JAIC 1986, Volume 25, Number 2, Article 1 (pp. 65 to 72)
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Journal of the American Institute for Conservation
JAIC 1986, Volume 25, Number 2, Article 1 (pp. 65 to 72)

DETERMINATION OF THE SPECIFIC RATE CONSTANT FOR THE LOSS OF A YELLOW INTERMEDIATE DURING THE FADING OF ALIZARIN LAKE

Robert L. Feller, Ruth M. Johnston-Feller, & Catherine Bailie



3 FORMATION OF A YELLOW INTERMEDIATE

UPON CALCULATION OF THE CONCENTRATION of pigments necessary to match the spectrophotometric curves of the faded states of the test panels of alizarin lake and titanium white formulated in poly(vinylacetate), applied at complete hiding, the color-matching computations indicated that the addition of a yellow component was usually required. Figure 1 shows a typical result: along with the expected decrease in the amount of alizarin lake with increased exposure, there was also at first a rising, then a declining, concentration of a yellow component present in many of the exposed panels. Such behavior can be expected as the result of a sequence of chemical steps that may be represented by the equation

Fig. .
in which initial colorant A (in our case, alizarin lake) forms component Y (in our case, a yellow substance for which the yellow, unlaked, form of alizarin was used for calculation purposes). This in turn yields a further product, C. In our situation, C must have been colorless because no pigments other than alizarin lake, titanium white, and a yellow were needed to match the color of the faded states of the test panels.4 Other organic colorants also are known to go through an intermediate colored form before passing on to a colorless, completely faded, condition.

In our initial report,2 we demonstrated that the apparent rate of disappearance of the alizarin lake tended to follow equation ii. We further reported that the fading of alizarin and its yellow intermediate appeared to follow a sequence of events such as shown in equation iii and cited a specific first-order rate constant, k2, for the disappearance of the yellow. However, the method used to obtain the rate constant for this second step was not explained. We wish now to describe the technique used to obtain values of k2 by a simple graphical procedure.


Copyright 1986 American Institute of Historic and Artistic Works