Almost every dyeing failure traces back to one decision made at the very start: picking a dye that can’t chemically bond to the fibre you’re dyeing.
No technique compensates for that. Acid dye on cotton washes straight out. Reactive dye on polyester barely tints it. The colour looks fine coming out of the pot and disappears in the first wash.
This guide covers each dye class, what it bonds to, what it needs, and how to choose. It’s part of our wider fabric dyeing section.
Why Fibre Chemistry Decides the Dye
Dyes don’t coat fabric. They bond with it — and the bond available depends on the fibre’s chemistry.
Cellulosic fibres (cotton, linen, viscose, rayon, hemp, bamboo, Tencel) carry hydroxyl groups. Reactive dyes bond to these covalently.
Protein fibres (wool, silk, and also nylon, which behaves similarly despite being synthetic) carry amino groups that attract acid dyes ionically.
Polyester is hydrophobic with almost no reactive sites. Nothing bonds to its surface — disperse dyes get inside the polymer structure instead, which requires heat and pressure.
The first question in any dyeing project is what the fabric is made of. If the label’s gone, a burn test is the usual method: cellulosics burn steadily and smell like paper, protein fibres smell like burning hair and self-extinguish, synthetics melt into a hard bead.
Reactive Dyes
Works on: cotton, linen, viscose, rayon, hemp, bamboo, Tencel. Also silk and wool with an acidic bath. Needs: soda ash (alkali) plus salt.
Reactive dyes are the standard for cellulosics and the most widely used class in industry. What sets them apart is the bond: a reactive group in the dye molecule chemically reacts with the fibre to form a covalent bond. The dye becomes part of the fibre rather than clinging to it.
That’s why reactive-dyed cotton holds colour through years of washing. Brilliant clear shades, an enormous colour range, excellent wash fastness, and reasonable cost.
How it works in practice: salt drives the dye onto the fibre by reducing electrical repulsion between the negatively charged dye and negatively charged cellulose. Soda ash then raises the pH to trigger the bonding reaction. Both are needed, and they do different jobs.
Procion MX is the best-known home dyeing brand in this class — a cold-water reactive dye, which is exactly why it dominates tie-dye and batik. Because it works without heat, it won’t melt the wax in batik work.
Worth knowing: reactive dyes have poor fastness to chlorine bleach. Bleaching a reactive-dyed garment strips colour. Also, reactive dyes used on silk or wool with vinegar instead of soda ash function as acid dyes, and mixed colours can shift noticeably on protein fibres — a black that works on cotton may go a different direction on silk.
Acid Dyes
Works on: wool, silk, nylon, and other protein fibres. Needs: an acid — usually vinegar or citric acid — plus heat.
Acid dyes bond ionically to the amino groups in protein fibres. They produce the most brilliant colours achievable on wool and silk, which is why they dominate hand-dyeing for those fibres.
They will not dye cotton. On a cotton-wool blend, an acid dye colours only the wool and leaves the cotton pale — occasionally used deliberately for heathered effects, usually just a mistake.
Nylon is the surprise in this category. It’s synthetic, but its amino groups make it behave like a protein fibre, so acid dyes work well on it.
Worth knowing: for deep blacks on silk, wool or nylon, premetallised (metal complex) acid dyes such as the Lanaset range generally outperform reactive dyes, which tend toward brighter but less deep colours.
Disperse Dyes
Works on: polyester, acetate, and other hydrophobic synthetics. Needs: high heat, usually around 130°C under pressure.
Disperse dyes exist because polyester couldn’t be dyed any other way. They’re only slightly water-soluble, suspended as fine particles, and under heat they diffuse into the fibre’s polymer structure rather than bonding to its surface.
That temperature requirement is why home-dyeing polyester generally fails. A domestic stovetop can’t reach the temperature needed to open the polyester structure. Products sold for synthetics work by extended simmering and produce pale, limited results.
Disperse dyes are also what makes sublimation printing possible — the same dyes convert from solid to gas under heat and bond into synthetic fabric.
