The Basics of Indigo Dyeing

When you start digging into the “how to” of naturally dyeing fiber it starts to look a tad bit like college chemistry, which personally I did neither well on nor enjoyed. Certain dyes can react to the metal of your dye pot, to the air, to the calcium content in your water. There is a moment when reading about dyes that the beginner becomes overwhelmed, and wishes to abandon everything. Fear not brave dyer, you are not alone! To keep from tossing the yarn out with the dye-water start by breaking natural dyeing down. Right now… Indigo!

Indigo- this is the “I” or ROYGBIV, Indigo is a deep blue while “Blue” is better called Cyan. When in dye it is creates what Crayola calls “Denim Blue” not surprising considering Denim is dyed blue with Indigo!

Indigo is a  legume that creates great ground cover by forming  shrubs. It is often used to improve soil quality as legumes add nitrogen with the nodules attached to the roots. The plants are part of the Indigofera genus witch has over 750 species many which produce indigo dye. Depending on climate it is an annual, biennial, or perennial.

Indigo is known as a vat dye, it is not water soluble.

There are 3 basic steps to indigo dyeing.

1. The bath must be alkaline removing the oxygen this is the “Reduction”
2. The molecular structure of the indigo must be changed so they penetrate and adhere the fiber
3. Oxidation- the air bringing the dye back to an insoluble Blue state

There are many methods of dyeing with indigo. They all basically must make the bath alkaline, allowing the indigo to will dissolve. Urine, ammonia, baking soda, lye, are all used for this purpose. The molecular structure of the indigo must be changed in order for the dye to penetrate and adhere to the fiber.

Making the Extract Stock Solution in Preparation for Dyeing –information provided by Michele Wipplinger of Earthhues

1. Put 2 oz. of natural indigo into a quart glass jar with a wide mouth- you can use this mix over time if you pick a jar that will seal air tight.
2. Add 1/4 cup warm water (80° F) and stir to make paste.
3. Add one more cup of water and stir. The solution should be opaque and blue.
4. Add two TBS of an alkali (e.g. sodium hydroxide or lye) to dissolve the indigo. Stir carefully.  Always wear gloves, mask and protective eye-wear when measuring and using lye. Do not breathe the stock solution vapors after adding the lye to the jar.
5. Next dissolve two TBS thiourea dioxide into nearly boiling water (one cup), add to the stock solution and stir until dissolved. Wear a protective mask and avoid breathing the stock solution vapors.  Add enough warm water to reach the neck of the quart jar and stir gently.
6. Allow this stock solution to sit for 15 minutes so it can dissolve and reduce. The solution will change from a dark blue to a translucent green-yellow with a coppery scum on the top. Check to see if the stock is ready by dribbling some solution on the side of a white plastic cup and note the change from a transparent green-yellow to a dark opaque blue once oxidized.

This stock can be kept indefinitely if stored in a dark cool place and sealed securely. Natural indigo extract always has a distinctive grassy odor because it is often composted with local organic matter prior to exporting. If the stock turns blue over time, add a scant 1 TBS of dissolved thiourea dioxide. Check the pH to see that the stock remains pH 11, if not add 2 tsp of lye and stir well. If some of the stock evaporates over time simply add warm water and 1 TBS of dissolved thiourea dioxide, stir well. Wait for 15 minutes until the stock once again reduces and changes color.

