CRITERIA FOR TREATMENT: REVERSIBILITY
In order to be as clear as possible about the meaning of the term “reversible,” it is important that conservators confine its use to the description of a process rather than of a material. The idea that a material can be reversible is not logical. The incorrect use of “reversibility” to mean “solubility” as applied to resins has however become common. It is certainly tempting to refer to a material like Acryloid¯B-72, for example, as a “reversible material” in order to indicate a group of properties which we find desirable. These include chemical inertness, unchanging solubility over long periods of time, durability, and lack of color changes. However, the use of the term “reversibility” to cover all of these properties may produce serious error if it leads us to assume that any treatment using a soluble material is a reversible one, or that any use of an “approved” conservation material constitutes a proper treatment.
In order to avoid this confusion, I propose that Robert Feller's terminology for describing the photochemical stability of thermoplastic resins2 be put into common use and extended to other conservation materials. Acryloid¯B-72 and the polyvinyl acetate resins can then be referred to as Class A materials, with a useful lifetime of over one hundred years, polybutylmethacrylate (e.g., Elvacite¯ #2044) as a Class B material, with a useful lifetime of twenty to one hundred years, etc. This terminology is particularly meaningful because, unlike many terms we take from other fields, it describes our particular needs for long-term stability, requirements much more stringent than those of the industrial fields from which we appropriate materials.
In this paper, I shall reserve the term “reversibility” to denote the property of a treatment that allows a knowledgeable conservator to “turn back the clock” on a treatment. In functional terms, this does not require that the object be identical to what it was,3 only that we can return it to a state where our treatment choices are as broad as they were before the treatment in question was performed.