This page is for anyone who specifies, buys or checks a dyed fabric and needs to know why a dye that works on one fabric can fail completely on another. At work, a buyer or merchandiser reads a fabric spec that names a fibre content, a dyeing mill reads the same spec and picks a dye chemistry, and both sides need to agree that the dye chosen actually bonds with the fibre named. This is the product flow: what the fabric is decides what can colour it, before any cost or timeline question is even asked.
Without this skill, a buyer can approve a fabric spec without knowing whether the dye class the mill plans to use will actually work on that fibre, and a merchandiser can treat "dyed fabric" as one single step instead of a fibre-matching decision. The most common failure is assuming one dye chemistry works on every fibre. It does not. Reactive, disperse, acid and vat dyes are each formulated for a specific fibre chemistry, and a blended fabric — cotton mixed with polyester, for example — can need two different dye classes applied one after the other, not one dye class doing double duty.
You need: lesson 2.1 (how dyeing works) is useful background, since it introduces fixation, but this lesson can be read on its own.
By the end of this lesson you can:
Not covered here: how the dyeing process itself works, fibre-to-fixation (lesson 2.1); batch dyeing versus continuous dyeing (lesson 2.3); dye-lot minimums (lesson 2.4).
| Term | Meaning |
|---|---|
| Dye class | A category of dye chemistry matched to a specific fibre type, such as reactive dyes for cotton, disperse dyes for polyester, or vat dyes for denim. |
| Reactive dye | A dye class that bonds chemically with cellulosic fibres such as cotton, linen and viscose. |
| Disperse dye | A dye class formulated for synthetic fibres such as polyester, which have no chemical sites for a reactive dye to bond with. |
| Acid dye | A dye class used on protein fibres such as wool and silk, and on some synthetics such as nylon. |
| Vat dye | A dye class, including indigo, used where a surface-only, wash-fast colour is wanted; the dye does not need to reach the fibre's core. |
| Indigo dye | A vat dye that colours only the outer surface of a yarn, not all the way through its core; the reason Order B's own denim fades along stress points with wear. |
A dye class is not a colour; it is a category of chemistry, and each category bonds with one kind of fibre. Reactive dyes bond chemically with cellulosic fibres — cotton, linen, viscose — and are the standard dye class for Order A's own cotton fabric. Disperse dyes are formulated for synthetic fibres such as polyester, which have no chemical sites for a reactive dye to grip. Acid dyes are used on protein fibres such as wool and silk, and on some synthetics such as nylon. Vat dyes, including indigo, are used where a surface-only, wash-fast colour is wanted, as with Order B's own indigo-dyed denim, already fixed as a vat dye. Each class is matched to specific fibre chemistry; the four are not interchangeable, and choosing the wrong one for a fibre means the dye will not bond.
| What can differ | The shapes you will meet | Why it matters |
|---|---|---|
| Reactive dye | Used on cellulosic fibres: cotton, linen, viscose. Order A's own cotton fabric uses this class. | It is the dye class you will meet most often on cotton apparel, both export and domestic orders. |
| Disperse dye | Used on synthetic fibres, chiefly polyester. Not used on either of this academy's two fixed orders, but common across the wider industry, including polyester-blend wovens and knits. | A cotton-formulated dye will not bond with polyester; a fabric spec naming polyester needs disperse dye, not reactive dye. |
| Acid dye | Used on protein fibres — wool, silk — and on some synthetics such as nylon. Not used on either fixed order. | Wool and silk garments, and some nylon items such as swimwear or hosiery, need acid dye; reactive and disperse dyes will not bond with these fibres. |
| Vat dye (including indigo) | Used where a surface-only, wash-fast colour is wanted. Order B's own denim uses indigo, a vat dye, already fixed for this academy. | Vat dyeing is why indigo-dyed denim fades along stress points with wear: the dye colours the yarn's surface, not its core. |
| Blended fabric (two fibres) | A cotton-polyester blend, for example, contains both a cellulosic fibre and a synthetic fibre. Neither this academy's Order A nor Order B is a blend of this kind, but blends are common industry-wide. | A blend needs two dye classes applied in sequence, one for each fibre present — never one dye class alone (the trap, below). |
Northline Apparel's buyer is finalising a season's range for export, made to order (MTO), and needs to confirm which dye class applies to two different fabrics in that range. The first is Order A's own cotton knit, single jersey 180 GSM, Ne 30/1 combed cotton — a cellulosic fibre, so it takes reactive dye, sold to Delta Garments by Colorline Dyeing. The second is a polyester-blend woven the buyer is sourcing separately from another mill for a different style in the same range; a fabric spec line reading "100% polyester" tells the buyer this fabric needs disperse dye, since polyester has no chemical sites for a reactive dye to bond with. The buyer records both dye classes against their respective fabric specs before approving either for bulk production.
The worked numberThis lesson needs no new calculation. It reuses Order A's fixed cotton fibre and Order B's fixed indigo-dyed denim as the two real, fixed examples, and the worked output is correctly matching each fabric to its dye class.
