3 CAUSES OF DISCOLORATION

Two of the many possible explanations for the discoloration or darkening of porous or powdering paint resulting from consolidation are: (1) the differences in the refractive indices of pigment and vehicle; and (2) reverse migration of polymer from the interior to the surface with solvent evaporation. These effects are considered to be of little significance in this study.

Feller and Kunz (1981) initiated their study in part to demonstrate that the darkening phenomenon resulted primarily from the filling of void spaces. A series of polymers of different refractive indices were shown to be similar in their darkening effects for paint formulated with ultramarine blue at different PVCs. They concluded that individual refractive indices of pigment and resin were relatively unimportant in the darkening that occurs in impregnated porous paint as opposed to the untreated state.

A high surface concentration of consolidant has been explained as a result of the migration of a resin solution that has penetrated a porous object back to the surface of the object with solvent evaporation as a consequence of capillary action (Wilks 1987). This process is often termed “reverse migration.” Reverse migration has been extensively discussed in relation to stone consolidation by Domaslowski (1987–88). He considers the migration process to be insufficiently known and understood by both conservators and scientists. He states that the resin migration effect depends on the quality of solvents, dimensions of resin molecules, solution viscosity, stone structure, and drying conditions after impregnation.

He found that the most important factor in limiting reverse migration is the solvent “quality,” or the tendency of the dissolved polymer to remain dissolved in a solvent. Poor solvents for polymers resulted in polymer deposition and lack of migration, provided that the polymer sufficiently penetrated the stone pores. (In some instances the solution viscosity is too high to allow sufficient penetration into the stone, leaving a higher concentration of resin at the surface.)

Domaslowski found that migration did not follow a sequence equivalent to solvent volatility rates, except that migration was reduced for very low volatility solvents. He also found that slow, long-term drying may reduce migration in solvents of weak and moderate quality. To achieve slow drying, resin migration was prevented by placing stone impregnated with solution in an enclosed container with vapors of “white spirits.” It is important to be aware of reverse migration because we are not taking the position in this article that working in a solvent vapor–saturated atmosphere reduces reverse migration and subsequent discoloration resulting from high surface concentrations of resins. Instead, our working assumption is that surface concentrations of resins are avoided when solvent loss through evaporation is inhibited (sec. 4).

Johnston-Feller (1990) suggested that surface concentrations of the resin remaining after treatment with a consolidant are due primarily to lack of penetration, lack of efficient wetting of pigment particles, and uneven distribution of resin solution within the paint. It was further suggested (Wicks 1990) that this lack of penetration is the result of an increase in solution viscosity due to solvent evaporation during the time it takes for a solution to penetrate a porous paint and that this effect could be reduced by minimizing or eliminating solvent loss until desired penetration and spreading of the solution were achieved.

Schiessl (1989) has discussed certain conditions of a matte paint that may result in discoloration from consolidation with a solution of a resin and that cannot be minimized by inhibiting solvent loss during application. He has commented on the formation of “tide lines,” observing that this phenomenon sometimes results either from of the solubilization of dirt, fungus, and bacteria or from the physical redistribution of small particles in a paint that has a wide range of pigment particle size. As can be observed through a microscope, application of a solution moves the smaller particles, leaving the larger particles in place.