Worth knowing: disperse dyes stain cotton during processing, and those stains are hard to remove — a real problem when dyeing poly-cotton blends, usually managed with anti-staining agents.
Direct Dyes
Works on: cellulosics. Needs: salt; no alkali fixation step.
Direct dyes attach to cellulose without the chemical reaction reactive dyes require. They’re simpler and cheaper, and that’s their appeal.
The tradeoff is fastness. They bond through weaker forces, so colour bleeds and fades with washing. A cationic fixative applied afterwards improves this noticeably but doesn’t match reactive dyeing.
Reasonable for items washed rarely — wall hangings, craft projects. Not a good choice for clothing.
Vat Dyes
Works on: cellulosics. Needs: a reducing agent and oxidation.
Vat dyes work through a redox process. The dye is insoluble normally, gets chemically reduced into a soluble form that penetrates the fibre, then oxidises back to insoluble inside the fibre — physically trapped there.
The result is outstanding fastness, including resistance to bleach. That’s why vat dyes are used for workwear and why denim behaves as it does.
Indigo is the famous example. Its particular behaviour — sitting on the outside of the cotton yarn rather than penetrating fully — is precisely what produces the fade patterns denim is valued for.
The process is more complex than reactive dyeing and the colour range is narrower.
Sulphur Dyes
Works on: cellulosics. Needs: a reducing agent.
The economical option for deep, dull shades on cotton — particularly blacks, browns and navies. Good wash fastness, moderate light fastness.
Not used for bright colours. The effluent is a genuine environmental concern at industrial scale.
Basic (Cationic) Dyes
Works on: acrylic, modacrylic. Also silk and wool with a mordant.
Basic dyes carry a positive charge and bond to negatively charged sites. On acrylic specifically they produce exceptional brightness — better than any other class.
On natural fibres, brightness is good but light fastness is generally poor.
Natural Dyes
Works on: most natural fibres, with the right mordant. Needs: a mordant, usually a metal salt such as alum.
Natural dyes come from plants, insects and minerals: madder (red), indigo (blue), weld (yellow), cochineal (crimson, from insects), walnut hulls (brown), oak galls (dark tones and a tannin source).
Most require a mordant — a metal salt that bridges between fibre and dye. Without it, most natural dyes wash out. Alum is the common and relatively safe choice; iron shifts and darkens colours; some historical mordants (chrome, tin) are genuinely toxic.
The appeal is real: subtle, complex colours synthetic dyes struggle to reproduce. The limitations are equally real: less predictable results, generally lower fastness, far more material per kilo of fabric, and the persistent misconception that natural automatically means safe.
Matching Dye to Fibre
| Fibre | Best choice | Also possible |
|---|---|---|
| Cotton, linen, hemp | Reactive | Vat, direct, sulphur, natural |
| Viscose, rayon, modal, Tencel, bamboo | Reactive | Vat, direct |
| Wool | Acid | Natural (with mordant), reactive with acid |
| Silk | Acid | Reactive, natural |
| Nylon | Acid | Disperse |
| Polyester | Disperse | — (nothing else works) |
| Acrylic | Basic (cationic) | — |
| Spandex/elastane | Acid, disperse | Usually a small blend percentage |
Our dyeing processes by fabric section covers fibre-specific procedures in detail.
Blends
Blends need two dye classes under different conditions, which is where dyeing gets genuinely difficult.
Poly-cotton needs disperse for the polyester and reactive for the cotton. Two-bath dyeing runs them separately — more controllable, better results, more time and energy. One-bath applies both at once — faster and cheaper, but conditions compromise both dyes.
Cotton-spandex dyes with reactive dyes; the small spandex percentage takes little colour but the fabric must be kept below temperatures that degrade elastane.
Wool-acrylic can be dyed with acid dyes, which will colour both components.
Silk-linen ideally gets reactive first, then an acid bath.
Union dyes (the household brands like Rit and Dylon) contain multiple dye classes in one package specifically to handle mixed fibres. They’re convenient and forgiving, but colours are less intense and fastness is lower than using the correct single-class dye.
