The choice of mordant is the single most powerful decision a natural dyer makes after choosing the dyestuff itself. Four traditional options—alum, iron, copper, and tannin—each move color in a predictable direction, and each behaves differently on protein fibers like wool and silk than on cellulose fibers like cotton and linen. This guide compares all four so you can match mordant to fiber and to the color you actually want, rather than the color a recipe happened to produce for someone else.
Understanding Mordants: The Key to Lasting Natural Color
A mordant is, at its most fundamental, a bonding agent. As the natural dye supplier Natural Dyes explains, mordants are metallic salts that facilitate the bonding of the dyestuff to the fibre, and colourfast dyeing usually requires one. The chemistry matters for fiber selection: protein fibers and cellulose fibers do not accept mordants in the same way, which is why a process that works beautifully on wool can disappoint on cotton.
One definitional point is worth settling early, because it shapes everything that follows. Tannins are not technically mordants at all—they are not metallic salts—but Natural Dyes notes they are routinely discussed as part of the mordant process for cellulose fibres, appearing in recipes alongside metallic mordants. Treating tannin as a fourth "mordant" is a practical convention, not a chemistry textbook classification. The same source also flags a caution worth preserving: some older recipes still call for heavy-metal mordants such as chrome, which the supplier does not recommend because of toxicity and disposal challenges. This guide deliberately stays with the four traditional, widely used options.
Alum: Brightening and Enhancing Hues
Alum is the workhorse of traditional mordanting, and its virtue is restraint. In a study cited in a published review of bio-mordants, alum applied to cotton garments dyed with onion-peel extract produced yellow colors—letting the dye's own hue read clearly rather than pushing it somewhere new. That transparency is why alum is the default choice when a dyer wants the dyestuff to speak for itself.
But alum is not one chemical, and fiber type decides which form you reach for. Botanical Colors, in a guide written by dyer Cara Piazza, recommends aluminum potassium sulfate, aluminum sulfate, and aluminum triformate for protein fibers—describing all three as easy to use and delivering excellent results—while recommending aluminum acetate or a tannin-alum-chalk method for cellulose fibers. Both cellulose routes are multi-step processes. Natural Dyes' own testing reinforces the protein-fiber strength of the alum family: the supplier reports that aluminum triformate is an excellent mordant for wool and silk, creating a strong bond between mordant dyes and the fibre, and works as a cold, one-step procedure on cellulose—though its lightfastness across many natural dyes improves when a tannin process precedes it.
