Dye

Archaeological evidence shows that, particularly in India and the Middle East, dyeing has been carried out for over 5000 years. 

The dyes were obtained from either animal, vegetable or mineral origin with no or very little processing. By far the greatest source of dyes has been from the plant kingdom, notably roots, berries, bark, leaves and wood, but only a few have ever been used on a commercial scale.

Natural dyes

Animal Origin

Mordant dye

As the name suggests these dyes require a mordant. This improves the fastness of the dye on the fiber such as water, light and perspiration fastness. The choice of mordant is very important as different mordants can change the final color significantly. Most natural dyes are mordant dyes and there is therefore a large literature base describing dyeing techniques. The most important mordant dyes are the synthetic mordant dyes (chrome dyes) used for wool, these comprise some 30% of dyes used for wool and are especially useful for black and navy shades. The mordant used is potassium dichromate applied as an after-treatment.

Vat dye

These dyes are essentially insoluble in water and incapable of dyeing fibers directly. However, reduction in alkaline liquor produces the water soluble alkali metal salt of the dye. In this leuco form these dyes have an affinity for the textile fiber. Subsequent oxidation reforms the original insoluble dye.

Reactive dye

First appeared commercially in 1956, after their invention in 1954 by Rattee and Stephens at the ICI Dyestuffs Division site in Blackley, Manchester, UK. They are used to dye cellulosic fibers. The dyes contain a reactive group, either a haloheterocycle or an activated double bond, that, when applied to a fiber in a weakly alkaline dyebath, forms a chemical bond with an hydroxyl group on the cellulosic fiber. Reactive dyeing is now the most important method for the coloration of cellulsic fibers. Reactive dyes can also be used to dye wool and nylon, in the latter case they are applied under weakly acidic conditions.

Disperse dye

Originally developed for the dyeing of cellulose acetate. They are substantially water insoluble. The dyes are finely ground in the presence of a dispersing agent then sold as a paste or spray dried and sold as a powder. They can also be used to dye nylon, triacetate, polyester and acrylic fibers. In some cases a dyeing temperature of 130 deg C is required and a pressurised dyebath is used. The very fine particle size gives a large surface area that aids dissolution to allow uptake by the fiber. The dyeing rate can be significantly influenced by the choice of dispersing agent used during the grinding.

Azoic dye

A dyeing technique in which an insoluble azo dye is produced directly onto or within the fiber. This is achieved by treating a fiber with a diazo component and a coupling component. With suitable adjustment of dyebath conditions the two components react to produce the required insoluble azo dye. This technique of dyeing is unique in that the final color is controlled by the choice of the diazo and coupling components.

Reading the Classification

The classification tables on this page sort the historic natural dyes by origin and by how they attach to fiber, and that second sorting is the one a dyer still works by. Vat dyes such as indigo and woad are insoluble until reduced in the vat, then oxidize inside the fiber. Mordant dyes — madder, weld, logwood, cochineal and many others — need a metal mordant to form a fast complex with the fiber. Direct or substantive dyes such as turmeric and safflower attach without a mordant but are generally less fast. Modern synthetic dye classes follow the same logic by fiber instead of by source: reactive dyes bond chemically to cotton, acid dyes suit wool, silk and nylon, and disperse dyes are made for polyester — the class that also supplies sublimation printing.

Fastness: The Quality Every Dye Is Judged By

A color is only as good as its fastness — its resistance to washing, light, rubbing and perspiration. The definition's mention of the mordant points at the oldest fastness problem: color that rinses out in the first wash is decoration for a day, not dyeing. Every advance in dye chemistry, from mordanting and vat reduction to reactive bonding, is an answer to that problem, and commercial dyeing is specified and tested against fastness standards before a fabric ever reaches a garment factory. That is also why the dye-versus-pigment distinction at the head of this page matters commercially: the two colorants behave differently in washing, in hand and in cost, and a specification that confuses them produces the wrong fabric.