Alum

Aluminum is the most plentiful metal in the earth’s crust, and it is a very companionable molecule. It readily bonds to oxygen and silicon, seizing into rocks including feldspar and bauxite. In spite of its abundance under foot, even after millennia of metallurgy aluminum wasn't isolated until 1824. For a time, the rarity of this newfangled, non-corroding metal saw it valued more highly than gold or silver. There was even a craze for aluminum jewelry before less expensive production methods were developed in 1886.

Aluminum was given its name in 1812 by the chemists wrestling to prise pure metal from the earths and minerals in which it was lodged. That name comes from the French alum, derived from Latin alumen for white salts used in medicine, water purification, and dyeing since ancient times. These alum salts are brewed by the geologic activity of aluminum-rich earth, particularly in volcanic regions, or they accrue in rings around evaporating desert pools. Grecian islands, Italian volcanoes, and the deserts of western Egypt were historical sources of alum salts. Long before the identification and isolation of elemental aluminum, alum salts were  a hugely important trade good. 

“Nature produces no perfect alum, but affords the materials for it in sundry ores, pyrites, stones, slates, earths, waters, and bitumens...” [1] Access to alum purified from such natural sources was essential to historical dyers. One plentiful source of alunite for extracting alum lies in Tolfa, outside of Rome. In the 15th century this lay within the Papal States, and a bustling export of alum grew under the auspices of Pope Pius II. A surprisingly avaricious series of pontiffs vied to monopolize the alum trade in Europe, threatening the leaders of Christian nations with excommunication should they patronize Eastern sources. 

One king did find himself excommunicated, but not for alum-associated reasons: Henry VIII of England. Initiating generations of politico-religious squabbling with the Popes, Henry’s great wool-producing nation was cut off from both Papal and Spanish alum. England began seeking a domestic source of alum-bearing rock in the mid-1500s, and the technological know-how to purify it. It took nearly one hundred years for the material and method to scale up to meet the needs of the nation, utilizing the bounteous shale of North Yorkshire. This area remained the center of alum production in England for over 250 years. The industry was so large that locals couldn’t keep up with its demand for urine - the source of ammonia - and large tubs of urine had to be transported north from London’s beer-drinking masses.

Why is alum so vital to the art of dyeing? It’s a water-soluble salt of aluminum. Potash alum is the naturally occurring variety, with the modern name potassium aluminum sulfate. It is just one type of alum (along with aluminum sulfate, aluminum acetate, aluminum triformate, and beyond) which makes a useful mordant thanks to its aluminum content. In the dyepot, companionable aluminum easily bonds to most natural dyes. It conjoins with these colorful compounds by chelation: attaching to two points of the organic dye molecule. Aluminum (under the right conditions) also attaches to natural fibers, drawing dye from solution and lodging it within a textile. Aluminum mordants effect strong color uptake and retention, making soluble dyes insoluble and helping to stabilize them against the depredations of UV light. 

All mordants are metallic salts, but aluminum salts are not the only variety. They are the most commonly used mordants because alum salts are relatively abundant, easily purified, inexpensive, and have no color of their own. The salts of other metals including iron, copper, tin, and chrome can also act as mordants. In addition to forming the chemical link between dye and fiber, these metals alter the shade of natural dyes.

Of course, dyers throughout history haven’t needed to know the chemical constituents or bonding mechanisms of alum to see its utility. They didn’t even need a concept of elements or molecules. Alum simply vivifies color and fixes it in place. As Vannoccio Biringuccio wrote in 1540, “alum is no less necessary to dyers of wool and woollen cloth than bread is to humankind.”

[1] The Dyer's Assistant in the art of dying wool and woollen goods by James Haigh, 1800.

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.

Pigment

Pigment is colorful particulate. Pigments are colorful because they absorb and reflect portions of the visible spectrum of light. Their particles are insoluble, maintaining their integrity in water. This is because pigments are (by and large) inert, discreet, stable compounds. Unlike wanton changeable dyes, pigments withstand the depredations of acids, alkalis, and sunlight. For their beautiful colors and steadfast nature, pigments are prized by artists.

Because pigments are so stolid, they don’t bond to other molecules (say, canvas or paper). To become paint, pigment particles are enrobed in adhesive: a binder. It may be fat, sap, paste, glue, oil, wax, protein, plastic - any type of sticky adhesive to hold the pigment in place. 

When pigment meets binder, it suddenly seems less inert. The effects of particle size, shape, and surface become apparent. Some pigments are necessarily large because, if ground too finely, their color would be scattered away in all directions. Morphology is the term for shape, which may be spherical, fibrous, platelet, needle-like, rod-shaped, crystalline, aggregate, or… amorphous. If a pigment particle has a reflective surface, it can appear lustrous, nacreous, or sparkly. When pigment meets binder, its absorbency, texture, granulation, and hiding strength become apparent. This last one, the opacity or transparency of a color, is governed by how pigment and binder each refract light. 

Over hundreds of thousands of years, most of the pigments humans have used to express ourselves and beautify our world are inorganic: rocks and minerals. Much effort has gone into seeking, extracting, and refining these colors. Gradually, recipes for complex formulations were developed: white lead, vermillion, verdigris, and beyond. These are classified as premodern synthetics because they are concocted by humans, either to reproduce in more pure and abundant form a naturally-occurring color, or to contrive a compound never found in nature.

When the Synthetic Revolution kicked off in 1856, the palette of pigments and dyes available to artists ballooned. New synthetics are being invented to this day - very sophisticated and brilliant hues. Pigments of the preindustrial world may not necessarily be as pure, constant, or rule-abiding as their modern counterparts. They don’t always follow expectations or slot neatly into categories. Handmade and historical pigments are characterful, subtle, beguiling, eloquent expressions of nature and artisanry.

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.