JAIC 1982, Volume 21, Number 2, Article 3 (pp. 43 to 58)
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Journal of the American Institute for Conservation
JAIC 1982, Volume 21, Number 2, Article 3 (pp. 43 to 58)

THE INFLUENCE OF MORDANT ON THE LIGHTFASTNESS OF YELLOW NATURAL DYES

Patricia Cox Crews



3 RESULTS AND DISCUSSION


3.1 Instrumental Evaluation

Statistical analysis of the instrumentally measured color changes showed that every variable—mordant, dye, and length of exposure—significantly affected the amount of color change that occurred. However, mordant affected color change more than any other variable, while dye affected color change less than any other variable (see Table III). There were significant interactions between mordant and dye, mordant and length of exposure, and dye and length of exposure. With one exception, the interactions were far less significant than main effects. Consequently, the main effects, as well as the interactions, will be discussed.

ANALYSIS OF THE EFFECTS OF MORDANT, DYE, AND TIME ON COLOR CHANGE


3.2 Mordant

The elemental composition of the mordant had a significant effect upon color change. The greatest amount of color change occurred with alum and tin mordants, while the least amount of color change occurred with chrome, copper, and iron mordants. The overall color change with alum and tin mordants was at least three times larger than with any of the other mordants (see Table IV).

EFFECT OF MORDANT ON COLOR CHANGE

The range of colors produced by the same dye using different mordants was remarkable, but each mordant had an individually limited range: only alum and tin yielded bright, clear yellows; chrome produced yellow-golds or oranges; copper produced yellow-greens; while iron produced shades of brown. Dyers in the past were faced with a difficult choice—favored colors versus lightfastness. The chrome mordants offer a compromise. They provide dyeings with good lightfastness and only slightly duller yellows or gold tones than those obtained with alum or tin mordants.


3.3 Dye

The effect of dye on color change was far less important than the effect of mordant on color change. Indigo, the most lightfast natural dye, had the least amount of color change as expected. Indigo was used without a mordant. Consequently, its mean color change was reported, but it could not be included in the general statistical analysis because all other dyes were used with mordants (see Table V).

EFFECT OF DYE ON COLOR CHANGE

Indigo and turmeric were included in the study for comparison purposes. Indigo was noted for its good lightfastness, while turmeric was noted for its poor lightfastness. Fustic, which had a somewhat better colorfastness reputation than turmeric, was not significantly different from turmeric in amount of overall color change.

Fustic, marigolds, and turmeric had the greatest amount of overall color change. They were also the dyes that produced the brightest yellow colors and were the dyes most widely used by industry.24 The cherry leaves produced some of the most fast dyes, but the yellows were very pale by contrast. Comparison of the colors produced by the most lightfast versus the least lightfast dyes makes it clear why the least fast dyes continued in use; the bright, rich yellows by fustic and turmeric were not found with any other dye.


3.4 Length of Exposure

Length of exposure, of course, had a significant effect upon the amount of color change. To determine how length of exposure affected color, a regression analysis was conducted. Regression analysis measures rate of change and determines whether the change is linear, quadratic or some other type of trend. A significant linear trend would indicate that the amount of color change was proportional to the length of exposure, while a quadratic trend would indicate that there was a faster rate of change during some parts of exposure. Regression analysis of color change over length of exposure for the dyes in this study showed that nearly all effects of length of exposure on color change were linear. However, the quadratic effect was also significant because there was a slightly faster rate of change for the first exposure period (see Table VI).

EFFECT OF TIME ON COLOR CHANGE


3.5 Mordant X Dye

The interaction of the mordants with the dyes was significant, as shown in Table III. This means that the effect of mordant on dye depended to a certain extent upon which dye was used. However, further analysis of the interactions showed that the main effect of mordant was of overwhelming importance compared to the interactions of mordant with dye. In other words, the greatest amounts of color change were attributed to alum and tin mordants and the least amount of color change occurred with chrome, copper, and iron mordants as indicated by the significant main effects. In fact, every dye had the most color change when used with an alum or tin mordant, as shown in Figure 1. However, when comparing all dye-mordant combinations, there was one dye (choke cherry) with an alum mordant that had less color change than three dyes with chrome mordants (see Figure 2), yet even choke cherry followed the general trend of having the most color change when it was mordanted with alum or tin.

