Chemistry of the Dying Process, With and Without a Mordant

Ancient people may have gotten the idea to dye fabrics when they noticed stains on their clothing after eating or working with different plants, berries, and mollas.

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They began to use these plants and animals to dye fabrics. Ancient dyes can be classified chemically into three groups: blue and purple indigoid vat dyes, red anthraquinonoid mordant dyes, and yellow flavonoid dyes.


Comes from a legume with tiny reddish flowers, exceptional quality and colorfastness important item of trade between India and other parts of the world for at least 2,300 years, Europeans had their own source of blue, woad, but in never truly rivaled indigo for, richness of color the “woadites” lobbied for laws in Britain to prohibit the importation of indigo, which they called the “devil’s weed”–they were successful until the 17th century, could not be grown in Europe, but was grown for a time in the Carolinas and Georgia, until cotton came along. Dye is contained in the leaves the stems and leaves are crushed, steeped in water for 12 hours and fermented the solution containing the white dye is aerated by beaters until a blue color develops.

This was the standard blue dye until 1856 when a synthetic blue dye was produced to make a dye that would adhere to fabric, the indigo solution was reduced by removing the oxygen from the pot. This changes the substance to “indigo white.” The reducing agent thiorea dioxide (also called Thiox or thiourea dioxide) removes oxygen and changes indigo to indigo white—“which will adhere to material, but it’s not blue, it’s white!” Thayer said. But when fiber is pulled from the vat, the dyeing solution oxidizes as soon as it hits the air and becomes indigo blue again.

When adding fiber or fabric to a dye vat, make sure it’s squeezed, Thayer advised, “because you don’t want to introduce air into the bath. Hold it together until it’s under the bath, then open it up. And wear gloves” if you don’t want your hands to turn blue. When you stir wool in the vat, do so gently. Likewise, when removing dyed fiber or fabric from the vat, put your hand under it and allow it to gently “float” out of the pot so that oxygen is not introduced into the reduced solution, Thayer suggested.

Because the indigo needs to be freshly cut to work well, and that used at the Fair had been cut a few days earlier, only a light blue resulted.

Dyeing with Fresh Indigo

  1. Pick 16 oz. of fresh indigo leaves, put them in a bucket and add just enough hot tap water to cover the leaves.
  2. Heat the solution to 160 degrees F. over a period of two hours. Don’t heat it too quickly.
  3. Strain the liquid and squeeze liquid from the indigo leaves into the strained liquid.
  4. Add 2 Tbsp. baking soda to the liquid and stir a little.
  5. Pour the liquid from one bucket to another for a few minutes, or until the solution turns dark green/blue. This oxidizes the dissolved indoxyl, changing it to indigo.
  6. Dissolve 2 Tbsp. Thiox in warm water, pour it into the dyebath, cover and set the pot in a larger container of water that is just hot enough to keep the dyebath at 100 to 120 degrees F. for about an hour.
  7. Meanwhile, soak 2 to 4 oz. of yarn in hot water.
  8. When the dyebath has turned yellow, add the wet yarn—carefully, to minimize adding oxygen to the solution. Leave it in the dye for 20 minutes. Remove it gently and let it oxidize by hanging it on a wooden rack. The yarn will turn blue as it reacts with oxygen in the air.
  9. Let the yarn dry for as long as it soaked in the bath. One dipping and airing is usually enough to richly color wool yarn, but for intense colors on cotton or silk, repeat the soaking and airing two or more times. You can put successive batches of yarn into the same dyebath, getting lighter colors each time, until the yarn no longer turns blue. Then discard the dyebath. It is safe to pour down the drain. Scrub stains from the pot.
  10. After the final airing, wash and rinse the yarn.

Dyeing with Natural, Powdered Indigo in Urine

  1. Collect about 4 gallons of urine in a bucket and let it sit for a week or so.
  2. Put 1 oz. of natural, powdered indigo in a cloth bag and suspend the bag in the urine.
  3. Every day for a month or as long as it takes, rub the bag and put it back in the urine, being careful not to oxidize the urine in the bucket.
  4. Gently lower wet wool into the solution and let it sit for up to 24 hours.
  5. Remove the wool gently and let it oxidize in the air for about the same length of time that it was soaking in the dye.
  6. Wash the wool in soap until the urine smell is gone.

Those who don’t want to use urine can try the baking soda and Thiox directions with indigo cakes.

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The Real World of Chemistry 6th ed by Lois Fruen Kendall.

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