Metal mordanting in dyeing with natural colourants

Metal mordanting, or the application of metal salts, is a common method of improving the light fastness in dyeing with natural colourants. This review presents the results from a survey of the literature on metal mordanting to assess what levels of correlation exist between mordant-induced effects (changes in colour and colour depth) and the changes in light fastness, what is known about the mechanisms of mordant-induced improvements of light fastness, and how the salt levels used in coloration processes compare with the limits on metal levels in wastewater and on the dyed substrates. No strong interrelationships are found between the mordant-induced effects and light fastness improvements. Knowledge about mechanisms of mordant effect on light fastness appears, in large part, to be derived from empirical correlations. However, as light fastness is affected by a multitude of factors, the correlations do not always hold true. It is found that residual metal contents in spent dye/mordanting liquors are generally not reported. However, with rough calculations, it is estimated that, with even the lowest reported salt concentrations, the metal contents in spent liquors exceed environmental release limits. The metal contents on dyed substrates are also not generally reported, but similar estimations show that the contents of heavy metal on dyed substrates (when copper and chromium salts are used as mordants) also exceed limits. On the basis of these observations, the authors make suggestions for elements to be included in investigations on furthering the use of natural colourants in textile dyeing.