A COMPARATIVE STUDY OF SILVER CLEANING ABRASIVES
GLENN WHARTON, SUSAN LANSING MAISH, & WILLIAM S. GINELL
IT IS difficult to define the ideal system for cleaning all silver objects. Selection of the abrasive, carrier fluid, and abrasive support cloth must be made by the conservator based on the condition and intricacy of the object to be polished. A lightly tarnished object, for instance, may simply be cleaned with a cloth and an aqueous surfactant solution without the use of any abrasive. A more intricate or irregularly tarnished object may require a combination of techniques to remove the tarnish. It is hoped that the results of this study will aid in the selection of materials that meet the demands of the individual object.
The abrasives that removed tarnish successfully and caused the least damage to sterling silver were calcium carbonate, gamma alumina, and chromium oxide. Calcium carbonate performed well in all tests, except that in some instances a few shallow scratches were produced on the silver surface. Presumably these scratches result from poor particle size classification by the manufacturer. Gamma alumina, which produced fewer scratches than calcium carbonate, was found to leave the silver with a very highly reflective surface. This exceptionally high reflectance may be considered objectionable. Chromium oxide was less favored than calcium carbonate and gamma alumina by some conservators at LACMA because of the extra labor involved in removing the bright green residue from crevices in the silver. Chromium oxide, however, had a high Figure of Merit and did not impart a residual color to the silver.
It was also found that less damage is caused by polishing with a less abrasive material for a longer period of time than a more abrasive material for a shorter period of time. If tarnish removal using calcium carbonate, chromium oxide, or gamma alumina proves to be too difficult and chemical cleaning cannot be performed, alpha alumina should be considered. Although alpha alumina removes more material and produces more scratches on the silver surface than the above abrasives, the overall performance of this material was superior to the other nine abrasives tested.
At LACMA the conservators rated diatomaceous silica very high. They found that it removed tarnish successfully and was easily removed from the crevices after polishing. On microscopic examination at GCI, it was found that diatomaceous silica produced an observable scratch pattern and removed more silver than calcium carbonate. The additional scratches produced by light polishing may not be discernible on some historic silver surfaces that have previously existing scratch patinas. With persistent polishing, however, this uniform pattern of light scratches will eventually replace the existing scratch pattern. If diatomaceous silica is used as a polishing abrasive, the conservator must be careful not to replace the random scratches of age with a new, perhaps oriented, finely divided scratch pattern.
From the evidence gathered on carrier fluids, it can be concluded that deionized water with a small amount of nonionic surfactant is preferable to reagent grade ethanol with or without ammonium hydroxide. With an alcohol carrier fluid, the abrasives, on the average, removed about twice as much tarnish, but the silver was scratched about three times as much and about four times as much silver was removed as with the aqueous carrier fluid. Because these differences may be due partly to evaporation or excessive spreading of the alcohol into the abrasive support cloth, a combination of water and alcohol could provide a more effective medium.
Selection of components of a polishing system will determine not only the amount of material removed and the scratch pattern on the silver surface but also the amount of specular reflectance. Historically, the surfaces of most silver objects were highly polished when the objects left the silversmith's shop. The surfaces were usually planished then polished with finely ground pumice, rouge and/or whiting. Over time, the silver surface was altered by cycles of tarnishing and polishing. This alteration inevitably rendered the surface slightly less reflective through the pitting action of oxidizing reactions, the scratching of abrasives, and random scratches produced over time by handling. In selection of materials to be used for cleaning old silver, the conservator must determine whether the objective is to maintain the “historical appearance” of a slightly less reflective surface or to retrieve the original high reflectance of a recently fabricated piece. If the high reflectance imparted by aluminum oxide is not desired, yet calcium carbonate does not remove the tarnish, the conservator may choose to use gamma alumina or, if necessary, alpha alumina, then lightly repolish with calcium carbonate to reduce the reflectance.