The Treatment is Clear

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CCI Newsletter, No. 23, June 1999

The Treatment is Clear

by Lyndsie Selwyn, Senior Conservation Scientist, Conservation Processes and Materials Research

It must surely be a common problem. Old friends are coming for dinner and the silver candlesticks they gave us for a wedding present are about to be used for the first time in years. The relief of finally locating them quickly turns to dismay as we unwrap them and see... tarnish! Exposure to sulphur-containing gases has caused a thin film of solid silver sulphide to form on the surface and the result is a dull lackluster finish. To restore the original brilliance of the silver, the sulphide film must be removed.

At the turn of the century (1907) an industrious homemaker could find the following recipe for silver polish¹: The best polish for silverware —that is, the polish that, while it cleans, does not too rapidly abrade the surface—is levigated chalk... The usual metal polishes, such as... finely ground pumice stone... cut away the surface so rapidly that a few cleanings wear through ordinary plating. More recently (1992), a conservator might find this advice²: Make a slurry... of precipitated chalk in distilled or deionized water and apply in small quantities to tarnished silver with cotton and cotton swabs.

As we can see, the 1907 opinion is still valid 85 years later. The only modification is the stipulation that the chalk be precipitated from solution rather than levigated (ground into a fine powder). This stems from the fact that precipitation gives better control of the particle size than grinding, and avoids the large particles that can leave behind visible scratches during polishing (small particles also leave scratches, but if the particles are small enough the scratches are invisible to the unaided eye). One reason for the enduring use of chalk is that it is a soft material (hardness about 3). Harder abrasives such as pumice (hardness about 6) remove more material with each stroke, but also cause more damage: if polishes containing the harder abrasives (such as "all-purpose" metal polishes) are used on silver plate, they rapidly remove the entire silver layer, revealing the underlying base metal; if they are used on sterling silver, they can rub away delicate engravings.

This is one case where time has simply polished the advice!

References

1. Henley's Twentieth Century Formulas, Recipies and Processes. 1907.
   
2. National Committee to Save America's Cultural Collections. Caring for Your Collections. New York: Harry N. Abrams, Inc., 1992, p. 116.