ULTRASONIC MISTING. PART 1, EXPERIMENTS ON APPEARANCE CHANGE AND IMPROVEMENT IN BONDING
STEFAN MICHALSKI, & CAROLE DIGNARD
The first, and unexpected, result from this study was that some pigments achieve remarkable cohesion as paints when applied without any binder, especially the clay-containing pigments. These did not need consolidation, and, besides, they discolored the most when any consolidation treatment was used. Thus good mechanical strength in a matte paint cannot be taken as any indication of the presence of binder. If anything, it could be taken as a sign of color sensitivity to water. A painting technology based on natural earth colors could certainly produce artifacts of adequate strength without recourse to any binders at all.
For weak paints, however, ultrasonic misting can deliver small quantities of consolidant at low impact with no noticeable change to surface texture. It is definitely possible to improve paint strength without noticeable change in color (given a quite stringent criterion) and sometimes without even a just-perceptible change. Such levels of consolidation, 1–5% BVC, do not create robust paints, but they do improve the ability of weak paints to withstand vibration, handling, gentle contact, or gentle dusting. If the only powdery paint being treated is calcium carbonate (white) or, as is often the case, the greatest expanse of paint is calcium carbonate, and it is thick enough to maintain opacity, then levels of consolidation approaching 10% BVC would remain visually acceptable and produce a robust film. One would have to use caution so as not to apply such levels to adjacent colored areas, however, since they would cause unacceptable color change.
To avoid skinning, practitioners should try to vary not only the consolidant but also the solvent. The notion of elution applies not only to initial application of solution mist but also to application of solvent alone to push undesirable skins and tidemarks down into the paint or porous substrate. Some practitioners call this, appropriately enough, the application of a “chaser” solvent.
A particular form of treatment vulnerability that emerged from this study was the effect of water itself on claylike minerals, particularly green earth. These natural earth colors have a long history of use in many cultures, so they can be expected to occur frequently. As noted already, such clays have high initial strength and may not need consolidation. Whether this initial strength is lost with age (humidity fluctuations?) is not known. At the opposite extreme, calcium carbonate and ultramarine have no claylike characteristics and low initial strength. The fortunate coincidence is that the same lack of water sensitivity that makes them weak also makes their color tolerant of aqueous treatments.
If one had to select the best consolidant system overall from our limited study, it would be gelatin in water. Given the darkening role of each wetting and drying cycle on the claylike pigments, the fewer the number of gelatin treatment cycles, the better. Thus the greater mist concentrations possible with the nebulizer technology described by Arnold (1996) would have another advantage besides speed: it would reduce the number of wet-dry cycles and hence darkening. One must establish first that the higher viscosity would not inhibit penetration. In the consolidation of carbon black, however, results with gelatin were dismal; successful consolidants in carbon black were methylcellulose in water or B-72 in ethanol.