CRITERIA FOR TREATMENT: REVERSIBILITY
3 REVERSIBILITY AND CLEANING
Cleaning is obviously not reversible; the exact material removed cannot be replaced. It is therefore vital that the conservator be sure that the material being removed is not original to the maker of the piece or important to any historic use of it, or that any information that such material can provide is not lost during the process. Cleaning of archaeological bronzes or ethnographic materials must be considered carefully before significant material is removed. However, once it is ascertained that the materials being removed are not a purposeful addition by the artist or user, the cleaning does not detract from the integrity of the piece. If there is a possibility that the removed material might at some time be analyzed to provide information about the history of the piece, it can be saved. Cleaning does not necessarily destroy information.
Even though cleaning is not technically reversible, the capability of reversing the visual effect can be important. An argument of the partial and selective cleaners4 in the recurring controversy on the cleaning of paintings is that varnish layers must not be completely removed so that a thin wash of discolored resin is left on the surface to harmonize color schemes which have become unbalanced because of the different rates of deterioration of the pigments. This is unnecessary, since cleaning is visually reversible, and the technical identity of the material being removed is unimportant as long as we are sure that it is not the artist's design layer. All discolored varnish which can be safely removed should be, since it will usually continue to cross-link or oxidize, causing additional color change and a decrease in solubility. Varnishing and inpainting of losses should be carried out before any decision can be made about the color balance of the painting. If, after an appropriate period of consideration, the color still seems inappropriate, a toned varnish can be applied to the whole or to parts of the painting. This process is actually a step in compensation. The use of a modern toned resin rather than the existing varnish for this purpose will provide less need for re-treatment, since the natural resin will continue to darken, while a Class A conservation material will retain exactly the color that is applied.