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Indigo is different than any other dye. It does not require a mordant. Rather it is dyed through a living fermentation process. The process "reduces" the Indigo, changing it from blue to yellow. In this state, it dissolves in an alkaline solution. The fabric is worked in the solution, or "solution". When brought out to the air, it is a bright green. Slowly the air changes it to the beautiful deep and rich blue of Indigo.
Indigofera is a legume. The plant looks similar to alfalfa, but is usually larger. It is an excellent rotational crop for increasing soil fertility. In southern Mexico, where some of the current Indigo of commerce originates, it is naturalized and grows in fallow fields.
Indigo dye must be prepared from the fresh plant in an exacting and elaborate process that takes about a month. The Indigo plants are harvested and brought to a central location. They are soaked in water and allowed to ferment. This separates the dyestuff from the plant. The solution is then beaten to oxidize and precipitate the Indigo. Excess water is poured off and the sludge is dried. This sludge, packed into balls or patties and fully dried, is the Indigo dye of commerce.
Processed Indigo is in the form of a hard, dark blue colored cake. It must be ground to be used for dyeing. Very small amounts can be ground in a mortar and pestle. In most cultures, Indigo dyeing is or was a specialty. The dye process is unique, and the facilities require a stable set-up. solutions made of great clay pots set in the ground are commonly used in warmer climates. If more heat is needed, pits for burning charcoal are placed between clusters of the solutions.
Indigo dyeing is practiced today in Japan, Southern China, Tibet, India, Indonesia, Indo China, Africa, especially Nigeria, Southern Mexico and Guatemala, and it has recently been reintroduced to Turkey. Traditional fermentation methods are used. However, many of these cultures now use synthetic Indigo, manufactured from coal tar or petroleum.
An additional beauty of dark Indigo is that when ironed or pounded, the blue cloth takes on a beautiful coppery sheen - the same sheen that is seen on the well reduced Indigo solution, when it is ready for dyeing.
The basic solution - The cut plant is tied into bundles, which are then packed into the fermenting vats and covered with clear fresh water. The indigo plant is allowed to steep till the rapid fermentation, which quickly sets in, has almost ceased, the time required being from 10-15 hours. The liquor, which varies from a pale straw colour to a golden-yellow, is then agitated. The colour of the liquid becomes green, then blue, and, finally, the indigo separates out as flakes, and is precipitated to the bottom of the vats. The indigo is allowed to thoroughly settle, when the supernatant liquid is drawn off. The pulpy mass of indigo is then boiled with water for some hours to remove impurities, filtered through thick woollen or coarse canvas bags, then pressed to remove as much of the moisture as possible, after which it is cut into cubes and finally air-dried.
NOTE: requires advance preparation of about one week.
(above amounts are by weight ounces, not volume ounces.) Combine in about a three gallon pot of warm water.
Always add these amounts in proportion. A larger solution can be made, for example with: 1 lb. ground indigo, 1/2 lb ground madder, 1/2 lb ground bran and 3 lbs washing soda in about a 10 gallon plastic tub. However, I advise starting small, till you are comfortable with the process. The size of the pot is determined by the amount of fobre you need to dye at one time. A three gallon pot is good for fabric skeins of 4 to 6 oz., while a 10 gallon or larger tub will be needed for yards of fabric.
It is necessary to keep the solution warm, but not hot, around 100 - 110° Fahrenheit. It is the same temperature for raising bread or making yogurt. It should feel pleasantly warm to the hand. To keep it warm, a light bulb in a reflector can be put under the solution, with a blanket over it to keep in the heat. (See illustration, next page.) In a warm climate no additional heat is needed, but be sure the solution is out of direct sun so it does not overheat.
It takes time for the solution to ferment and it does no good to try to rush the process. The first time, it takes about a week for the solution to ferment and be ready to dye. With "renewals" the time needed is a bit less, four or five days. It takes time to do the dyeing. The fabric is "dipped" several times to build up a dark color, with airing between each dip.
The solution itself lasts a long time. In traditional cultures there are solutions over 100 years old. The solution is "renewed" with more Indigo and the other ingredients in proportion, whenever the dye value weakens. Then let sit a few days to re-ferment. Indigo dyeing by natural fermentation is a slow-steady process.
Stir the solution once a day to integrate the undissolved Indigo, madder and bran that settles to the bottom, back into solution without incorporating air into the solution. So stir gently. Keep the solution covered - air is the enemy of a good Indigo solution. The level of liquid in the pot should just allow room for dyeing, without spilling. The less air between surface and lid the better. Use a domed lid, turned upside down, to leave less than 2" of air at the top of the solution.
The solution is ready for dyeing when:
Begin Indigo dyeing!
The solution lasts indefinitely as long as it is maintained with a certain amount of Indigo, and all other ingredients as given, in proportion. Dyeing is begun, with the darkest color dyed first, then medium, then lights. Between dyeings the solution must rest overnight or an extra day. This is because, during dyeing a certain amount of the Indigo is oxidized in the solution. Allowing it to rest lets it re-reduce that Indigo. An oxidized (blue colored) solution won't dye well. The Indigo color will only wash out and rub off too quickly.
When the solution is "exhausted", and will only dye light shades, it is time to renew it. All ingredients are again added, again in correct proportion. The solution is let to ferment for several days, and is ready to dye when it shows the proper signs. In this way a solution can be kept going for many years.
Taking a break - Simply turn off the heat source, and keep the solution cool for several weeks. Stir it vigorously on occasion. When ready to dye again, warm it up, renew it with the ingredients, and proceed as before. It is not good to leave a solution unused for too long, as it is a living process and may then get cranky about starting up again. Also it is important to exhaust the solution before leaving it, or it may over-ferment and ruin any Indigo remaining in it. Over time a deposit of sludge will develop at the bottom of the solution. You may want to gently lower a screen into the solution before dyeing, to keep your fabric from pickinging it up during the dye process. Be sure to remove the screen after the day's dyeing, so you can stir the solution before closing it.
For greens, dye your fabric Indigo first, then rinse well and overdye with alum mordant and your chosen yellow dye. For purples, dye the Indigo first, rinse well, then mordant and dye over with any red dye.
The indigo solution is very alkaline. It is important to rinse out all the alkalinity. Just to be on the safe side, I always double rinse my indigo dyed textiles. First I rinse well just after dyeing, then I let air overnight. Next day I soak in two successive waters for about an hour each time, rinse again, wring and dry.