CRITERIA FOR TREATMENT: REVERSIBILITY
8 CRITERIA FOR THE REMOVAL OF CONSERVATION MATERIALS
Once we have standards of reversibility for different parts of a treatment, by what means can we judge in advance the future removability of the materials we add? In order to establish a method for evaluating the reversibility of treatments, it would be interesting to try to define what makes the removal of added conservation materials possible. To a first approximation, these are the main points of consideration: 1) the solubility of the components of the piece under treatment; 2) the solubility of the conservation material; 3) the physical nature of their interface; 4) the amount of material to be removed. The theoretical relation between numbers (1), (2) and (3) is sometimes a simple one: in order to remove a material by dissolving it, it must be soluble in a solvent that does not soften the substrate, and it must be physically accessible to the solvent. Acryloid¯ B-72, for example, cannot be removed safely from an acrylic emulsion painting if the paint is soluble in the same solvents that dissolve the B-72.
The nature of the interface can be important apart from any question of solubility: if the bond between a coating layer and a substrate is weak, mechanical removal is possible, and sometimes preferable, regardless of the solubility of either material. This is the case with the removal of a deteriorated glue lining from the reverse of a canvas painting. In situations like this, putting the glue into solution is less desirable than mechanical removal, since the solution would penetrate the porous textile, from which it probably could not be removed. Another example where mechanical removal might be preferable is in the case of removing excess adhesive from glass or porcelain. As long as the object surface is strong, mechanical removal may be harmless. Dissolving the adhesive would spread the solution over the surface, making the remnants difficult to remove, and the solvent could penetrate into breaks to soften the adhesive already in position. Easy solubility of an added conservation material may therefore be irrelevant to the reversibility of a treatment.
Solubility is also a negligible factor with a material like gesso. Many gessoes can be softened considerably with cold water. Those made with clear sheet gelatin soften much more easily on the palette than those made with rabbit-skin glue, but when used as a filling material, gesso usually becomes impregnated with varnish, paint medium, or consolidants, making the original differences in solubility irrelevant. Removal is aided to varying degrees by water, but the removal is largely mechanical. Removal is also aided by the fact that dried gesso is more friable than most ceramics and most paint films. Since it has very weak adhesive qualities, its removal tends not to pull off any original surface.
Consideration of the amount of material to be removed (4) is an important but often neglected point, thus illustrating the gap in the field of conservation between practice and theory. A small amount of a material can often be removed with a skilled hand and a sharp scalpel. Large amounts of material create quite different problems. If a material is to be removed by dissolution, the dissolved material must be removed without its dripping or flowing into places it is not wanted, like pores or cracks. If a great deal of material is to be removed mechanically, problems of contaminating the object with crumbs and dust may be encountered.