Fig. 1. Mean Color Change for Each Mordant Grouped by Dye Mordant: A = Alum, B = Chrome, C = Copper, D = Iron, and E = Tin Dye: 1 = Coreopsis, 2 = Dock, 3 = Onion, 4 = Clover, 5 = Apple, 6 = Poplar, 7 = Marigold, 8 = Turmeric, 9 = C. Cherry, 10 = S. Cherry, 11 = Mullein, 12 = Grape, 13 = Peach, 14 = Mimosa, 15 = Fustic, 16 = Goldenrod, and 17 = Smartweed.

Fig. 2. Mean Color Change for Each Dye Grouped by Mordant Mordant: A = Alum, B = Chrome, C = Copper, D = Iron, and E = Tin Dye: 1 = Coreopsis, 2 = Dock, 3 = Onion, 4 = Clover, 5 = Apple, 6 = Poplar, 7 = Marigold, 8 = Turmeric, 9 = C. Cherry, 10 = S. Cherry, 11 = Mullein, 12 = Grape, 13 = Peach, 14 = Mimosa, 15 = Fustic, 16 = Goldenrod, and 17 = Smartweed.

The interaction of dyes with mordants was not simple. As a result, general trends attributable to dyes could not be easily discerned because dyes with the greatest amount of color change did not always have the greatest amount of color change with each individual mordant. For example, fustic had the second largest color change of any dye with a tin mordant, while it had the least color change of any dye with copper and iron mordants (see Figure 2). Turmeric was another example of inconsistent color changes. Turmeric with a tin mordant had the most color change of any dye-mordant combination. However, turmeric with an alum mordant had only a moderate amount of color change. These inconsistencies in amount of color change for some dyes could explain some contradictory reports in the literature regarding the lightfastness of fustic.25 When chrome mordants came into use making a more lightfast yellow color possible with fustic, its improved lightfastness was no doubt noted, yet its poor colorfastness with alum and tin mordants had been reported for many years.

Analysis of individual dye-mordant combinations showed other dyes with large amounts of overall color change but individual dye-mordant combinations with very little color change, just like fustic. This emphasizes, once again, the greater importance of mordant on lightfastness than dye.


3.6 Mordant Relative to Length of Exposure

The effect of additional amounts of exposure to light on color change depended upon which mordant was used. A regression analysis, which measured the rate of color change that occurred with additional exposure, was conducted. It showed that the color change associated with each mordant was primarily linear, that is the color change was usually proportional to length of exposure. However, the rate of color was not the same for all mordants. The rate of color change was much faster with alum and tin mordants than for chrome, copper, and iron mordants. In fact, the tin mordant had a slope, B = .1557, which was five times larger than the slope with the copper mordant, B = .0319 (see Figure 3). This showed that dyes used with tin mordants faded five times faster than dyes used with copper mordants.

Fig. 3. Color Change as a Function of Length of Exposure by Mordant.


3.7 Dye Relative to Length of Exposure

The interaction of dye with length of exposure was also significant, indicating that the effect of additional amounts of exposure to light depended upon which dye was used. It was, however, the least significant interaction with an F value of only 2.60 (see Table III). Regression analysis showed that the rate of color change for each dye was usually proportional to the length of exposure. However, some dyes faded at a faster rate than other dyes. The slopes, which indicate the rate of color change over time, ranged from as low as .0476 to as high as .1447 for turmeric (see Table VII). A flatter slope or a smaller B value would indicate a gradual color change over length of exposure, while a steeper slope or a larger B value would indicate a very rapid color change. Turmeric's slope was 3-4 times larger than dock, smartweed, cherry leaves, mullein, or grape leaves, which indicates that turmeric would fade four times faster than some of the other dyes mentioned.

REGRESSION ANALYSIS OF COLOR CHANGE ON LENGTH OF EXPOSURE FOR EACH DYE


Copyright 1982 American Institute of Historic and Artistic Works