Indigo is probably the most widely used dyestuff of all time – indeed denim jean material is still coloured using indigo dye as it is extremely wash fast. It was used many centuries before the Christian era in the Far East. Marco Polo saw indigo being prepared in China during the 13th century; at that time European dyers were obtaining blue colours from woad (Isatis tinctoria) which contains the same indigo molecule. Traditional methods of indigo dyeing can be observed today in Africa, Mexico, India and Japan; indigo vats can be found in almost any country in the developing world, as dyers have adapted age-old techniques to their local situations. Indigo dye is used extensively for batik and shibori.

indigo blue vat blue

George Weil offers a range of indigo including natural, synthetic and pre-reduced. Both the natural indigo and synthetic indigo are available in kits which also contain Thiourea Dioxide, Soda Ash, a stirring rod and detailed instructions.

Sources

Indigo is obtained from plants which contain the indican molecule. These belong to different plant families; the most common are of the genus Indigofera, but they also belong to others including the buckwheat family, the Lonchocarpus cyanescens found in Africa, or the Indonesian Marsdenia, and of course Woad. Although the indican molecule is contained in all these plants, producing the traditional blues requires more than merely steeping fibres with the plants in a pot full of water.

Synthetic Indigo

Synthetic indigo, prepared in the laboratory, contains the identical molecular structure to the natural indigo, but it has a much higher percentage of indigo per weight than the natural form. It is necessary to use only 1/4 to 1/3 as much synthetic indigo as natural. The powdered form is easiest, as the lump indigo must be ground or pounded to reduce it for use.

Extracting the Colour from Indigo

Although the indigo powder is blue, the indigo molecule does not produce its blue colour until it is oxidised. The indigo blue powder must be dissolved in an alkali bath with the combined oxygen removed. This is done by adding Thiourea Dioxide to the indigo vat during its preparation. Indigo is only soluble in an alkaline solution made by dissolving sodium carbonate (as Soda Ash or Washing Soda) or caustic soda in water. The resultant solution is a yellow-green in colour.

The alkalinity of the solution is controlled by the amount of Soda Ash dissolved. This can be tested with the Universal Indicator Paper. The pH needs to be between 9-11. At pH 11 it is easiest to reduce the indigo and will be best for dyeing cellulose fibre such as cotton, linen and viscose. A level of pH 9 is gentler on silk and wool and the Soda Ash should be added in increments to test the alkalinity until this level is achieved. For your first indigo bath it helps to work at the higher range but not above pH11 and wash the dyed fabric in vinegar as the final rinse.

The Dyeing Process

The indigo dyeing process involves making the soluble, yellow-green indigo in the bath turn blue while still attached to the fibre. This is accomplished by immersing the wetted (but not dripping) fibre, yarn or fabric into the indigo bath and leaving it under the surface initially for a period of 5-10 minutes. It is then removed very carefully and slowly to prevent dripping, splashing or the introduction of oxygen into the bath. It turns from yellow-green to blue as it takes in oxygen from the air.

Indigo builds colour, so the more immersion-oxidation cycles, the deeper the colour. This must be done with care to prevent introduction of oxygen to the dye bath. Subsequent immersion times must be kept to a minimum to ensure that the newly attached molecules of indigo are not stripped off by the solution. Indigo dyed materials are also very wash-fast, since the indigo blue is not water soluble, except in alkali.

Prepared for Dyeing?

Before dyeing, it is important to check whether the item is ‘Prepared for Dyeing’ (PFD) or requires scouring to remove any grease, oil or starch. Run a few droplets of cold water onto the fabric. If they soak in quickly, no scour is necessary. To remove starches, size and oils, add 5mls of Synthrapol (a non-ionic detergent) along with 2-3 litres of water for each 100gms of material. Stir gently over a 15 min period, and then rinse thoroughly in warm water. It is possible to use household detergent, but the alkaline residue may affect the final colour or wash fastness.

INDIGO DYE RECIPE:

This Indigo Dye Recipe has been tried and tested by our customers for over 30 years! (See our Blog post Preparing an Indigo Dye Vat where we did some indigo dyeing here at George Weil.)

Equipment

• 1 large jar with lid, approx. 1 litre
• Stir sticks with at least one able to reach the bottom of the dye bath
• 1 large dye bath with lid (our dyers bucket holds 12 litres and is ideal for this purpose)
• Rubber gloves; apron or old clothes
• Sink or tub for rinsing with water supply
• Clothes line for airing

Making a Stock Solution using Thiourea Dioxide

This solution is for up to 3 kg fibre.

