface Finishing, 1988, 75 (2), pp. 58-61.
9. Lins, A., McMahon, N., The inhibition of silver tarnishing, Current problems in the Conservation of Metal Antiquities, Tokyo National Institute of Cultural Properties, 1989, pp. 135-162.
10. Singh, I., Sabita, P., Alterkar, V.A., Silver tarnishing and its prevention. A Review, Anti-Corrosion Methods and Materials, 1983, 30 (7), pp. 4-8.
11. Heath, D., (Head of Metal Conservation Section, Conservation Department), Personal Communication.
12. Martin, G., Preventive conservation guidelines for developments, Science section report of V&A, report number 56/93/glm, 1993.
13. Bradley, S.M., Hydrogen sulphide scavengers for the prevention of silver tarnishing., Environmental monitoring and control. The Scottish Society for Conservation and Restoration, Preprints of the symposium, 1989, Dundee, pp. 65-67.
14. Blades, N., Measuring pollution in the Museum environment V&A Conservation Journal, 1995, 14, pp. 9-11.