A COMPARATIVE STUDY OF SILVER CLEANING ABRASIVES
GLENN WHARTON, SUSAN LANSING MAISH, & WILLIAM S. GINELL
TO EVALUATE the results and compare the overall relative performance of the abrasives, a Figure of Merit was calculated. Figure of Merit is defined as the ratio of Tarnish to the absolute value of the mean of Scratch and Delta. Figure of Merit provides an overall assessment of the data collected by comparing the amount of tarnish removed to the damage (Scratch and Delta) incurred during the polishing process. The rationale behind this manipulation of data was that a desirable polishing system should remove as much tarnish as possible with the least amount of surface scratching and silver loss. Thus, the higher the Figure of Merit value, the better the overall performance of the abrasive. The calculated values of the Figure of Merit are shown in table 1 for each of the abrasives tested.
TABLE 1 Calculated Figure of Merit for Abrasives
A control sample was run to determine the polishing effects of the carrier fluid and cloth alone. The control sample was sulfided, then polished on the polishing device with ethanol/ammonium hydroxide for 3 minutes with no abrasive on the cloth (see table 1).
To determine the extent and depth of residual abrasive on the silver surfaces, five samples were examined by SEM and two samples were analyzed by Auger spectroscopy. The samples examined by SEM were polished with diamond, gamma alumina, tin oxide, calcium carbonate, and rottenstone. Except for diamond, abrasive particles were observed embedded in the surface of all of the samples, even though they were cleaned ultrasonically with deionized water and nonionic surfactant for several minutes after polishing. Although the silver surface prior to sulfidation was relatively smooth, depressions or pits were observed in the elongated copper-rich phases after sulfidation and polishing and most of the embedded abrasive particles were found in these areas. Electron beam microprobe analysis showed that the copper-rich phases were also sulfur-rich, indicating incomplete removal of tarnish in these areas. The presence of sulfur in the copper-rich, rather than the silver-rich, phase after polishing can be ascribed to a marked reactivity rate difference between copper and silver in sulfide solutions (Graedel et al. 1985) and hence a thicker tarnish film in the elongated areas. It is likely that most, if not all, of the abrasives used in this experiment left residual particles embedded in the silver surface. However, it is unlikely that any of these particles would cause significant chemical reactions over time.
Because of the decidedly yellow color of the silver samples polished with tin oxide, an Auger analysis of a specimen was performed. On removal of successive surface layers by sputtering, tin and oxygen contamination was found to a depth of about 300 Angstroms below the surface. By comparison, aluminum contamination of a gamma alumina-polished sample was limited to the upper 25 Angstroms.
Before the abrasives were used to polish the Gilbert Collection at LACMA, they were tested on sterling silver scrap. Most of the abrasives were ruled out early in the project either because they were too abrasive or did not remove tarnish efficiently. Rouge and rottenstone, which were tested at GCI to establish parameters for known harsh abrasives, were not tested at LACMA. The nine LACMA conservators who participated in this project were asked to rate the abrasives according to three criteria: speed of tarnish removal, surface appearance after polishing, and ease of removal of the abrasive from crevices after drying. Their comments were valuable in the final assessment of the abrasives for practical use.
The following section summarizes the findings at LACMA and GCI on the six abrasives that had the highest Figures of Merit.
4.1.1 Chromium Oxide
In an alcohol/ammonium hydroxide suspension, chromium oxide removed tarnish rapidly and effectively with very little effort. A moderate amount of silver was removed during the process. Chromium oxide was not tested in water suspension. Although chromium oxide received the highest Figure of Merit, it was not favored by some of the conservators because of its green color. In addition, they unanimously reported that it was difficult to remove all of the abrasive residue from the crevices after drying. This observation may be due to the fact that a green residue is more visible than a white one.
4.1.2 Magnesium Oxide
In alcohol/ammonium hydroxide, magnesium oxide removed tarnish rather slowly and only with significant rubbing. Eventually the tarnish layer was removed, but a moderate amount of silver was also removed. Magnesium oxide was not tested using water as a carrier fluid; however, water suspensions are available commercially for metallographic polishing.
