CRITERIA FOR TREATMENT: REVERSIBILITY
9 COMPLICATIONS IN THE PRACTICAL APPLICATION OF THEORIES OF SOLUBILITY
In practice putting a material into solution involves more than bringing a solid and a liquid into physical contact. The statement that one material is soluble in another may be technically correct, but it does not in itself indicate the conditions required for dissolution. It might be useful to look at the ways conservators make adhesive solutions in vitro, and to compare that process with procedures used to dissolve those same adhesives in situ, that is, on the object, in preparation for removal. Researchers who test conservation materials have recognized that there is a difference between chemical or technical solubility and solubility in practice, and have coined the term “removability” for use in a particular testing situation.6
One factor not covered in charts of solubility is the time required for dissolution. Making a resin solution in the laboratory, for example, often requires several days. Some objects can be soaked, or at least exposed to fumes for long periods; most cannot. The range of times available for safe exposure of certain objects to solvents is so limited that the actual chemical solubilities of materials may be irrelevant to conservation treatments. The time ranges that are involved in many treatments are very limited compared to procedures common in chemistry laboratories. On the other hand, these time limitations allow us the safe use of solvents that, technically speaking, could dissolve the material of which the object is made.
Another factor in producing a solution is agitation, probably because it helps promote removal of the dissolved surface layer, which in turn provides better access by the solvent to the as-yet undissolved material beneath. Yet the amount of abrasion to the substrate caused by agitation can make such a procedure harmful. Normal cleaning procedures involving the removal of resins from the surface of an object may require significant amounts of friction to shorten the time the resin takes to dissolve. Anything sensitive to abrasion, like the surface of soft ceramics, rigging lines on ship paintings, or very lean contemporary paint films, makes us aware of the potential danger of even a small amount of abrasion in our cleaning procedures.7 A common mistake of conservation students when finding small amounts of color on their swabs during cleaning tests on lean paint films is to assume that they are dissolving the film rather than abrading loosely bound particles. A dry swab may remove the same amount of color. Dissolving resin films off the surface of extremely abrasion-sensitive objects by simply dripping a solvent over the surface and wicking up the liquid will remove some resin, but not as much as the use of a cotton swab. On the other hand, the removal of a resinous coating does not necessarily entail the complete chemical dissolution of the resin. In most cases putting only a small percentage of the resin into solution is enough to break it up so that it can be wiped away; resins which swell rather than dissolve are also removable. Many removals are actually combinations of softening or breaking up a material with a solvent, and mechanical removal.
Another factor which promotes the solution of materials is heat. Preparation of starch paste or a gelatin solution requires elevated temperatures. Few works of art can withstand the range of temperatures necessary to dissolve gelatin or make starch paste. Fortunately, these materials often soften enough with moisture to be mechanically removed; removal may be aided by temperatures significantly lower than those used in adhesive preparation. However, conservators who do not specialize in works on paper may be surprised that the removal of starch paste linings may require prolonged immersion in water, and that quite hot water may be needed. The wide reputation of starch paste as a “safe” adhesive does not imply ease of removal. However, paper is so sensitive to materials in its surroundings that the chemical compatibility of starch paste with paper and its long-term stability are overriding criteria for the choice of adhesive.
Heat may be useful in the removal of some materials where heat was not used in their formulation or application. Because of the relationship between the ease of solubility and the second-order transition temperature,8 it may be that slight heating of a resinous coating would increase the rate of penetration of the solvent through the film, and therefore, the speed of dissolution. In my experience, slight heating is not a common tactic in removing difficult films, but it may be one that should be tried more often.
The relationship between reversibility and solubility in polyvinyl acetate emulsions is a controversial topic. There seems to be no agreement in the conservation literature on the actual degree of solubility of these materials.9 The large number of ingredients used to formulate proprietary emulsions, individually untested by conservators, results in a startling variety of softening temperatures, pH, and other properties10 and makes the understanding of these properties extremely difficult. For practical purposes, dried films can be softened in a wide variety of solvents, but never dissolved to form a liquid of low enough viscosity to make removal easy. They tend, even when softened, to remain sticky, so their safe removal from fragile surfaces is virtually impossible. The removal of softened emulsion from the edges of a soft or grainy ceramic almost inevitably involves some loss of original material.
There seems to be a great deal of confusion about why polyvinyl acetate emulsions are often difficult to remove. Some of the difficulty of removal is due to the nature of the resin, not usually to cross-linking11 the resins used in formulating PVA emulsions are of a much higher molecular weight than the polyvinyl acetate resins conservators ordinarily use, so that their properties are quite different. Difficulty in removal is also caused by changes in behavior with time, due to the loss of water and other volatiles rather than to changes in the resin. The temperature required for heat-seal bonding when an emulsion film is touch-dry is therefore far lower than that required when most of the volatile materials have evaporated from the film. Color changes seem to be due to materials other than the polyvinyl acetate resin, and are not associated with cross-linking. For all these reasons, equating cross-linking, discoloration, and loss of solubility in emulsions prevents a realistic understanding of their properties and appropriate uses. Because the properties of dried emulsion films continue to change for several years after application, the later removal of heat-seal emulsion linings, unlike the removal of heat-seal resin linings, can be much more difficult than expected.
Acryloid¯ B-67 is another material where the assumption that difficulty in dissolution is due to cross-linking leads to serious errors. The glass transition temperature of B-67 is above room temperature. At room temperature it has a glassy dense surface. Removal with xylene may be difficult, not because the resin is insoluble—it is not—but because the surface is relatively impermeable to solvents. Slight warming should remedy this problem. Acryloid¯ B-67 is a fine conservation material, one which, I believe, is under-used due to misunderstandings of its solubility characteristics.
In short, it can be extremely difficult in a given case to project our knowledge of the behavior of newly-applied materials into the future and predict the reversibility of any particular conservation treatment. Only by providing a wide margin of safety and using materials which fulfill the most stringent aging tests can the reversibility of a treatment well into the future be assured.