Recent AIC presentations (2017 Annual Meeting) indicate that there are numerous misunderstandings about the use of solvent adhesives, particularly Paraloid B-72. The increasingly popular use of Paraloid B-72 is based primarily on its notable and favorable conservation attributes. Paraloid B-72 is the most stable, reversible and dependable resin now used in conservation. It was introduced as an adhesive by the author just over 30 years ago and also has very commendable working properties, including excellent adhesion and fast setting time, but these can easily be compromised by improper preparation and improper application.
This paper discusses the many ways that Paraloid B-72, or other solvent-based adhesives should be prepared, modified or manipulated to obtain easy and efficient application as well as consistent and excellent results. Preparation is critical to having a dependable solvent-based adhesive. B-72 can be very easily made up with only a few minutes of preparation, and then allowing about 8 hours for the resin to dissolve in solvent. The choice of solvent is very important, and acetone has proven to be the best solvent, on its own, or in some cases with a small amount (5-10%) of ethanol. The ratio or percentage of resin:solvent can be modified to control the application and setting time for different uses. This then allows the conservator to control the application of a thin or thicker adhesive. One additive is recommended in the initial preparation, and that is the addition of a small amount of hydrophobic fumed colloidal silica, which aids in uniform application, stabilization of the mixture, film formation and solvent evaporation. Fumed silica is an inert material, classified as a rheological agent (to control flow characteristics). It is not necessary to evaporate off any solvent after the B-72 resin has dissolved in the acetone, as the initial amount of solvent can easily be calculated for producing an adhesive of specific viscosity (or thickness). For glass, a thinner solution of approximately 60% weight/volume is recommended because glass is non-porous and non-permeable, while a thicker solution of 72% works better on more porous substrates, such as low-fired ceramics, porous stone, wood, bone and ivory. For best results, including application and maintaining a consistent fluid mixture, the prepared adhesive should be poured into adhesive tubes, specifically designed for solvent adhesives. This also improves the ease-of-use and accuracy of assembly.