Although very few scratches were visible after polishing, the conservators noticed that dried MgO was somewhat difficult to remove from crevices. After drying, extremely abrasive agglomerates formed (possibly magnesium carbonate), which scratched the silver during removal. Because this characteristic was observed at the bench with LACMA tests, it is not reflected in the GCI Figure of Merit values.
In view of the tendency of Mg0 to form abrasive agglomerates, MgO cannot be recommended as an abrasive for polishing silver.
4.1.3 Gamma Alumina
Gamma alumina is the synthetic, tetragonal crystalline form of aluminum oxide. Gamma alumina suspensions in water are available commercially. In a water-surfactant slurry, gamma alumina (particle size 0.05 μm) removed tarnish successfully, but at a slow rate. In alcohol/ammonium hydroxide, tarnish was removed more effectively, but the silver was more visibly scratched. Silver loss in both cases was small. After drying, gamma alumina was reported to be slightly harder to remove from crevices than other abrasives.
At LACMA, gamma alumina was considered a good material for polishing lightly tarnished silver, and the conservators unanimously reported very little or no visible scratching. Significantly fewer scratches were noted after the 6 minute tests than after the 3 minute tests. This result could have been due to the evaporation of alcohol or the clogging of the abrasive support cloth, which resulted in burnishing of the silver surface.
4.1.4 Alpha Alumina
Alpha alumina is a synthetic, hexagonal crystalline form of aluminum oxide that occurs in nature as corundum. Alpha alumina suspensions in water are available commercially. In alcohol/ammonium hydroxide, alpha alumina (particle size 0.3 μm) removed tarnish rapidly and efficiently but also removed a fair amount of silver in the process. In water/surfactant, tarnish was removed less efficiently, but scratching and silver loss were significantly reduced.
In general, however, alpha alumina produced more visible surface scratches than gamma alumina and was reported by the LACMA conservators to be slightly more difficult to remove from crevices after drying than some of the other abrasives.
It was observed that both alpha and gamma alumina produced more highly reflecting surfaces than any of the other abrasives. The conservator should be aware of this property when using these materials.
4.1.5 Tin Oxide
In alcohol/ammonium hydroxide, tin oxide removed tarnish effectively but more slowly than other abrasives. Less silver was removed, however, than with most other abrasives that were effective in tarnish removal. In water/surfactant, tin oxide removed tarnish slowly, produced few scratches, and removed little silver. Very few scratches were observed on samples polished with tin oxide. Samples polished with water were distinctly yellow in appearance. Those polished with the alcohol/ammonium hydroxide were only slightly yellow. Tin oxide samples were given low Tarnish values because of this yellow hue.
Removal of tin oxide from crevices after drying did not present a problem. Because of the extensive contamination of the samples with tin oxide (as determined by Auger spectroscopy) and the surface color change of the silver, this material should be used with caution for cleaning museum silver.
4.1.6 Calcium Carbonate
In alcohol/ammonium hydroxide, calcium carbonate effectively removed most of the tarnish and only a moderate amount of silver. In water/surfactant, tarnish was removed less efficiently, but the silver was scratched significantly less and much less silver was removed. Although few scratches were observed at GCI, some were moderately deep. To ensure that the abrasive supply was not contaminated, a second batch of calcium carbonate was tested and similar results were obtained. Laboratory-grade calcium carbonate is not graded for size, and agglomerates up to 48 micrometers were found. Precipitated chalk or whiting obtained from other sources should be evaluated.
The conservators noted that the abrasive was more difficult to remove from crevices after drying than other abrasives and that a slight “dusty coating” remaining on the silver surface must be wiped off after polishing.
4.2 CARRIER FLUIDS
The relative effect of the carrier fluid on abrasive performance is shown in table 2, where the ratio of Tarnish, Scratch, and Delta values for alcohol and water are given for four abrasives. These data indicate that cleaning with alcohol/ammonium hydroxide as the carrier fluid resulted in the removal of more tarnish, produced a surface that had more scratches, and removed more silver than a water/surfactant slurry for each of the abrasives tested. The relative increase in scratches and in material removal was significantly greater than the increase in tarnish removal.
