1. Why Color Rubbed On Cloth Washes Away but Dyed Color Stays
Simply putting color on cloth is not difficult. If you rub red flower petals on fabric, a red mark remains. However, it usually disappears when washed. "Putting color on" and "dyeing" are different things. Dyeing means putting the color source (pigment) deep into the fibers so it does not come out when washed. The materials for this are called dyes. Things that dissolve in water and soak into fibers are dyes, while things that do not dissolve in water and sit on the surface as fine particles are called pigments [7]. (We discussed how black ink separates when put in water in a note about ink.)
Traces of dyed cloth are older than you might think. A 2009 paper in the science journal Science reported that wild flax fibers, which were spun, knotted, and dyed with plant pigments, were found in layers about 30,000 years old in Dzudzuana Cave in Georgia [1]. However, in 2010, other researchers questioned the identification of these fibers as flax [2]. This report alone does not tell us if the color remained after washing, which is what we mean by dyeing. In this note, we will look at the methods used to keep color in cloth for each type of dye.
2. How Indigo, a Blue Pigment That Does Not Dissolve in Water, Dyes Cloth
The source of the blue color in indigo is a pigment called indigo. It does not dissolve in water. Colors that do not dissolve are hard to wash out. But if they do not dissolve, they cannot enter the fibers on their own. Traditional Japanese indigo dyeing solves this problem cleverly. According to Tokushima Prefecture, traditional indigo dyeing uses "sukumo," made by fermenting indigo leaves. You pile up indigo leaves, add only water, and manage the temperature for about 100 days to make Awa indigo (sukumo) [3].
Making the dye liquid is called "building the indigo vat." Sukumo is put into an alkaline liquid (like lye). Through the action of microbes, the indigo changes into a water-soluble form called a leuco form, which is yellowish [4][5]. When cloth is dipped in this liquid and pulled out, it touches the air and bonds with oxygen, turning back into insoluble indigo [4]. The pigment in the dye liquid is not blue; it becomes blue only after soaking into the fibers and touching the air. One dip makes a thin color, so the process of dipping and exposing to air is repeated to make it darker [4]. It is like unlocking a door to enter, then locking it from the inside. But really, it is a chemical change where oxygen bonds with the pigment, changing it from a water-soluble form to an insoluble one.
The history of indigo dyeing is also old. In 2016, it was reported that cotton cloth about 6,000 years old found at the Huaca Prieta site in northern Peru used indigo [6]. In Japan, cotton cultivation spread around Osaka, increasing demand for indigo dye, which pairs well with cotton. Tokushima Prefecture explains that Awa indigo came to dominate the national market in the 1700s [3]. Indigo was a dye where the pigment itself was insoluble. So, how was color kept in cloth for pigments that dissolve easily in water?
3. How Madder and Lye Use Metal Ions to Hold Color in Fabric
Dyes that are dissolved and put into cloth before returning to an insoluble form, like indigo, are different from other dyes. Some plant pigments do not stay in fibers just by dipping the cloth in them. For these, mordanting is used. Metal ions enter between the pigment and the fiber, bonding with both, and firmly holding the pigment in the fiber [7]. It is like double-sided tape sticking to both the pigment and the fiber. But really, it is not tape; it is an electrical attraction holding them together.
Madder is a prime example. Red pigments (like alizarin) are taken from its roots. According to the J. Paul Getty Museum, dyeing cloth with madder requires a mordant. Metal ions like aluminum bond with the root components, helping them attach to the fiber and shifting the color toward red [8]. The metal used for the mordant changes the color, even with the same plant. In general plant dyeing, alum (aluminum) often results in yellow to red colors, while iron often results in deep green to deep blue colors [7].
In Japan, instead of metal salts, lye (water dissolved with ash) has been used. Aluminum ions in the lye bond with the pigment, making the color vivid and fixing it. It is explained that camellia ash has a higher proportion of aluminum oxide than straw ash and has long been used for purple dyeing [9]. The Man'yoshu poetry collection mentions this in a poem: "Purple is dyed with ash; at the Tsu-ba-i-chi market, who is the child I met?" (Book 12, poem 3101, author unknown). The phrase "ash is added for purple" leads to asking the name of a person met in the crowd at the "Tsu-ba-i-chi" market (a place name using the same characters as camellia). The explanation says that "adding ash" brings out the word "camellia" in the place name [10][11]. People in an age without chemical terms likely knew from experience that camellia ash dyed purple beautifully.
4. Why Dyes From Shellfish and Insects Were So Expensive
The price of color changes greatly depending on the material. Shellfish purple, taken from Mediterranean sea snails, is an extreme example. The pigment in shellfish purple is chemically related to indigo, containing pigments like 6,6'-dibromoindigo (indigo with bromine attached) [12][13]. The dyeing method is similar to indigo: it is reduced to a yellow, water-soluble form, soaked into the cloth, and then oxidized by air [12].
Very little can be obtained. It is reported that the chemist Paul Friedländer collected about 12,000 Murex brandaris snails in 1908, extracted about 1.4 grams of pigment, and published this in 1909 [13][29]. That is 12,000 snails for a weight slightly more than a 1-yen coin. As for cloth dyed with shellfish purple, three pieces of wool from a copper smelting site in the Timna Valley in Israel (late 11th to early 10th century BCE) are the oldest known in the southern Levant (the southern part of the eastern Mediterranean coast). The paper introduces shellfish purple as "the most prestigious dye in many ancient societies" [12]. In the Roman Empire, it is said that Emperor Theodosius II issued a law in 424 CE restricting dark reddish-purple shellfish purple to the imperial family [14].