1. Fill up the dye kettle with warm water; 120°-130 F for wool and silk, 90 – 100F for cotton or linen.
2. If you are dyeing protein fibers (silk or wool), follow this next step. In a separate container or jar, soak 3 TBS of hide glue in lukewarm water until the grains swell, about 20 minutes.  Add about two cups of very hot water and stir until dissolved.  You will add this hide glue solution once your vat is sharpened.
3. Add the one half cup of dye stock, stir well and note the color of the dyebath. If after 15 minutes it remains an opaque blue color it needs to be sharpened.
4. Check the pH of the dyebath with your pH sticks.
   • Wool and silk should have (and can have if using hide glue) a pH 10-11. Cotton and cellulose fibers have a pH of 11.
   • If the pH is low, increase it by dissolving ½ cup soda ash in 1 quart of hot water.  When it is dissolved, add all of it to the vat and re-check the pH of the dyebath.
5. Next add a little thiourea dioxide that has been dissolved in very hot water.  Use 1 teaspoon of thiourea dissolved in 1 quart of very hot water.  Add it about ½ cup at a time and wait for 15 minutes for the water to change color, or reduce. If the indigo water does not change, add another ½ cup of the thiourea solution and wait for it to reduce.
6. Once the oxygen is reduced, the indigo water changes color from blue to a green-yellow. Carefully note the color because it is an important chemical change that indicates vat readiness.
7. The dye bath should be a clear green-yellow (not clear yellow!) with the appropriate pH and temperature for each fiber type before dyeing. If the dye bath is too yellow it is over reduced and dark indigo will be impossible to attain. As well, the excess thiourea will smell quite strongly of sulfur. If this occurs just paddle the dye bath to add oxygen until the color of the dye bath is correct. Once this has occurred dyeing should commence.
8. Add the dissolved hide glue solution to the vat and stir well.
9. Add the clean wet cloth, warps or skeined yarn to the dye bath. Keep the goods submerged the entire time and gently move them around under the water the entire dye period. Hold the goods in the dye bath from a few seconds to three minutes (maximum of five minutes) depending on the depth of shade required, the amount being dyed and the number of previous indigo dips. Basically the first dips should only be for 30 seconds to one minute. All subsequent dips can be from one to five minutes. Keep track of the number of dips.
10. Remove the cloth or yarn gently from the dye bath trying not to drip into the dyebath.  Do not squeeze or wring your yarn or cloth. Allow the goods to oxidize (flat) in the shade for 20-30 minutes. Gently open up strands of yarn in the skein to allow oxygen to reach the inside of the fibers. After oxidizing dip again, repeating this sequence until the desired shade of blue is achieved. Keep in mind that at least two values of color will be lost to rinsing and drying. Therefore always dye two to three values deeper than required.
11. When do you add fresh indigo stock?  Often you will need to add ½ to 1 cup stock if you notice that your dips are not getting any darker.
12. Where possible, oxidize 24 hours after the last dip and before washing.

Finishing Process

1. The finishing process includes two steps: neutralizing and washing.
   Neutralize all yarns after indigo dyeing by rinsing in either tannic acid (5 tea bags per pound) for cotton or acetic acid (1/4-cup vinegar per pound) for wool and silk. Soak (110° F) for 15 minutes until the rinse water is between pH 6 – 7.
2. Wash the indigo dyed goods in very hot water (170° F) with a neutral soap (Orvus paste or shampoo) for 20 minutes. Often it requires two to three hot water washings with fresh water to remove the excess indigo. End the process with a series of warm water rinses (no soap) until the color runs clear and the goods do not crock (rub off).
3. Because hide glue has been used to protect the cloth or yarn, they should be soft and supple after the indigo dyeing. Silk should have retained its sheen and strength and wool will be more lustrous than before the dyeing, and it too should be full and lofty.Remember that the process of extracting and dyeing with indigo is an art. It is necessary to continuously experiment and make changes until you arrive at your own effective system.

Notes:

The following color reference below will help guide your assessment of the vats readiness for dyeing.
Opaque blue:  The indigo bath is not ready for dyeing because of the oxygen in the water.
Clear blue-green: There is just a little too much oxygen in the dyebath. It is getting closer to being ready.
Clear greenish-yellow: The indigo bath is perfect; there is no oxygen left.
Clear yellow: There is an excess of the reducing agent (thiourea dioxide). Do not dye yet!
Paddle the bath to reintroduce some air until it turns greenish-yellow.