TABLE 2 Effect of Carrier Fluid on Abrasive Performance
Although more tarnish was removed using abrasives in an alcohol carrier, water seems to be preferable because less silver damage (Scratch and Delta) was observed. Possible reasons for this result are: 1) loss of alcohol by evaporation or wicking into drier parts of the cloth, which would result in a more concentrated abrasive slurry, or 2) that the superior suspension properties of a water-surfactant solution produces a more dilute, stable slurry. In general, the conservators found that although tarnish removal was slower when a relatively thin slurry was used, less scratching seemed to occur and the cleaning process was more easily controlled. Because of these findings, and until more extensive studies on suspension fluids have been carried out, it is recommended that an aqueous polishing slurry with surfactant be used and that it be kept relatively dilute during the polishing operation.
Each of the seven conservators who responded to the questionnaires on carrier fluids expressed concern about the evaporation rate of alcohol during polishing. Although several comments indicated that alcohol/ammonium hydroxide was a slightly more effective carrier, other remarks indicated that it was more difficult to control and rendered the polishing system too abrasive. The conservators were also asked to compare alcohol as a carrier fluid both with and without ammonium hydroxide. The primary reason for adding ammonium hydroxide to the polishing fluid is to aid in removal of greasy deposits. Two of the seven conservators preferred the addition for this reason, but the others could not detect any difference in performance. All were concerned about the ammonia fumes. In view of the little, if any, improvement offered during polishing, ammonium hydroxide is not recommended for use as an additive to alcohol. It is also recommended that greasy films be removed with an aqueous detergent or a solvent prior to polishing.
4.3 ABRASIVE SUPPORT MATERIALS
The performance of several different abrasive support materials were compared during the tests at LACMA. These included cotton wool swabs, women's nylons, cotton diapers, and a lint-free polyester fabric. Nylons were rejected because the brown dye proved to be soluble in alcohol. None of the other three support materials were found to scratch the silver more than the least effective abrasive used. Cotton swabs were found to leave lint on the silver surface but were useful for reaching into deep recesses and crevices. Cotton diapers were found to leave less lint and to polish large surfaces more efficiently than swabs but tended to dry out more rapidly. The diapers chosen for this experiment were found to be quite thin; a heavier fabric would offer more support during polishing. The disposable, polyester fabric was favored by most of the conservators because it performed as well as the diapers but did not leave any lint. Based on the findings at LACMA, cotton swabs, cotton diapers, and polyester cloths used in these tests can all be recommended for silver cleaning.
A control test was run at GCI in which a cotton diaper was used to clean a sulfided silver sample using an alcohol/ammonium hydroxide fluid without abrasive. No visible scratches were produced, and very little, if any, tarnish or silver was removed.
4.3.1 Polishing Time
The relative effects of doubling the cleaning time on the values of Tarnish, Scratch, and Delta were determined. The ratios of 6 minutes to 3 minutes data for the abrasives that were used in the alcoholic slurry are shown in table 3.
TABLE 3 Effect of Polishing Time on Abrasive Performance
The results show that very little additional tarnish was removed, and, on the average, no additional scratches were produced by doubling the cleaning time from 3 to 6 minutes. Delta, however, did increase for those abrasives that were capable of removing tarnish and silver.
Samples polished with gamma alumina for 6 minutes, however, had significantly fewer scratches, and significantly more material was removed than those polished for 3 minutes. This reduction in number of scratches over time could explain why the silver polished with gamma alumina at LACMA appeared to have an exceptionally high reflectance.
A question posed by one of the conservators during the project was whether it was desirable to polish with a mild abrasive for a long period of time or a more aggressive abrasive for a shorter time. It is apparent from these data that use of a mild, tarnish-removing abrasive for a longer time is less damaging to the silver surface than a more aggressive material for a shorter time.