1 INTRODUCTION

TARNISH REMOVAL from silver objects is usually referred to as “silver polishing,” although the term “silver cleaning” would be a more accurate description of the intent of the operation. The purpose of silver polishing is to produce a smooth and highly reflective surface, while, in silver cleaning, surface deposits are removed and the underlying silver surface, which may or may not be reflective, is revealed.

Tarnish can be removed from silver chemically, electrochemically, or mechanically. In mechanical processes, abrasives are generally used. In addition to tarnish, however, abrasives remove a small amount of silver. Under certain conditions, abrasive cleaning is the method preferred by conservators, yet information on the relative performance of typical abrasives is not generally available.

In conjunction with the planned cleaning and lacquering of 180 objects in the Arthur Gilbert Collection of silver at the Los Angeles County Museum of Art (LACMA), a joint project was carried out at the Getty Conservation Institute (GCI) and LACMA to evaluate abrasive cleaning materials. The objective of the project was to evaluate the effects produced on tarnished sterling silver by various combinations of abrasives, carrier fluids, and abrasive support cloths with respect to tarnish removal, silver removal, and scratch production. Other evaluation factors considered were ease of application, effects of cleaning time, residual abrasive particles on the silver surface, and appearance of the silver after cleaning. Although various carrier fluids and abrasive support cloths were considered, the principal emphasis of this project was comparison of abrasives.

A search for information on relatively safe abrasives for tarnish removal revealed that Fales (1967), Heller (1983), Holland (1977), Majewski (1973), Reisman (1983), and a report on silver care products byConsumer Reports(1978) recommend procedures and report on casual bench tests, but no reports on systematic investigations were found.

The selection of abrasive slurries was limited to those that could be prepared in the conservation laboratory using readily available, generic materials. Commercial products were not investigated because potentially damaging components may be contained in proprietary cleaners. These components include corrosion inhibitors, silicones, wetting agents, chemical cleaning agents, emulsifiers, antifoaming agents, perfumes, and dyes, along with the active abrasives. Moreover, unannounced compositional changes by the manufacturer could lead to changes in performance of the cleaner.

At GCI, cleaning tests were conducted on sterling silver discs using 13 abrasives, 2 carrier fluids, and a single type of abrasive support. The discs were mounted, polished, artificially tarnished, and then cleaned reproducibly on a mechanical device. The amount of silver removed during cleaning was determined by measurements of dimensional changes of a group of indentations in the silver surface. The amount of tarnish removed and scratches produced were rated visually. Selected samples of cleaned surfaces were examined by scanning electron microscopy (SEM), optical microscopy, electron beam microprobe, and Auger spectroscopy to characterize the surface features produced.

Nine conservators at LACMA participated in the evaluation project. They provided subjective comments on the speed and effectiveness of tarnish removal, depth and quantity of scratches produced, surface appearance of the silver after cleaning, ease of abrasive removal after drying, and their preferred cleaning technique.

It should be noted that most of the objects in the LACMA collection did not require abrasive cleaning. Acidified thiourea solutions were generally used. Under certain circumstances, such as the absence of a proper fume exhaust system, the presence of associated organic or other sensitive materials, or the deliberate retention of tarnish at the bottom of engraved lines or in deep recesses, chemical cleaning is not appropriate and abrasive cleaning is preferred.