1. Mix together 25g indigo dye powder and 15g Thiourea Dioxide in the jar
2. Gradually add half a cup of warm water to make a paste
3. Let the paste stand, covered, for 10-20 minutes
4. In a separate container add 150g of soda ash (sodium carbonate) slowly to about 1/2 litre of water, stirring until it is dissolved.
5. Add the soda ash solution to the indigo dye “mix” in stages. This is when the dye vat should be tested for the pH level, a pH of 9 is best for wool and silk while a pH level of 11 is best for cotton, linen and viscose. Stop adding the soda ash solution when the correct pH level is achieved.
6. Replace the lid and let the solution stand until the liquid clears. This usually takes 30-60 minutes. If possible keep the temperature of the solution around blood heat.
7. Test the liquid by dipping the plastic rod. As you pull it out, the liquid should be yellow-green, turning blue in around 30 seconds. If, at this time, there are white specks on the plastic rod they should be dissolved by the addition of a small amount of soda ash (sodium carbonate). If blue specks are seen, the indigo dye solution still contains some oxygen and needs a little more Thiourea Dioxide.

There is also an alternative method of making a solution using yeast and sugar.

Preparing the Indigo Vat

This process requires good ventilation, preferably outside.

1. Fill the dye bath two thirds full with water (approx. 6-8 litres), heated no higher than about 50°C (140°F.)
2. Add ½ teaspoon Thiourea Dioxide to the water and stir. Cover and let the Thiourea Dioxide reduce the oxygen in the water; this takes about 20 minutes.
3. Gently add about half your stock solution of indigo dye to the vat. Any white precipitate in the stock solution may contain undissolved indigo, which won’t help your dye in the vat, so avoid disturbing this layer if it has formed.
4. Stir very gently to avoid introducing oxygen and cover the vat for another 20-30 minutes.

Adding the Material to the Indigo Dye Vat

1. Immerse the wetted fibre, yarn or fabric into the vat. Lower it gently with your sticks or rubber-gloved hands. With a rod placed across the top of your bath, you can loosely tie through a skein of yarn. This will enable you to “work” the yarn under the surface without immersing your whole arm in the vat.
2. As you remove the fibre from the vat, squeeze it gently as it comes out of the surface to remove surplus liquid. Do not allow oxidised liquid to drip back or stir air into the vat.
3. Hang up the dyed fibre to allow it to oxidise as the colour changes from yellow-green to blue.
4. Keep immersing and oxidising until the desired blue is obtained.
5. When you are happy with the shade, lift out and squeeze out excess liquor. Thoroughly wash your dyed fibre in a bowl to which two tablespoons of vinegar have been added, rinse it well and hang it out to dry.

Unless you are intending to dye all 3 kg at once, you will not need to add all the indigo stock solution at the beginning. As your dye bath gets depleted (i.e. the blue does not get darker) you can add more stock solution. If the vat bath turns blue, it means there is free oxygen in it, so add more Thiourea Dioxide, one teaspoonful at a time, and let the vat rest for 15-20 minutes after each addition. The vat will keep for 2-6 weeks if you cover it and add additional Thiourea Dioxide to reduce any introduced oxygen (add about 1 teaspoon at a time). Store the vat in a warm place if possible.

Hazards and Problems when Indigo Dyeing

• Thiourea Dioxide can be unpleasant. Handle it carefully and do not inhale it. Working outdoors and carefully measuring with spoons will help. Be cautious if there is a breeze, as the powder becomes airborne easily. This chemical can ignite, so keep it away from sparks, fire etc. Store in a labelled covered dark glass jar away from children or other unsuspecting people or pets.
• Indigo powder adheres to your skin. Wear rubber gloves and old clothes.
• If you find that your indigo dyed material seems to have the colour rubbing or “crocking” off, the fabric did not have the indigo attached well. Your alkali bath may have been too weak to dissolve the indigo completely, or the fibre itself may not have been ready for the indigo vat; it should be well-wetted but not dripping wet. Soak the yarn, fibre, fabric for 30 minutes in warm water, then gently squeeze out the excess water and let it drip through a colander for another 5-10 minutes. Then it is wet-damp but not dripping.
• The fibre may also have been too “slick” for the indigo, which can occur when too much Thiourea Dioxide is added to the vat or the indigo may simply not have penetrated the fibres. Sometimes it helps to “work” the material while it is immersed, by gently squeezing the liquid through the fibre. Wear rubber gloves and be careful not to introduce oxygen into the bath.
• If you wish to try the Indigo Dye Recipe, sensible precautions should be taken when handling dyes and chemicals, particularly as powders:
  • Avoid inhaling dusts, they can produce an asthma type reaction. People with known respiratory problems should not handle synthetic dyes, and particularly the fibre reactive dyes, in powder form. A dust mask should be worn when working with the powders or exposed to an aerosol from spraying dye solutions made up in water
  • Avoid splashing solutions into the eyes, swallowing the materials or prolonged skin contact. A simple ‘non-contact’ approach (most people use gloves to avoid dyeing themselves) plus normal, good, hygiene is sufficient precautions for the occasional user
  • Store in clearly labelled containers well away from children, pets and foodstuffs. Treat dye powders and solutions with the same caution as domestic poisons (eg strong cleaners, bleaches or medicines)
  • Dispose of spent solutions containing residues of the dyes responsibly. Dilute and pour onto waste land or into the sewage system. They have no known effect on the environment when used in the quantities recommended in the literature