How to Prepare Cotton Fabric for Natural Indigo Dyeing

Why Preparation Decides the Colour You Get

Indigo behaves unlike almost every other natural dye. It does not chemically bond to cotton the way madder or weld might; instead, the pigment sits in and around the fibres, building up in thin layers each time you dip. That means anything sitting on the surface of your cloth — sizing, starches, oils, fabric softener, even the natural pectins and waxes in raw cotton — will block the pigment and leave you with the pale, mottled blue that so many home dyers blame on their vat. The good news is that almost all patchiness traced back to a vat is actually a preparation problem, and it is entirely fixable.

Two steps do the heavy lifting: scouring, which strips away everything the cotton came with, and mordanting (or a tannin pre-treatment), which primes the fibre for the layers to come. Neither is difficult. Both reward patience.

First, Work Out What You Are Dyeing

New cotton straight from a shop is usually the hardest to prepare, because it is full of sizing and optical brighteners designed to make it look crisp and white. Secondhand shirts, sheets and tea towels have often been washed many times, which helps, but they may carry softener residue, perfume oils or synthetic stitching that will not take the dye.

• Check the fibre content. A "100% cotton" label with 5% elastane will dye unevenly. Pure cotton, linen, rayon and hemp all take indigo well.
• Cut away what will not dye. Remove polyester labels, zip pulls, interfacing and any non-cellulose trim before you begin.
• Do the water-drop test. Place a droplet on the surface. If it sinks in within a second or two, the cloth is absorbent. If it beads up and sits there, scour it — and possibly scour it again.
• Weigh your dry fabric. All the quantities below are given as a percentage of the weight of the fibre (WOF), so a kitchen scale is your most useful tool here.

Scouring Cotton: The Step Most People Skip

Scouring is simply a long, hot, alkaline wash. It removes pectins, waxes, manufacturing oils and sizing in one go, leaving the fibre thirsty and ready.

For every 100g of dry cotton, you will need roughly:

• 2g soda ash (sodium carbonate) or about 5g washing soda, which is the same chemical with water of crystallisation attached
• 3–5g soap flakes or a grated plain white soap with no perfume or additives
• Enough soft water to cover the fabric freely — hard water leaves calcium deposits that dull the finished blue, so rainwater or filtered water is worth the effort

Dissolve the soda ash and soap in hot water, add the wetted fabric, and bring the pot to around 80–90°C — a gentle simmer, not a rolling boil, which can abrade the cloth. Hold it there for 45–60 minutes, stirring occasionally with wooden tongs so the fabric does not sit in creases. Then rinse in gradually cooler water until it runs completely clear and no suds remain. If the water still feels slippery or the fabric still repels droplets, repeat the whole process.

Do You Need to Mordant for Indigo?

Strictly speaking, no. Indigo is a vat dye and attaches mechanically rather than through a metal salt, so a mordant is not what makes the blue stick. If you plan to layer other natural dyes underneath or over the top, however, mordanting first is essential — and many dyers find that a tannin and alum pre-treatment gives indigo a slightly deeper, cooler tone and helps it survive repeated washing.

The classic sequence for cellulose is a tannin bath followed by alum:

• Tannin: simmer 5–10% WOF of tannin-rich material — oak gall, sumac, pomegranate rind or strong black tea — with the damp fabric at 60–80°C for 45 minutes, then rinse lightly.
• Alum: dissolve 10–15% WOF of potassium aluminium sulphate in hot water, add the fabric, and hold at 60–80°C for 45 minutes.
• Rinse and use promptly. Mordanted cloth keeps best if it is used within a few days.

Getting Fabric into the Vat Correctly

Indigo vats are alkaline, and wet cloth accepts them far more evenly than dry cloth. Take the scoured or mordanted fabric straight from a rinse and squeeze it gently — do not wring it hard, as sharp creases become visible streaks of lighter blue.

Open the fabric out fully before it goes in, lower it in without splashing (oxygen is the enemy of a good vat), and work it gently under the surface for two to three minutes. Lift it out, let it drain, and then let it oxidise in the air for at least 15–20 minutes before the next dip. The green you see on lifting turns blue as it meets the air; resist the urge to rush this stage. If you need to pause between dips, wrap the damp cloth in a plastic bag rather than letting it dry out.

Fixing Patchy, Pale or Streaky Results

• Blotchy, lighter in the middle: almost always insufficient scouring or sizing left in the weave. Re-scour and start again.
• Pale overall: usually too few dips rather than a preparation fault. Indigo builds slowly — six to eight dips is normal for a rich navy.
• Streaks along creases: the fabric was scrunched or wrung too hard. Keep it open and handle it flat.
• Colour rubs off when dry: the pigment has not fully set. A gentle rinse in cool water followed by air drying out of direct sun will help, though a little crocking is normal with indigo.
• Yellowish or dull undertone: hard water, or a vat that has been over-reduced. Try rainwater next time.

Once you have scoured properly, the rest of the process becomes remarkably forgiving. Keep notes of your weights, times and dip counts, and within a few batches you will have a repeatable recipe for deep, even, lasting blue.