A COMPARISON OF SELECTED UV FILTERING MATERIALS FOR THE REDUCTION OF FADING
Patricia Cox Crews
3 RESULTS AND DISCUSSION
FOUR TYPES OF ULTRAVIOLET FILTERS were evaluated for their effectiveness in reducing fading of wool dyed with selected natural dyes and one synthetic dye. Unprotected wool fabrics dyed with the same dyes and exposed to the xenon-arc lamp served as controls. Color difference (DE) was measured after 80 AFUs of xenon light exposure to determine the comparative effectiveness of the filters in reducing fading or color change.
The mean color difference values for each filter after 80 AFUs of xenon light exposure are given by dye in table 1 along with the results of the Fisher's LSD tests because one-way analysis of variance procedures performed on each dye/mordant combination showed that UV filter was a significant independent variable. All dyes exhibited the least amount of color change when protected by the amber-tinted filter of polyester film; and most dyes were better protected by the silver-coated filter than the filters with the light yellow tints. This result was not surprising since the filter with the reflective silver coating transmitted approximately 50% of the radiation throughout the visible spectrum, whereas the filters with the light yellow tints transmitted over 80% of the visible radiation. Only weld (alum) exhibited a significantly greater color difference (DE) after light exposure behind a UV filter (DE = 15.63 with light yellow polyester filter versus DE = 14.27 for the control). This unexpected result occurred because the wool control yellowed upon exposure to the UV radiation in the unfiltered light more rapidly than it faded. Natural dyes are faded by both UV and visible radiation,9 and wool is yellowed by UV radiation.7 The control specimens colored yellow with the weld dye retained more of their yellow color because of the yellowing of the wool substrate whereas the weld-dyed specimens exposed to light behind UV filter underwent no yellowing, only fading caused by the visible radiation. However, this interpretation of the results should not be construed as meaning that it is better to expose objects colored with weld dyes to unfiltered light containing the ultraviolet component.
Table 1 Fisher's LSD Test on Mean Color difference Values for Each Filter after 80 AFUs of Xenon Light Exposure
The percent reduction in mean color difference for each filter is given by dye in table 2. Unlike most natural dyes evaluated, the AATCC Blue Wool L2 dyed with synthetic dyes was protected equally well by all filters (percent reduction in fading was exceptional at approximately 80%). Clearly, Blue Wool L2 should not be used in lightfastness tests to compare and/or predict the protective effects of UV filters for museum textiles as has been done in the past.8,10 Apparently, Blue Wool L2 is not sensitive to the presence or absence of visible light (at least during the exposure period evaluated in this study) unlike most natural dyes, which are subject to fading by both ultraviolet and visible radiation.
Table 2 Percent Reduction in Fading Attributed to Each Filter after 80 AFUs of Xenon Light Exposure
The average percent reduction in fading attributed to the amber-tinted polyester filter was 51% when the exceptional figure for the Blue Wool L2 was excluded. The average percent reduction in fading attributed to the other filters was 28% (silver-coated polyester), 16% (light yellow acrylic), and 15% (light yellow polyester). These findings support earlier work by Crews,3 who found that amber-tinted polyester filters were much more effective than almost colorless polyester filters.