EVALUATION OF APPEARANCE AND FADING OF DAYLIGHT FLUORESCENT WATERCOLORS
SANDRA A. CONNORS-ROWE, HANNAH R. MORRIS, & PAUL M. WHITMORE
The appearance of the fluorescent watercolors in this study was found to be dependent on the concentration of the paint application. As with glazes, dark applications of these watercolors (i.e., high concentrations of color) will experience a shift in hue due to the widening of their absorbance band. They may also experience self-quenching, which results from the absorbance of fluorescent emission by the colorant itself and further alters the apparent hue and chroma. Some watercolors (Tropic Gold, Sunset Orange, Sunset Red, Ice Green, and Sunshine Yellow) experience significant spectral shifts at high concentrations because of the process of energy transfer between two dyes used to create each of these watercolors.
As one should expect, the appearance of these watercolors is also dependent on the light source used to illuminate them. High CCT light sources (i.e., simulated daylight) contain the appropriate wavelengths to produce both reflected and luminous color. Low CCT sources (i.e., incandescent lamps) will not produce a luminous appearance because they do not contain the appropriate wavelengths to excite fluorescent emission. Black light sources will produce the luminous appearance from the fluorescent emission alone.
Removal of near-ultraviolet wavelengths from a simulated daylight source—a common preservation measure—does not significantly alter the appearance of the watercolors studied. Designed as daylight fluorescent watercolors, they are primarily excited by visible wavelengths, and the removal of UV wavelengths does not significantly reduce the intensity of their fluorescent emission. Nine of the 12 watercolors studied showed light sensitivities comparable to Blue Wool 1 and Blue Wool 2 when exposed to simulated daylight. Only Ice Pink, Tropic Pink, and Ice Green showed moderate light sensitivity comparable to Blue Wool 3. Removal of UV wavelengths slowed the fading rate for several watercolors—Sunset Red, Ice Pink, Tropic Pink, Sunshine Yellow, Ice Green, Fuchsia, Sunrise Pink, Raspberry, and Tahiti Red—to 24–83% of the rate under unfiltered daylight.
Three types of change occurred to these watercolors during prolonged exposure to simulated daylight. Seven of the 12 watercolors tested (Fuchsia, Raspberry, Sunrise Pink, Tahiti Red, Ice Pink, Tropic Pink, and Ice Yellow) faded by a simultaneous loss in the ability to fluoresce and absorb light as the colorant was destroyed. Four other watercolors (Tropic Gold, Sunset Orange, Sunset Red, and Ice Green) lost one fluorescent emission peak (at longer wavelengths), while a second emission peak emerged at shorter wavelengths. This change occurred because of a disruption in energy transfer between two dyes (from the more rapid destruction of the acceptor dye) in the colorant mixture and resulted in a significant shift in hue as the watercolor faded. The 12th watercolor (Sunshine Yellow) also seemed to contain a mixture of fluorescent dyes; however, both dyes lost their ability to fluoresce simultaneously instead of showing the loss of one emission peak followed by the emergence of another.
Prolonged exposure to black light produced fading results similar to those found upon exposure to simulated daylight. However, three of the watercolors (Tropic Gold, Tahiti Red, and Sunset Orange) experienced an increase in emission as a result of black light exposure. Also, the colorant component responsible for the green fluorescent emission (at 510 nm) found in several watercolors (Tropic Gold, Sunset Orange, Sunset Red, Ice Green, Ice Yellow, and Sunshine Yellow) was relatively stable to black light exposure. Because they all degrade to the same fluorescent green residue, it is difficult to distinguish the actual color of these watercolors after only a slight degree of fading under black light exposure.
Matching the color of faded fluorescent watercolors with the original paint was possible in some cases. The spectra for 6 of the 12 watercolors (Fuchsia, Raspberry, Sunrise Pink, Tahiti Red, Ice Pink, and Tropic Pink) matched well at both high (slightly faded) and low (severely faded) color concentrations. Dilute applications of the other six watercolors (Ice Yellow, Tropic Gold, Sunset Orange, Sunset Red, Ice Green, and Sunshine Yellow) did not match their faded counterparts because the faded watercolor had experienced a significant loss of fluorescent emission. For these six, compensation of faded passages may be better achieved with paints containing nonfluorescent colorants.