A common mistake is treating a cotton-polyester blend as needing only one dye class, since it looks and feels like a single fabric. It does not. A 60% cotton, 40% polyester blend actually needs two different dye classes applied in sequence: reactive dye for the cotton portion and disperse dye for the polyester portion, since neither dye class alone bonds with both fibres present. Applying only reactive dye leaves the polyester portion uncoloured; applying only disperse dye leaves the cotton portion uncoloured. Neither Order A's cotton knit nor Order B's denim is this kind of blend, but a blend like this is common across the wider industry, and the mistake costs a mill a re-dye or a visibly wrong colour result.
Dye is not one thing. A dye that colours cotton will not colour polyester, and a dye that colours polyester will not colour cotton. Each dye class is made to work with one kind of fibre. Order A's cotton fabric uses one dye class, called reactive dye. Order B's denim uses a different dye class, called vat dye, because indigo colours only the surface of the yarn. A fabric made of two fibres, like cotton mixed with polyester, needs two different dyes, one after the other, not one dye doing the whole job. Before approving a dyed fabric, always check what it is made of first.
Facts:
The document the buyer reads, the fabric spec line, exactly as sent by the mill:
You are Northline Apparel's buyer. Does this fabric need one dye class or two? Decide before you open the answer.
Two dye classes are needed: reactive dye for the cotton portion and disperse dye for the polyester portion, applied in sequence.
Two facts decide this together.
First, the fibre content. The spec line names two different fibre types in one fabric: cotton (60%) and polyester (40%). Cotton is a cellulosic fibre; polyester is a synthetic fibre. These two fibre types do not share a dye class.
Second, dye chemistry only bonds with one fibre family. Reactive dye bonds with cellulosic fibres like cotton but has no chemical site to grip polyester. Disperse dye bonds with synthetic fibres like polyester but will not colour cotton. Neither dye alone reaches both fibres present in this fabric.
If the buyer approved this fabric for dyeing with only reactive dye, the polyester 40% would come out of the dye bath uncoloured or a visibly different, lighter shade than the cotton portion — a colour failure the buyer would only discover after the fabric was already dyed, wasting the dye-lot minimum quantity and the time to redo it.
Framed here without a fixed dollar figure, since neither fixed order is a blend: applying only one dye class to a blended fabric leaves the other fibre's portion uncoloured, or a visibly different shade, producing a colour result the buyer or mill must then re-dye or reject. A re-dye repeats the fabric preparation and dyeing steps already run once, doubling the mill's dyeing time and chemical cost for that lot, and risks missing the fabric delivery date inside the order's timeline. This is a real cost, not separately quantified for this lesson, that the fibre-to-dye-class matching method above is designed to prevent before dyeing starts, not after.
Use the dye class selector workbook to record the fibre type and dye class for each fabric your order uses. Fill it from the fabric spec, not from the fabric's feel or appearance.
| Question | Order A (filled in) | Your fabric |
|---|---|---|
| Main fibre named on the spec | Cotton (Ne 30/1 combed) | |
| Cellulosic, synthetic, protein, or blend? | Cellulosic | |
| At or above Colorline Dyeing's 300 kg dye-lot minimum? | Yes (Order A: 3,075 kg) | |
| Recommended dye class | Reactive dye | |
| More than one fibre present? | No |
The full workbook is available for download at [templates/T04-C02-dye-class-selector.xlsx](../../templates/T04-C02-dye-class-selector.xlsx).
1. What dye class does Order A's own cotton fabric use?
2. What dye class does Order B's own indigo-dyed denim use?
3. A fabric spec reads '55% cotton, 45% polyester blend'. Does it need one dye class or two?
| EN | AR | FR | TR | Meaning |
|---|---|---|---|---|
| Dye class | فئة الصبغة | Classe de teinture | Boya sınıfı | A category of dye chemistry matched to a specific fibre type, such as reactive dyes for cotton, disperse dyes for polyester, or vat dyes for denim. |
| Reactive dye | صبغة فعالة | Teinture réactive | Reaktif boya | A dye class that bonds chemically with cellulosic fibres such as cotton, linen and viscose. |
| Disperse dye | صبغة متشتتة | Teinture dispersée | Dispers boya | A dye class formulated for synthetic fibres such as polyester, which have no chemical sites for a reactive dye to bond with. |
| Acid dye | صبغة حمضية | Teinture acide | Asit boya | A dye class used on protein fibres such as wool and silk, and on some synthetics such as nylon. |
| Vat dye | صبغة وعائية | Teinture de cuve | Küp boya | A dye class, including indigo, used where a surface-only, wash-fast colour is wanted; the dye does not need to reach the fibre's core. |
| Indigo dye | صبغة النيلة | Teinture indigo | İndigo boya | A dye that colours only the outer surface of a yarn, not all the way through its core; the reason indigo-dyed denim fades along stress points with wear. |
You now know that a fabric's fibre content decides which dye class can colour it. The next lesson looks at a different question: once the right dye class is chosen, does the mill dye that fabric in a batch with other orders, or run it continuously through a dyeing line? Back to the course map