THE INFLUENCE OF MORDANT ON THE LIGHTFASTNESS OF YELLOW NATURAL DYES
Patricia Cox Crews
4 VISUAL EVALUATIONS
4.1 Gray Scale Classifications
A gray scale consists of pairs of gray chips in gradations of color differences from 1 to 5. Gray scale ratings are made by comparing the dyed specimens to the scale under standard lighting and the classification or rating is determined by that pair of gray chips on the scale which show a contrast equal to that between the original dyed specimen and the exposed specimen.26 The gray scale classifications for all dye-mordant combinations are shown in Table VIII. The gray scale classifications of the three observers did not vary from each other more than one step on the Gray Scale for Color Change. Consequently, the average gray scale classifications reported in Table VIII are representative.
COLOR CHANGES AFTER EXPOSURE TO XENON LAMP FOR 80 AFUs
Color changes are dramatic: a class 1 occurred only with tin and alum mordants. Fustic with a chrome mordant was rated class 4 and with copper and iron mordants was rated a class 5, no change. In fact, it was the only dye rated a class 5 with any mordant. Yet the colorfastness of fustic with an alum or tin mordant after exposure to 80 AFUs was only class 2.5 and class 2, respectively. These gray scale evaluations illustrated, once again, some of the reasons for contradictory reports in the literature about the lightfastness of fustic.
The visual assessments of color change corresponded fairly well with instrumental measurements. However, there were some dye-mordant combinations visually assigned to the same class which had 10–15 units difference in instrumentally measured color changes. For example, following exposure to 80 AFUs, mimosa with copper and tin mordants was assigned to class 3, yet the color change measured instrumentally was only 2.5 units for mimosa with a copper mordant, while the color change was 12.4 units for mimosa with a tin mordant. Onion, peach, and turmeric were other dyes which were visually assigned to the same classes, but which had large differences in instrumentally measured color changes.
These discrepancies illustrate the problems inherent in attempts to compare and correlate visual and instrumental measurements of color changes. Human perception of color change is often different than that reflected by instrumental measurement of color change. At present, there are no color-difference equations that will yield numerical values for color differences of the same size as those perceived by the eye for all colors.27 This is the major criticism of color difference formulas and for this reason the American Association of Textile Chemists and Colorists recommends visual evaluation.28 Nevertheless, many researchers in textile mills and laboratories successfully use instrumental methods for measuring color difference. Consequently, I decided to use both types of measurements in this study despite the problems with the comparison of data.
4.2 Lightfastness Ratings
Another way to assess lightfastness of textiles is by exposing dyed specimens with blue wool standards, L 1 - L 8. The lightfastness rating is determined by ascertaining which of the blue wool standards has faded to the same extent as the dyed specimen.29 The lightfastness ratings for all dye-mordant combinations are shown in Table VIII. Fustic with copper and iron mordants had lightfastness ratings of L7, and they were the only dye-mordant combinations above L6. Only fustic (Cr), grape (Cr), mimosa (Cr), clover (Cu), smartweed (Cu), and indigo had lightfastness ratings of L6. These ratings show the superiority of chrome, copper, and iron mordants in producing lightfast colors. The lightfastness ratings had deviations from the instrumental measurements similar to those described for the gray scale classifications. Some dye-mordant combinations given the same ratings had as much as 10 units difference in instrumentally measured color changes.