For red, there are insects. If you dry the cochineal scale insects that live on prickly pear cacti, you get a red pigment called carminic acid [15]. It was used in Mexico for a long time, and Spain began exporting it to Europe in the 16th century [15]. According to the Science History Institute, around 1600, about 150,000 pounds (about 68 tons) were sent to Spain each year, sometimes worth more than half its weight in silver. It is written that only one other export product surpassed cochineal in the wealth it brought to the Spanish royal family over three centuries [16]. Cochineal pigment is still used in food and cosmetics today [15]. In explanations of food additives, it is noted that it is used in drinks, sweets, processed foods like kamaboko (fish cake), and ham/sausage, and may be listed as "cochineal color" or "carminic acid color" [31]. A National Geographic article (2013) gives examples like yogurt and candy [17]. The ability to make these purples and reds from living things in factories was triggered by a failure.
5. How William Perkin Found Purple Dye While Trying to Make Medicine in 1856
During the Easter holiday in 1856, 18-year-old William Perkin, a student at the Royal College of Chemistry in London, was trying to make a medicine in his home laboratory: quinine for malaria. At the time, quinine could only be taken from the bark of the cinchona tree. His teacher, Professor Hofmann, said it would be good if it could be made artificially [18]. Perkin oxidized aniline, a colorless oil taken from coal tar, with potassium dichromate. What he got was not quinine, but a black precipitate. He extracted color from that black precipitate and tested it on silk, which turned purple [19]. He named this purple "mauve" (aniline purple) and applied for a patent on August 26, 1856 [19]. Against Hofmann's advice, he opened a factory in Greenford Green near London in 1857, using funds from his father, who was a builder [18]. He did not make the medicine. But the leftover from that failure became the world's first commercially successful synthetic dye [18]. Until now, we have looked at indigo, madder, shellfish purple, and cochineal, which are colors taken from plants, shells, and insects. From here, the era of making colors in factories begins.
6. How Factory-Made Dyes Replaced Madder and Indigo From Fields
Colors taken from fields and seas were replaced one by one by factory products. In 1868, German chemists Graebe and Liebermann found a way to make the madder pigment alizarin from substances taken from coal tar. Patents were quickly taken in Britain, dealing a heavy blow to the industry of growing madder [20]. Indigo took a bit longer. Baeyer found a way to synthesize indigo around 1880 [23]. It took time to make it in factories. The German company BASF spent 17 years of research and 18 million gold marks (an amount exceeding the company's capital at the time) to launch synthetic indigo in 1897 [21]. Production of natural indigo is reported to have dropped from about 19,000 tons per year in 1900 to about 1,000 tons in 1914 [30].
Japan's Awa indigo is also part of this trend. The area of indigo cultivation in Tokushima peaked at about 15,000 hectares in 1903. However, after that, imports of precipitated indigo from India and synthetic indigo from Europe increased, and the production of Awa indigo dropped sharply, according to Tokushima Prefecture [3]. Awa indigo is still produced today [3]. Synthetic indigo is the same substance, indigo, as natural indigo. Today, world production is about 50,000 tons of synthetic and only about 500 tons of natural indigo per year, and the main use of synthetic indigo is said to be denim and jeans [30]. So, how do today's clothes keep their color?
7. Why Today's Clothes Use Different Dyes for Different Fibers
Today's dyes for clothes are chosen for each type of fiber. According to the Nissenken Quality Evaluation Center, which tests fiber quality, reactive dyes are used for cotton and rayon, acid dyes for wool and nylon, cationic dyes for acrylic, and disperse dyes for polyester (cationic dyes for cation-dyeable types) [24]. Reactive dyes for cotton were first announced as "Procion" by the British company ICI in 1956 (exactly 100 years after the discovery of mauve). In an alkaline dye bath, the dye chemically reacts and bonds with the fiber (cellulose). Because the dye becomes part of the fiber, it is strong against washing [25]. Polyester has high crystallinity, making it hard for dye to enter. Disperse dyes are used, and the dye is pushed into the gaps in the fiber at high temperatures of about 130°C [24][26].
Even so, color can come out during washing. "Color transfer" is explained as a phenomenon where dyes with weak bonds to fibers, or excess dye on the surface, dissolve in water during washing and stick to other fibers. It is more likely to happen when the water temperature is high, when strong alkaline detergents are used, or when the washing machine is overloaded, causing strong friction [27]. Jeans are clothes that use the ease of color loss to their advantage. Indigo does not easily enter fibers, so with a method of dipping threads in the dye and exposing them to air, only the surface of the thread is dyed blue, while the center remains white ("core white"). As you wear and wash them, the blue surface wears away, revealing the white inside. This is the charm of color fading in jeans [28][22]. Jeans are clothes where the change of losing color is enjoyed.
Safety Note: Indigo does not easily enter the inside of fibers. With the method of dipping threads in dye and exposing them to air, only the surface is dyed, and the center remains white ("core white").
8. How to Investigate Indigo in Your Own Jeans With a Magnifying Glass
If you have jeans, look at the frayed hems or the threads in worn-out spots with a magnifying glass. Can you find threads that are white in the center? (Do not cut or unravel the threads. With new jeans, you may not see it yet.) The tags on clothes list the fiber combinations, such as "cotton" or "polyester." They do not list the dye names, but it is interesting to apply the correspondences from this note (cotton uses reactive dyes, polyester uses disperse dyes) and think about how that cloth was likely dyed. Look for names like "cochineal color" or "carminic acid color" in the ingredient lists of sweets and kamaboko [31]. You often will not find them. If you do, it is an example of red from insects still being used today.