THE INFLUENCE OF MORDANT ON THE LIGHTFASTNESS OF YELLOW NATURAL DYES
Patricia Cox Crews
MOST OF THE DYES tested in this study had only poor to moderate lightfastness. Many of the yellow dyes faded appreciably with exposure to as little as 5 AFUs and most yellow dyes had lightfastness ratings below L5 on a scale of L1 to L9. The dyes, fustic and turmeric, that had the greatest amounts of color change were the natural dyes most widely used in the European dye houses of the 18th and 19th centuries.30, 31 Consequently, conservators need not fear that North American textiles dyed with local plant materials will be less lightfast than textiles dyed industrially in the dye houses of that time. In fact, home-dyed and ethnic textiles may be more lightfast.
All of the dyes which had large amounts of color change were used with alum or tin mordants. Many dyes which had very little color change with chrome, copper, and iron mordants had large amounts of color change with alum and tin mordants. The effects of mordant on lightfastness have been known for some time in the textile industry,32 which is why chrome mordants are the only ones currently in use industrially.33 This work dramatically reinforces the importance of considering and carefully selecting a mordant. In fact, identification of the mordant present in a textile could be more important than dye identification in predicting lightfastness. To make the wisest display decisions, museum personnel should ideally know both dye and mordant present in the artifact.
Finally, museums should request or encourage the use of chrome, copper, and, iron mordants when making special purchases of tapestries and wall hangings for their collections from contemporary fiber artists. In this way, museums would begin practicing “preventive conservation”. While the copper and iron mordants produce only yellow-green and brown shades, the chrome mordants do provide dyeings with good lightfastness and only slightly duller yellows and golds than alum or tin mordants.