Natural Indigo Blue Dye Origin Dying Procedures Technology and Dye Recipes for Denim Fabrics

Dyeing with Indigo

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 fibres (silk or wool), follow this next step. In a separate container or jar, soak 3 TBS of hiding 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 colour of the dyebath. If after 15 minutes it remains an opaque blue colour, it needs to be sharpened.
4. Check the pH of the dyebath with your pH sticks.
   1. Wool and silk should have (and can have if using hide glue) a pH of 10-11. Cotton and cellulose fibres have a pH of 11.
   2. 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 colour, 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 colour from blue to a green-yellow. Carefully note the colour because it is an important chemical change that indicates vat readiness. The following colour reference below will help guide your assessment of readiness for dyeing.
   1. Opaque blue:  The indigo bath is not ready for dyeing because of the oxygen in the water.
   2. Clear blue-green: There is just a little too much oxygen in the dyebath. It is getting closer to being ready.
   3. Clear greenish-yellow: The indigo bath is perfect; there is no oxygen left.
   4. 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.
7. The dye bath should be a clear green-yellow (not clear yellow!) with the appropriate pH and temperature for each fibre 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 colour 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 for 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 fibres. After oxidizing dip again, repeating this sequence until the desired shade of blue is achieved. Keep in mind that at least two values of colour 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

The finishing process includes two steps: neutralizing and washing.

1. 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 colour 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.
4. 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.
5. Conclusion— I have long been fascinated with indigo – the brilliant natural blue dye that has been used throughout human history. Before the advent of chemical dyes, indigo dying was practised throughout Europe, most of Africa, the Middle East, most of Asia, and South and Central America. For me, this attraction has been purely aesthetic. Indigo is used so beautifully in traditional textiles. But once I started delving into the history of indigo (for the purposes of writing this article) I discovered that there is so much more than meets the eye. From being revered almost magical qualities, to a central role in slavery and colonization, to its resurgence in sustainable fashion, the history of indigo is worth getting acquainted with.