Mordant

Mordant is a wonderfully acerbic descriptor. A mordant wit is cutting; a mordant pain is gnawing. A mordant question is incisive. Also from the Latin mordēre, meaning to bite: morsels of food and remorseful moods. And mordant salts: the metal compounds which help natural dyes bite onto fibers. 

The craft of natural dyeing is the pursuit of the greatest spectrum of hues with the highest durability to laundering, wear, and light exposure. Some natural dyes have a friendly chemical propensity for latching fast to fibers with no assistance at all. But the majority of dyes lack the attachment points for that connection. Because dyes are water soluble, colors merely sitting on the surface are liable to rinse off and flow down the drain when washed. They require the helping hand of a mordant, which chemically fixes dye molecules onto the fiber substrate.

Mordants are metal salts, most commonly of aluminum, but also iron, copper, and tin. These metal salts dissolve in water, where they complexate with natural dyes - the metal ion attaches at multiple points to dye molecules. Dyes which bond to mordants in this way are called, conveniently enough, mordant dyes. The hue of mordant dyes may be altered by this attachment, depending on the choice of metal.

If only it were so simple to lodge the mordant and its colorful baggage in a fiber. This step usually requires careful attention to temperature, pH, and adjunct ingredients which assist with mordant uptake. These ingredients, including tannin, chalk, cream of tartar, and soda ash are not mordants in themselves, but create the chemical conditions for secure bonding to fibers.

Mordants effect strong color uptake and retention, vastly improving the washfastness and lightfastness of mordant dyes. They transform soluble dyes into insoluble complexes that won’t wash away during laundering, and bolster those colors against the depredations of sunlight. 

While mordants help dyes bite into fibers, they can become corrosive if applied overgenerously, fretting and fraying the fiber substrate. This corrosive quality is a linkage to mordant’s etymological family tree, full of other biting words. To test if a fabric has been mordanted, simply touch it to your tongue and taste for the bitter, astringent saltiness. It tastes something like the flavor of a mordant wit or remorseful mood.

Dye

Dye is a soluble colorant compound. Soluble because it drifts into translucent solution in a solvent (usually water, but sometimes alcohol, fat, or another solvent). Dye is colorful because it reflects some portion of the visible spectrum of light to our eyes, which excitedly signal our brains to interpret the perception.

Because they are soluble, dyes seep into the pores or fibers of a substrate with penetrating color. To be made washfast (impervious to rinsing away with more solvent), dyes must be chemically affixed to the substrate. The bonding mechanism depends on the type of dye (mordant, vat, acid, fiber reactive, disperse) and the type of substrate (cellulose, protein, synthetic). 

Natural dyes are derived from living sources: plants, fungi, insects, lichens, mollusks, algae, and bacteria. Natural dyes flow in solution through the bodies of plants and other living dye sources. Some dyes serve the purpose of conveying visible color, such as within the purple petals that beckon pollinators to a flower. Others dyes serve metabolic, immune, or reproductive purposes - or functions science hasn’t yet plumbed the depths of. Because natural dyes are organic molecules compounded within a body and lifespan, they are by nature excitingly reactive and changeable compounds.

Humans have sought dyes in the living world for many thousands of years; archaeological evidence suggests at least 34,000. Today, our relationship to natural dye sources is enduring evidence of countless generations of husbandry and craft. The history of dyeing encompasses deep connection to nature and astounding artisanry, as well as the abuse of living people and ecosystems with the burgeoning commodification of color. The history of dye is also the history of chemistry itself, stretching from empirical embodied knowledge, through the development of the experimental method, to molecular models and quantum physics.