This is why a poly-cotton shirt often emerges from a home dye bath looking washed out: the cotton took colour, the polyester didn’t, and the result is a diluted-looking shade.
Choosing for a Project
- Identify the fibre. Everything follows from this.
- Decide how permanent it needs to be. Reactive and vat for garments; direct is adequate for decorative pieces.
- Consider your equipment. Reactive dyes work cold, which suits tie-dye, batik, and anyone without a large heated vessel. Acid dyes need sustained heat. Disperse effectively needs industrial equipment.
- Check colour expectations. Reactive and acid give bright clear shades; vat and sulphur give deeper, duller ones; natural gives subtle complex tones.
Our general dyeing section covers product guides and practical questions.
Common Problems
Colour washed straight out. Wrong dye class for the fibre, or the fixation step was skipped. If the first rinse runs heavily coloured, the dye never bonded.
Blend came out pale and patchy. Only one fibre took the dye. Use a union dye, or dye in two baths with the appropriate class for each.
Shade doesn’t match the sample. Dye quantity is calculated as a percentage of dry fabric weight, not estimated by eye. Water chemistry, liquor ratio and temperature all affect the result too.
Colour shifted on silk compared to cotton. Mixed-colour reactive dyes contain several component dyes that behave differently on protein fibres. Single-hue dyes stay truer across fibres.
Can’t get a proper black. Blacks are the hardest shade. On protein fibres, premetallised acid dyes generally outperform reactive. On cotton, use more dye than you think — deep shades need 4–6% or more of fabric weight.
Where to Go From Here
- Dyeing Processes by Fabric — fibre-by-fibre procedures
- General Dyeing — product guides and practical questions
- Knit Fabric Dyeing — handling knitted structures in wet processing
- Fabric Dyeing — the full guide to processes and variables
Related areas:
- Fabric Types & Specs — identifying fibre content before you dye
- Fabric Printing — applying colour to specific areas instead of whole cloth
Frequently Asked Questions
What dye should I use on cotton? Fibre reactive dye — Procion MX is the common home brand. It forms a covalent bond with cellulose, giving the best wash fastness of any class. You’ll need soda ash to fix it and salt to drive it onto the fibre.
Can I dye polyester at home? Realistically no. Polyester needs disperse dyes at around 130°C under pressure, which a domestic stovetop can’t reach. Products marketed for synthetics work by extended simmering and give pale results. On a poly-cotton blend, only the cotton will take colour.
Will acid dye work on cotton? No. Acid dyes bond to amino groups, which cellulose doesn’t have. The colour will rinse out. Use a reactive dye for cotton.
Can I use Procion MX on wool or silk? Yes, but you have to change the recipe — use vinegar instead of soda ash, and apply heat. It then functions as an acid dye rather than a reactive one. Results are adequate, though purpose-made acid dyes give better colour on those fibres. Expect mixed shades to shift.
What’s a mordant, and do I need one? A mordant is usually a metal salt that bridges between fibre and dye. Natural dyes generally need one — without it most wash out. Synthetic reactive, acid and disperse dyes don’t need mordants; they have their own fixing chemistry.
What’s the difference between a dye and a pigment? A dye dissolves and bonds with the fibre chemically. A pigment is an insoluble particle that sits on the surface held by a binder. Dyed fabric keeps its original hand-feel; pigment-coloured fabric often feels stiffer but can colour fibres that are otherwise hard to dye.
Are household dyes like Rit and Dylon any good? They’re union dyes — several dye classes in one package, so they’ll put some colour on most fibres. Convenient and forgiving, especially for mixed or unknown fabrics. The tradeoff is less intense colour and lower fastness than using the correct single-class dye for a known fibre.
How much dye do I need? Calculate as a percentage of dry fabric weight, not by volume of water. Roughly 0.5–1% for pale shades, 2–3% for medium, 4–6% or more for deep shades and blacks. Weigh the fabric dry before you start.
